Atoms, Ions and Bonds: Reading a Photographic Formula

Here is a formula, copied from a 1928 Kodak publication and converted from its four-litre quantities to one litre: Elon 2.0 g, sodium sulfite 100 g, hydroquinone 5.0 g, borax 2.0 g, water to 1 litre. Four powders, one liquid. By the end of this page you will be able to say, without reading another word of description, which two of those four supply the electrons that make the picture, which one is there to be sacrificed, which one sets the alkalinity, and which part of each of them is doing nothing at all except keeping the books balanced. That last skill is the one this page really delivers, because a published formula is not a list of powders. It is a list of ions, and roughly half of them do not matter.

Atoms, and the one particle that does the chemistry

An atom has a nucleus of protons and neutrons, and around it a cloud of electrons. The proton carries one unit of positive charge and the electron one unit of negative charge; the neutron carries none. Almost all the mass is in the nucleus, because a proton or neutron is close to two thousand times the mass of an electron. A neutral atom has equal numbers of protons and electrons, so the charges cancel exactly.

The number of protons is the atomic number, and it is the atom’s identity: seventeen protons is chlorine and nothing else can be. Change the number of neutrons and you have a different isotope of the same element, with the same chemistry and a different mass. Change the number of electrons and you have an ion — the same element, still, but now carrying a net charge, and behaving completely differently.

All the chemistry in this course is electrons. Nuclei are spectators. When a developer reduces a silver ion to metallic silver, one electron moves. When silver nitrate meets potassium bromide and a crystal falls out, no electrons move at all and only the company changes. When light strikes a silver bromide grain, an electron is displaced. Photography is, from end to end, an argument about the whereabouts of electrons.

The periodic table as a map of behaviour

The table is arranged so that elements in the same group — the same vertical column — behave alike, because they have the same number of outer electrons. That is what makes it a map rather than a list. Group 1, the alkali metals, hold one loose outer electron and give it up readily to form a 1+ ion: sodium and potassium, the two counter-ions of nearly every photographic formula. Group 17, the halogens, are one electron short of a full outer shell and take one to form a 1− ion: chloride, bromide and iodide, the three anions of the silver halides. Group 16 takes two, giving the 2− ions, of which oxide and sulfide matter here.

That is why potassium bromide and sodium bromide behave so nearly identically in a developer, and why silver chloride, silver bromide and silver iodide are a family with a family resemblance and important differences — which the next page measures.

The elements this course actually uses

The chemical data set this course has fetched and checked currently covers 85 substances built from 24 elements. That is close to the whole vocabulary, and the encyclopaedia’s remaining entries add no new element to it. Here is what each of the commonest is doing.

Element	Where it appears	What it is for
Silver, Ag	Every silver process	The image. Ag⁺ in a crystal before development, Ag⁰ metal after it.
Bromine, Br; chlorine, Cl; iodine, I	Halides in emulsions, restrainers, bleaches	The partner ion in the light-sensitive crystal, and the thing that has to leave.
Sodium, Na; potassium, K	Almost every formula	Counter-ions. They make an anion available as a weighable, storable powder.
Nitrogen, N	Nitrate; ammonium; organic developing agents	Nitrate is silver’s soluble travelling companion; ammonium is a counter-ion that behaves like potassium.
Sulfur, S	Sulfite, bisulfite, metabisulfite, thiosulfate, sulfide	Preservation, acidity reserve, fixing, and toning.
Oxygen, O	Every oxyanion, and the air	The commonest element in the list, and also the enemy of every developer.
Carbon, C; hydrogen, H	Developing agents, gelatin, acetic and citric acids	The organic half of photography.
Boron, B	Borax, boric acid, metaborate	Mild alkalis and acid reserves, held in a narrow pH band.
Iron, Fe	Cyanotype, Van Dyke, kallitype, platinum and palladium	The light-sensitive metal of the iron processes, and the ferricyanide of bleaching.
Gold, Au; selenium, Se; platinum, Pt; palladium, Pd	Toning and the noble-metal prints	Image metals that replace or protect silver.

The remainder — chromium, aluminium, cadmium, mercury, lead, phosphorus, uranium — appear mostly in historical processes the course studies and does not perform, and each is met on its own page with its own hazard assessment.

Ions: atoms that have gained or lost

An atom forms an ion when the energy books come out favourably: a sodium atom is more stable having handed its single loose outer electron to something that wants one, and a chlorine atom is more stable having taken it. Neither happens in isolation. The transfer happens because both partners end up lower in energy than they started, and the resulting opposite charges then attract.

A cation is positive, having lost electrons. An anion is negative, having gained them. The mnemonic that survives is that the t in cation is a plus sign wearing a hat, and that an anion is a negative ion.

Charges you can predict, and one you cannot

For the main-group elements the charge follows from the column. Group 1 gives 1+; group 2 gives 2+; group 16 takes 2−; group 17 takes 1−. So Na⁺, K⁺, Mg²⁺, O²⁻, S²⁻, Cl⁻, Br⁻, I⁻, every one of them predictable from position alone.

The transition metals are the exception, and OpenStax says so plainly: they commonly form more than one charge, and you cannot read it off the table. Iron is the photographic case — Fe²⁺ and Fe³⁺ are both common and the whole of cyanotype depends on the difference. Silver is the useful exception to the exception: in every compound in this course it is Ag⁺, singly charged. That single fact is why a silver halide has the formula AgX rather than AgX₂, why one electron converts a silver ion to a silver atom, and why the arithmetic of an emulsion is as simple as it is.

The polyatomic ions of photography

Most of the anions here are not single atoms. They are small groups of atoms, covalently bonded to each other, carrying a charge on the group as a whole and travelling through a solution as one unit. Learn these eight and you can read most photographic formulas.

Ion	Formula	Charge	What it is doing in a photographic solution
Nitrate	NO₃⁻	1−	The counter-ion of silver nitrate. Soluble with everything; contributes nothing to the picture.
Carbonate	CO₃²⁻	2−	The alkali of most print developers, and a reservoir rather than a dose.
Hydrogencarbonate	HCO₃⁻	1−	Carbonate’s partner; the pair is what holds a developer’s pH.
Sulfate	SO₄²⁻	2−	Mostly what sulfite turns into when it has done its job.
Sulfite	SO₃²⁻	2−	Preservative, weak silver halide solvent, and a mild reducing agent.
Hydrogensulfite	HSO₃⁻	1−	Sulfite’s acid partner. Sold as bisulfite; the pair buffers near neutral.
Disulfite	S₂O₅²⁻	2−	Sold as metabisulfite. In water it becomes hydrogensulfite.
Thiosulfate	S₂O₃²⁻	2−	The fixing agent. It dissolves silver halide by binding the silver ion.

Two naming rules do most of the work, and one of them is a trap.

The suffix -ate means the oxyanion with more oxygen and -ite the one with less: sulfate SO₄²⁻ against sulfite SO₃²⁻, nitrate NO₃⁻ against nitrite NO₂⁻. OpenStax warns that the number of oxygens attached to a given suffix is not consistent between elements, so the rule tells you which of a pair is which and never the absolute count.

The prefix thio- signals sulfur where oxygen would otherwise be, and the formulas show it: sulfate is SO₄²⁻ and thiosulfate is S₂O₃²⁻ — same charge, same number of heavy atoms, one oxygen swapped for a sulfur. That single substituted sulfur is the atom that grips a silver ion, and it is therefore the reason photography has a fixing bath at all. Kodak’s 1928 primer shows the other side of the same fact: add a strong acid to thiosulfate and the free thiosulfuric acid falls apart into sulfurous acid and a precipitate of sulfur, the substituted atom dropping straight out of the molecule.

✕Sulfite is not sulfate, and the difference is the whole point

Sulfite, SO₃²⁻, is hungry for one more oxygen. That hunger is exactly why it is in your developer: it takes oxygen that would otherwise attack the developing agent, and becomes sulfate. Sulfate, SO₄²⁻, is what is left when the job is done — full, inert and useless as a preservative. A tub of the wrong one is not a slightly different developer. It is a developer with no preservative in it.

Ionic bonding, and why silver bromide is a lattice

When electrons are transferred outright, the oppositely charged ions attract each other electrostatically, and that attraction is the ionic bond. It is strong: OpenStax gives 769 kJ as the energy needed to pull one mole of solid sodium chloride apart into separate gaseous ions.

The crucial sentence for photography is the one OpenStax states most bluntly. The attraction between ions is isotropic — the same in every direction — so no particular sodium ion is bonded to any particular chloride ion. It is therefore incorrect to speak of a sodium chloride molecule. The ions arrange themselves instead into a tightly bound three-dimensional lattice, an alternating array that continues in every direction to the edge of the crystal. The formula NaCl does not describe a molecule; it describes a ratio.

Everything Part IV will say about grains and the latent image rests on that. AgBr is not a molecule of silver bromide. It is a ratio, one silver ion to one bromide ion, in a crystal that may contain billions of each. Mike Ware draws exactly that alternating array in his account of silver chloride: rows of Ag⁺ and Cl⁻ in strict alternation, with — and this is the part that matters photographically — extra ions of one kind adsorbed on the outside, because a precipitate made with an excess of silver ion carries silver ions on its surface and one made with excess halide carries halide ions. A grain therefore has a chemistry that depends on what was left over when it was made.

Silver bromide as a lattice, not a molecule

1. 1Silver ion, Ag⁺ — one positive charge; the source of the image metal
2. 2Bromide ion, Br⁻ — one negative charge; the ion that must leave during fixing
3. 3Adsorbed excess silver ion — left over from making; sits on the surface with its nitrate counter-ion
4. 4The lattice edge — where every reaction with the outside world happens

Drawn as a flat grid to show the alternation; the real arrangement is three-dimensional and this diagram is schematic rather than a crystal structure.

Why the ions can move once the solid dissolves

An ionic solid is a poor conductor, because the ions are locked in place. Melt it or dissolve it and it conducts well, because the ions are then free to move independently. That freedom is what a photographic solution is. When an ionic solid dissolves, water molecules surround each ion and insulate it from its former neighbours — an ion-dipole attraction between the charge on the ion and the partial charges on the water molecule — and the ion moves off into solution on its own account, hydrated, indifferent to what it was crystallised with.

That is the deep reason the sodium in a formula usually does not matter. Once the powder is in the water, there is no sodium sulfite in the beaker. There are sodium ions and there are sulfite ions, and they are not attached.

OpenStax adds the qualification that makes the silver halides interesting: even a sparingly soluble ionic compound is a strong electrolyte, because the very small amount that does dissolve dissociates completely. Silver bromide barely dissolves — but what dissolves is fully ionised, and it is that tiny population of free Ag⁺ and Br⁻ ions that fixing, restraining and the whole of the next page are about.

🔬The tension this picture cannot resolve, and where it is resolved

A purely ionic solid should not conduct electricity, because its ions cannot move. Yet the latent image requires an electron, freed by a photon, to travel through the inside of a silver bromide crystal to a trap. Something is moving in there that the ionic picture does not account for.

The resolution is that the ionic and covalent descriptions are the two ends of a continuum rather than two boxes, and that a real solid can sit anywhere between them; OpenStax makes the point generally and warns that the usual electronegativity rule of thumb has many exceptions. This course does not state how ionic the silver halides are, because it has not verified a figure from a solid-state source. Part IV owns the grain and the latent image, and that is where the electron’s journey is properly described. For now, hold both facts: a lattice of ions, and something inside it that moves.

Covalent bonding, and the molecules of photography

When neither atom can take an electron outright from the other, they share a pair instead, and that shared pair is a covalent bond. Sharing produces discrete molecules with definite sizes, and the formula of a molecule really does count atoms.

Sharing is rarely equal. If one atom pulls harder on the shared pair — if it is more electronegative — the pair sits closer to it, and the bond has a negative end and a positive end. That is a polar covalent bond, and OpenStax gives the electronegativity difference as a rough guide to where a bond sits: zero for H–H, 0.9 for H–Cl, 2.1 for Na–Cl. It also warns, in the same paragraph, that the guide has many exceptions, so do not treat a number as a verdict.

Water is a molecule with a bias

Water is two O–H bonds at an angle, each of them polar, with the oxygen end negative and the hydrogen ends positive. Because the molecule is bent rather than straight, those two biases do not cancel: the molecule as a whole has a negative end and a positive end. Everything water does in this course follows from that one asymmetry — dissolving salts by surrounding their ions, carrying ions through a gelatin layer, swelling gelatin in the first place, and refusing to dissolve the oily organic compounds that have no charge to offer.

Why water pulls an ionic solid apart

1. 1The solid — ions held in a lattice; nothing moves
2. 2A water molecule — bent, so its two polar bonds do not cancel: an oxygen end that is δ− and hydrogen ends that are δ+
3. 3The hydrated cation — water turns its oxygen ends inwards
4. 4The hydrated anion — water turns its hydrogen ends inwards

The ions are drawn the same size for clarity; in reality the hydration shell of a small, highly charged ion is tighter than that of a large one.

Developing agents are rings, and gelatin is a chain

The organic half of photography is covalent throughout. A developing agent such as hydroquinone or metol is drawn as a ring of six carbon atoms with groups attached, and those attached groups are where the chemistry happens: they are what give up the electrons a silver ion needs. Part VIII owns the mechanism; the reason the ring matters here is that a molecule with a definite shape can be adsorbed onto a crystal surface in a definite orientation, and an ion cannot.

Gelatin is a protein, which is to say a very long covalent chain of amino-acid units. Long chains tangle, hold water between them and set into a gel, which is why an emulsion can be a liquid when warm and a solid when cool. That is the whole of what this page needs to say about it: gelatin is the medium every reagent has to travel through, it is not a bystander, and Part V treats it properly as the emulsion’s other half.

Reading a formula, symbol by symbol

Anatomy of three real photographic formulas

1. 1Subscript after a symbol — how many of that atom or ion: Na₂ is two sodium ions
2. 2The anion group — S₂O₃ travels as one unit carrying a 2− charge
3. 3The hydrate dot — ·5H₂O — five water molecules built into the crystal and weighed with it
4. 4A subscript inside a group — NO₃ has three oxygens; the 3 says nothing about the silver
5. 5Brackets — a subscript outside brackets multiplies everything inside them

Charges are shown here for teaching; a neutral compound's formula normally leaves them out, because charge balance makes them redundant.

Subscripts count. A number after a symbol says how many of that atom the formula contains: Na₂SO₃ is two sodiums, one sulfur, three oxygens. A number after a closing bracket multiplies the whole group inside: Al₂(SO₄)₃ is two aluminiums and three complete sulfate ions, twelve oxygens in all. Brackets exist for exactly that purpose and for no other.

The dot is not multiplication and it is not a bond. In a hydrate such as Na₂S₂O₃·5H₂O, the dot separates the salt from the water molecules built into its crystal. Those waters are real, they occupy space in the lattice, and — the practical point — you weigh them. This is why the anhydrous and hydrated forms of the same substance are different entries in the encyclopaedia and different lines in a formula. The concentration and dilution page already taught the arithmetic of converting between them; this page supplies the reason the conversion exists.

Charge balance fixes the subscripts

A compound is electrically neutral, so its subscripts are not a choice. Silver is Ag⁺ and bromide is Br⁻, so one of each balances and the formula is AgBr. Sodium is Na⁺ and carbonate is CO₃²⁻, so it takes two sodiums and the formula is Na₂CO₃. Potassium is K⁺ and thiosulfate is S₂O₃²⁻, so potassium thiosulfate is K₂S₂O₃. Once you know the two charges you can write the formula without looking it up, and if a formula you have been given does not balance, one of you is wrong.

Checking a balanced equation, atom by atom and charge by charge

An equation is balanced when both sides have the same count of every element and the same total charge. Here is the reaction that makes an emulsion, in the form Kodak’s 1928 primer gives it:

AgNO₃ + KBr → AgBr + KNO₃

Precipitating silver bromide, molecular form

Count them. Left: one Ag, one N, three O, one K, one Br. Right: one Ag, one Br, one K, one N, three O. Charge: zero on both sides, since every species is a neutral compound. It balances.

Naming, and the traps that cost money

Three traps recur, and all three have cost photographers a session.

Sulfite against sulfate. Discussed above and worth the repetition: one is a preservative and the other is its used-up product.

Thio- means something specific. Thiosulfate is not “sulfate for photographers”; it is sulfate with one oxygen replaced by sulfur, and that substitution is the entire mechanism of fixing.

Sodium against potassium against ammonium. These are three different counter-ions for the same anion, and the anion is usually what the formula is buying. But usually is not always, and the sources are specific about when it matters. Kodak’s 1924 primer says potassium carbonate is used instead of sodium carbonate in some developer formulas, is more soluble, and has the disadvantages of being more expensive and of absorbing water very readily, so it must be kept in well-sealed bottles. That is a real difference between the two salts, and none of it is about the carbonate.

Ammonium is the outlier. As a counter-ion it is another singly charged cation and will balance the same anions — but ammonium thiosulfate fixes considerably faster than sodium thiosulfate, which is a difference in the fixing chemistry rather than in the counter-ion’s charge, and is why the two are separate entries in this course’s terminology and separate products on a shelf. Part XI does that comparison properly.

!Substituting one salt for another is arithmetic, not intuition

If you replace a sodium salt with the potassium salt of the same anion, you must reweigh, because the molar masses differ: sodium carbonate is 105.99 and potassium carbonate is 138.21, so 10 g of the sodium salt supplies the same amount of carbonate as 13.04 g of the potassium salt. Substituting gram for gram changes the alkalinity of the developer. The concentration page has the method; this page has the reason.

Molar mass, worked three times

Molar mass is the mass in grams of one mole of a substance, and for a formula it is nothing more than the sum of the atomic masses of everything in it. For an ionic compound OpenStax calls this the formula mass rather than the molecular mass, precisely because there is no molecule to have a mass. The arithmetic is addition; the discipline is in not forgetting anything.

Atomic masses used below, rounded as this course uses them: H 1.008, C 12.01, N 14.01, O 16.00, Na 22.99, S 32.06, K 39.10, Br 79.90, Ag 107.87.

1. Sodium thiosulfate pentahydrate, Na₂S₂O₃·5H₂O

2(22.99) + 2(32.06) + 3(16.00) + 5(18.02) = 45.98 + 64.12 + 48.00 + 90.08 = 248.18 g/mol

Formula mass of the pentahydrate

PubChem’s computed value is 248.19, so the arithmetic checks. The anhydrous salt, Na₂S₂O₃, is 45.98 + 64.12 + 48.00 = 158.10, and PubChem gives 158.11.

Now the question the darkroom actually asks. How much anhydrous salt is there in 100 g of the crystals?

158.11 ÷ 248.19 × 100 g = 63.7 g

Anhydrous content of the pentahydrate

Thirty-six per cent of what you weighed was water. A formula that calls for 240 g of hypo per litre and a bag labelled “sodium thiosulfate” that turns out to be the anhydrous salt are not the same fixer, and the difference is not small.

2. Silver nitrate, AgNO₃

107.87 + 14.01 + 3(16.00) = 169.88 g/mol

Formula mass of silver nitrate

PubChem gives 169.873. The useful derived number is the fraction that is silver: 107.87 ÷ 169.88 = 0.635, so 10 g of silver nitrate carries 6.35 g of silver. Since silver is what you are paying for, and what you will later recover from spent fixer, that fraction is worth remembering. The nitrate is 36.5 per cent of the mass and contributes nothing to any picture.

3. Sodium carbonate, three ways

The same alkali is sold in three forms, and the formula tells you the strength of each.

Form	Formula	Molar mass	Na₂CO₃ content
Anhydrous	Na₂CO₃	105.99	100 %
Monohydrate	Na₂CO₃·H₂O	124.00	105.99 ÷ 124.00 = 85.5 %
Decahydrate	Na₂CO₃·10H₂O	286.14	105.99 ÷ 286.14 = 37.0 %

Kodak’s 1928 primer, working from the same formulas nearly a century ago, gives the three commercial grades as containing 37, 85 and about 98 per cent of dry carbonate — the last being the dry powder, which the primer notes retains a small residual quantity of water. The two independent routes agree, and that agreement is the point: molar mass is not book-keeping, it is a check you can run on a supplier.

🧮Why this connects straight back to Part II

Molar mass is the bridge between a formula and a bottle. Grams per litre is what you weigh; mol/L is what the chemistry responds to; molar mass converts between them. Every calculation of that kind — per cent w/v, grams per litre, molarity, hydrate correction, C₁V₁ = C₂V₂ — is taught in concentration and dilution and is not repeated here. What this page adds is the ability to get the molar mass itself from the formula, which is the one input that page assumes you can find.

Spectator ions, and when the counter-ion matters

Write out the emulsion reaction again, but this time as the ions actually present. Both silver nitrate and potassium bromide are soluble, so in solution they are not compounds at all:

Ag⁺ + NO₃⁻ + K⁺ + Br⁻ → AgBr + K⁺ + NO₃⁻

Complete ionic form

Potassium ion and nitrate ion appear unchanged on both sides. They were dissolved before and they are dissolved after; nothing happened to them. OpenStax calls them spectator ions, and cancelling them leaves the reaction that actually occurred:

Ag⁺ + Br⁻ → AgBr

Net ionic form: the only thing that happened

Two ions out of four. That is the general shape of photographic chemistry, and it is why reading a formula as a list of ions rather than a list of powders is such a powerful habit.

⚗When may you substitute potassium for sodium, and when may you not?

You may, when the ion you care about is the anion and the counter-ion is a spectator — as here, where using sodium bromide instead of potassium bromide changes the spectator and not the precipitate. Reweigh for the different molar mass and the chemistry is unchanged.

You may not, when the source names a property of the whole salt rather than of its anion. Kodak names two: potassium carbonate’s greater solubility, which is why it appears in concentrated developer formulas, and its readiness to absorb water, which is a storage problem rather than a chemical one. And you may not when the cation is itself reactive — ammonium is not a spectator in a fixer, and Part XI explains why.

The test to apply is: does the source’s reason mention the cation? If it does not, the cation is a spectator and you may substitute with the arithmetic done. If it does, you are changing the formula.

The payoff: reading a formula cold

Return to the developer this page opened with. Elon — which Kodak Ltd’s 1949 handbook defines as “a specially purified form of monomethyl paraminophenol sulphate, a compound which is also known under other names, such as ‘Metol’” — 2.0 g, sodium sulfite 100 g, hydroquinone 5.0 g, borax 2.0 g, water to 1 litre. That is Kodak’s D-76, printed in the 1928 primer as 8.0, 400, 20 and 8.0 grams to the gallon of four litres.

• Elon and hydroquinone are organic molecules, covalent rings, present to supply electrons. They are the developing agents; the developer is the whole formula, which is why this course keeps those two terms apart.
• Sodium sulfite supplies the sulfite ion, SO₃²⁻, which is hungry for oxygen. Its sodium is a spectator. At 100 g per litre it is by far the largest ingredient, and Kodak’s own primer says why: at that concentration sulfite is also a mild solvent for silver bromide, which dissolves a little of each grain during development and reduces graininess.
• Borax supplies the borate ion and sets the alkalinity. Its sodium is a spectator. Kodak states the photographic consequence directly: adding carbonate to such a developer instead increases the rate of development and accentuates graininess.
• Water is the solvent, and it is not passive — it is what pulls every one of those salts into independent, mobile ions in the first place.

Four ingredients, and you have said what each is for, which parts are spectators, and what would change if you swapped one. That is what reading a formula means.

Recap

An atom is a nucleus and its electrons, and all photographic chemistry is electron chemistry. Atoms that lose or gain electrons become ions, with charges that are predictable from the periodic table for the main groups and not for the transition metals — silver being the convenient case, always Ag⁺. Ionic compounds are lattices, not molecules, so AgBr is a ratio; covalent compounds are molecules, and water’s bent, polar shape is what dissolves the lattices. A formula’s subscripts count atoms, brackets multiply groups, and a hydrate dot marks water you have to weigh. Molar mass is the sum of the atomic masses, and it converts a formula into a bottle. And when two solutions meet, usually only two of the four ions present do anything at all — which is why a published formula tells you what it is for, if you read it as ions.

Next: what makes one of those ionic solids dissolve and another fall out of solution as a crystal, and why that difference is how an emulsion is made.

Check your understanding

Question 1. A formula calls for 20 g of anhydrous sodium carbonate per litre, and all you have is the decahydrate, Na₂CO₃·10H₂O. How much of the decahydrate supplies the same amount of carbonate?

• 20 g, because it is the same substance
• about 7.4 g
• about 54 g
• about 200 g

Show the answer and why

Answer: about 54 g

The decahydrate is 105.99/286.14 = 37.0 per cent carbonate, so you need 20 ÷ 0.370 = 54.1 g of it. Weighing 20 g of the decahydrate would give you only 7.4 g of carbonate, a developer at about a third of its intended alkalinity. Kodak was giving the same three percentages — 37, 85 and 98 — in 1928.

Question 2. Why is it wrong to speak of a "molecule of silver bromide"?

• Because silver bromide is a mixture rather than a compound
• Because the attraction between the ions is the same in all directions, so no particular silver ion is bonded to any particular bromide ion; the solid is a lattice and AgBr states a ratio
• Because silver bromide is covalently bonded
• Because the formula should be written Ag₂Br₂

Show the answer and why

Answer: Because the attraction between the ions is the same in all directions, so no particular silver ion is bonded to any particular bromide ion; the solid is a lattice and AgBr states a ratio

Ionic attraction is isotropic. Each ion is pulled equally by all its neighbours of opposite charge, so the ions pack into a three-dimensional lattice rather than pairing off. The formula gives the ratio one silver ion to one bromide ion, and a single grain contains billions of each — which is exactly why a grain has a surface, an interior and defects, and why Part IV can talk about what happens at a particular site in it.

Question 3. Silver nitrate solution is mixed with potassium bromide solution. Which ions are spectators?

• Ag⁺ and Br⁻
• K⁺ and NO₃⁻
• Ag⁺ and NO₃⁻
• All four, since nothing dissolves

Show the answer and why

Answer: K⁺ and NO₃⁻

Silver ion and bromide ion combine and leave the solution as solid silver bromide; potassium ion and nitrate ion are dissolved before and dissolved after, so they cancel from the equation. The net ionic equation is Ag⁺ + Br⁻ → AgBr. This is why sodium bromide would work just as well: you would be changing a spectator.

Question 4. A supplier sends sodium sulfate instead of sodium sulfite. What has changed in your developer?

• Nothing chemically; only the weight needs adjusting
• The developer has lost its preservative, because sulfate is the oxidised form and cannot take up any more oxygen
• The developer becomes more alkaline
• The developing agents are replaced

Show the answer and why

Answer: The developer has lost its preservative, because sulfate is the oxidised form and cannot take up any more oxygen

Sulfite, SO₃²⁻, protects a developing agent by taking oxygen and becoming sulfate, SO₄²⁻. Sulfate is that reaction already finished. A developer mixed with sulfate has a full complement of everything except the ingredient that keeps it alive, and it will oxidise rapidly — which is the subject of the aerial oxidation page later in this part.

Question 5. What does the dot mean in Na₂S₂O₃·5H₂O, and why does it matter on the balance?

• It means the compound is a mixture of two substances that can be separated by filtering
• It separates the salt from water molecules built into the crystal, which are weighed along with the salt: 100 g of the pentahydrate contains only 63.7 g of the anhydrous salt
• It is a multiplication sign, so the formula means five times Na₂S₂O₃
• It indicates that the compound must be dissolved in exactly five parts of water

Show the answer and why

Answer: It separates the salt from water molecules built into the crystal, which are weighed along with the salt: 100 g of the pentahydrate contains only 63.7 g of the anhydrous salt

Water of crystallisation occupies real space in the lattice and contributes real mass. The molar masses are 158.11 anhydrous and 248.19 for the pentahydrate, so the anhydrous fraction is 158.11/248.19 = 63.7 per cent. Substituting one for the other gram for gram gets you a fixer at either 64 per cent or 157 per cent of the intended concentration, depending on which direction you got it wrong.

Question 6. Which statement about counter-ions is correct?

• Potassium may always be substituted for sodium, gram for gram
• Potassium may never be substituted for sodium
• Potassium may be substituted for sodium where the cation is a spectator, provided the quantity is recalculated for the different molar mass — but not where a source names a property of the whole salt, such as potassium carbonate being more soluble and much more hygroscopic
• The counter-ion determines the colour of the developed image

Show the answer and why

Answer: Potassium may be substituted for sodium where the cation is a spectator, provided the quantity is recalculated for the different molar mass — but not where a source names a property of the whole salt, such as potassium carbonate being more soluble and much more hygroscopic

The test is whether the reason a source gives mentions the cation. For the anion chemistry — carbonate as an alkali, bromide as a restrainer — the cation is a spectator and only the arithmetic changes. Where the source names solubility, hygroscopicity or cost, it is describing the whole salt, and Kodak names exactly those for potassium carbonate. And a cation is not always a spectator: ammonium thiosulfate is a faster fixer than sodium thiosulfate, which is why this course never treats those two as interchangeable.

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