1cAtomic structure

An atom is the smallest particle of an element that can exist. A molecule is two or more atoms chemically joined together.

That distinction sounds small and is worth holding onto. Oxygen gas is made of O₂ molecules, each containing two oxygen atoms. Examiners report candidates losing marks for calling one the other.

The three sub-atomic particles

Three things follow from that table, and questions test all of them.

Almost all the mass is in the nucleus. An electron's mass is about 1/1840 of a proton's — small enough that it is usually treated as negligible. So the mass number counts protons and neutrons only.

An atom is neutral. It has equal numbers of protons and electrons, so the +1 and −1 charges cancel exactly.

The nucleus is positive. It holds the protons (+1 each) and the neutrons (0), so its overall charge is positive.

Position matters in the answer

When a question asks about the structure of an atom, it usually wants position as well as mass and charge. "Protons and neutrons are in the nucleus; electrons are in shells around it" is the sentence that earns the position mark.

Examiners note that candidates often give mass and charge correctly and then lose the third mark by not saying where the particles are.

Two numbers describe any atom, and keeping them apart is the whole of this objective.

From those two you can work out everything else:

• Neutrons = mass number − atomic number
• Electrons (in a neutral atom) = atomic number

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

Both halves of that sentence matter. Same element means the same number of protons — that is what makes it that element at all. Different neutrons means a different mass number.

So chlorine-35 and chlorine-37 both have 17 protons. One has 18 neutrons, the other 20.

Why isotopes behave identically in reactions

Chemical reactions involve electrons, not the nucleus. Isotopes of an element have the same number of electrons arranged the same way, so they react in exactly the same manner.

The neutrons change the mass, which is a physical property, not the chemistry.

Answering the classic isotope question

A very common question asks for one similarity and one difference between two isotopes. Examiners report that candidates often answer without mentioning sub-atomic particles at all, which is what the question requires.

Write it in those terms:

• Similarity: they have the same number of protons (and the same number of electrons).
• Difference: they have different numbers of neutrons.

"They have the same chemical properties" is true, but it is a consequence rather than an answer about particles.

Most elements exist as a mixture of isotopes, so a single mass number does not describe a real sample. Relative atomic mass (Ar) is the weighted average across the isotopes actually present.

The method

1. Multiply each mass number by its percentage abundance.
2. Add those products together.
3. Divide by 100 — the total percentage abundance.

A worked example

Bromine exists as bromine-79 (50.5%) and bromine-81 (49.5%).

1. (79 × 50.5) + (81 × 49.5) = 3989.5 + 4009.5 = 7999
2. 7999 ÷ 100 = 79.99, so Ar ≈ 80.0

The error examiners report most

Dividing by the sum of the mass numbers instead of the sum of the abundances.

In the example above that would mean dividing 7999 by (79 + 81) = 160, which gives about 50 — a nonsense answer, because Ar must lie between the two mass numbers.

That gives you a free check on every answer of this type. Ar always sits between the lightest and heaviest isotope, and closer to whichever is more abundant. Bromine's isotopes are almost equally abundant, so 80.0 sitting midway between 79 and 81 is exactly what you would expect.

If your answer falls outside that range, you have divided by the wrong thing.

When the abundances are not percentages

Sometimes abundances are given as a ratio or as raw counts rather than percentages. The method is the same, but you divide by the total of the abundances given, whatever they are — not automatically by 100.