How to Count Every Atom in a Formula With Brackets

Seventeen — and the bracket is what makes it easy to get wrong. The subscript outside multiplies everything inside it, so the three oxygens inside become twelve.

Get that one rule right and every formula in the chapter becomes countable. This page covers everything in the ICSE Class 7 Chemistry chapter's first part: the postulates of atomic theory, atoms and molecules, atomicity, and counting atoms in a formula.

Dalton's atomic theory states that:

- All matter is made of tiny, indivisible particles called atoms.
- Atoms can neither be created nor destroyed.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements differ in mass and properties.
- Atoms combine in small whole-number ratios to form compounds.

That last postulate is why water is always $H_{2}O$ and never $H_{1.5}O$ — you cannot use part of an atom.

Two limitations are examined, and both concern the first and third postulates:

- An atom is not indivisible. It is now known to be made of smaller particles — protons, neutrons and electrons.
- Atoms of the same element are not always identical in mass. Some elements have atoms of differing mass, called isotopes.

The theory still earns its place, though, and this is the point to make in an answer. Its core idea — that matter is particulate and combines in fixed whole-number ratios — remains the foundation of every formula you write, even though the details have been corrected.

An atom is the smallest particle of an element that takes part in a chemical reaction. A molecule is the smallest particle of an element or compound that can exist independently and still show its properties.

The distinction is between what reacts and what exists.

Most atoms cannot survive alone — they are too reactive, so they join up. A single hydrogen atom does not sit around on its own; two combine to form a hydrogen molecule, $H_2$, and that is the particle actually present in a gas cylinder.

But it is the atom, not the molecule, that is exchanged in a reaction. When hydrogen burns in oxygen, the molecules break and their atoms regroup into water.

So the everyday water in a glass consists of $H_{2}O$ molecules, each of which can exist on its own. Split one and you have hydrogen and oxygen atoms, which will not stay as water.

The noble gases are the exception worth knowing. Helium, neon and argon are so unreactive that a single atom is stable on its own, so for them the atom and the molecule are the same particle.

Atomicity is the number of atoms present in one molecule of a substance.

Molecules come in two kinds:

- Molecules of elements contain atoms of only one kind — $O_2$, $H_2$, $O_3$, $P_4$, $S_8$.
- Molecules of compounds contain atoms of two or more kinds — $H_{2}O$, $C{O}_2$, $NaCl$, $HCl$.

Atomicity is read straight off the formula:

- $He$ — 1 atom, monoatomic
- $O_2$ — 2 atoms, diatomic
- $O_3$ — 3 atoms, triatomic
- $P_4$ — 4 atoms, tetratomic
- $S_8$ — 8 atoms, polyatomic

Monoatomic elements are the noble gases and the metals — sodium is written simply as $Na$. Diatomic elements include hydrogen, oxygen, nitrogen, chlorine and the other halogens.

Oxygen and ozone make the most useful pair here. Both are made of oxygen atoms alone, so both are molecules of an element, yet $O_2$ is the gas you breathe and $O_3$ is a different substance with a sharp smell. Atomicity alone changes the substance.

The misconception to clear is that atomicity counts kinds of atoms. It does not — it counts atoms. So $H_{2}O$ has an atomicity of 3, not 2, even though only two elements are present.

Multiply everything inside a bracket by the subscript outside it, then add up each element.

**$Ca(OH{)}_2$ — calcium hydroxide.** The bracket holds one O and one H, and the 2 outside doubles both:

- Ca — 1
- O — $1\times 2=2$
- H — $1\times 2=2$
- Total = 5 atoms

**$A{l}_2(S{O}_4{)}_3$ — aluminium sulphate.** The bracket holds one S and four O, and the 3 outside triples both:

- Al — 2
- S — $1\times 3=3$
- O — $4\times 3=12$
- Total = 17 atoms

**$CuS{O}_4\cdot 5H_{2}O$ — copper sulphate crystals. The dot means five molecules of water of crystallisation** are attached, so count the main part and the water separately, then add:

- Cu — 1
- S — 1
- O — $4$ from the sulphate $+5$ from the water $=9$
- H — $5\times 2=10$
- Total = 21 atoms

You can check the water separately: $5\times 3=15$ atoms of water plus 6 atoms in $CuS{O}_4$ gives 21.

The two frequent errors are both about reach. A subscript outside a bracket multiplies every element inside, not just the nearest one — so the O in $A{l}_2(S{O}_4{)}_3$ is 12, not 4. And a coefficient written in front of the whole formula multiplies everything: $2H_{2}O$ has 4 hydrogen and 2 oxygen atoms, 6 in all.