Sec 3 Chemistry — Atomic Structure

1 What Is Inside an Atom?

An atom is the smallest particle of an element that still has the chemical properties of that element. It is the building block of all matter.

Inside, the atom is not a solid ball. Its mass is concentrated in a tiny central region containing two kinds of particle, with electrons arranged around it in shells.

In this model, the nucleons — protons and neutrons — sit in the nucleus. The nucleus is surrounded by a "cloud" of electrons arranged in orbits called electron shells. The outermost shell is the valence shell, and the electrons in it are the valence electrons.

Particle	Charge	Relative mass	Symbol	Where it is	Why it matters
Proton	+1	1	p	Nucleus	Its number is what makes the element what it is — change the protons and you have a different element.
Neutron	0	1	n	Nucleus	Adds mass but no charge, so changing the neutrons leaves the element unchanged — that is what makes an isotope.
Electron	−1	1/1840	e−	Electron shells	Effectively massless, so it barely affects mass — but it decides the atom's chemical properties.

Three facts to state:

Protons are positively charged and neutrons are neutral, so the nucleus has a net positive charge.
An atom has an equal number of protons and electrons, so the positive and negative charges cancel and the atom is electrically neutral.
Not every atom contains all three kinds of particle — the simplest hydrogen atom has one proton and one electron and no neutron.

2 Writing an Atom Down — Nuclide Notation

Any atom can be written as a symbol with two numbers beside it. Everything you are asked about — protons, neutrons, electrons, isotopes — can be read off those three pieces of information.

23
11 Na

the nuclide notation for a sodium atom: 23 nucleons, 11 of them protons

Position	Name	What it counts	How you get it
Top number	Nucleon (mass) number	Total number of protons and neutrons	Add the protons and neutrons. Only these two matter, because an electron's relative mass is negligible.
Bottom number	Proton (atomic) number	Number of protons in the nucleus	Read it from the Periodic Table — it is what fixes the element's identity.
Symbol	Chemical symbol	Which element it is	Periodic Table, e.g. Na for sodium.

Exam habit: the two numbers let you find the third quantity. Neutrons = nucleon number − proton number. For sodium: 23 − 11 = 12 neutrons, and with 11 electrons its configuration is 2.8.1.

3 Electronic Configuration

Electron shells are numbered outwards, in increasing order — the first shell is the innermost. For the atoms of the first 20 elements, the capacity rule is:

The electronic configuration of an atom lists the number of electrons in each shell, written as a.b.c.d, where a, b, c and d are the electrons in the first, second, third and fourth shell. Fill the inner shells first.

Element	Protons	Configuration	What the last number tells you
Lithium	3	2.1	1 valence electron — it will tend to lose it.
Carbon	6	2.4	4 valence electrons — a half-full shell.
Oxygen	8	2.6	6 valence electrons — it needs 2 more to reach 8.
Sodium	11	2.8.1	1 valence electron, so it loses one and becomes Na⁺.
Chlorine	17	2.8.7	7 valence electrons, so it gains one and becomes Cl⁻.
Calcium	20	2.8.8.2	2 valence electrons in the fourth shell.

Exam habit: work from the number of electrons, not the neutrons. For an atom the electron count equals the proton count, so oxygen is 2.6 because 2 + 6 = 8 — the 8 neutrons have nothing to do with it.

4 Drawing the Full Electronic Structure

Given either the atomic structure (protons, neutrons and electrons) or the configuration, the full electronic structure can be drawn. Follow the same five steps every time:

Step	What to do
1	Write the number of protons and neutrons present in the nucleus, at the centre of the atom — e.g. "11p 12n" for sodium.
2	Draw the first electron shell as a circle bigger than the nucleus. It holds at most two electrons — draw those two directly opposite each other.
3	Draw the second shell bigger than the first, holding up to eight. Place its electrons singly, in order, going north → south → east → west (the 12–3–6–9 o'clock rule).
4	Repeat step 3 for any further shells — the third shell also holds up to eight.
5	Add a legend or key saying what symbol you used for an electron. Dots or crosses are both accepted.

Marks are lost on the easy parts: the nucleus with its two numbers, a shell drawn bigger than the one inside it, and the key. Draw the electrons singly rather than in pairs, and do not draw two electrons on top of each other.

5 Why Ions Form

When an atom loses or gains one or more valence electrons during a chemical reaction, to reach a stable (usually duplet or octet) electronic configuration, an ion is formed.

Why the noble gases sit it out: atoms of the noble gases already have stable electronic configurations. That is why they are chemically inert — they do not gain or lose electrons in chemical reactions, and they do not form ions.

6 Cations and Anions Side by Side

Whether an atom loses or gains electrons is decided by how many valence electrons it starts with, and that in turn follows its position in the Periodic Table.

	Cation	Anion
Formed by	Metal atoms, usually in Groups 1 to 13. They generally have one to three valence electrons and lose them.	Non-metal atoms in Groups 15 to 17, which have five to seven valence electrons and gain the rest to reach eight.
Protons vs electrons	More protons than electrons — the positive charge wins.	More electrons than protons — the negative charge wins.
Charge	Positively charged	Negatively charged
Example	A lithium atom (2.1) loses its 1 valence electron to form a Li⁺ ion.	An oxygen atom (2.6) gains 2 electrons to complete its shell, forming an O²⁻ ion.
Half-equation	Li → Li⁺ + e⁻	O + 2e⁻ → O²⁻

Exam habit: balance the half-equation. Electrons appear on the right when they are lost and on the left when they are gained, and the number must match the charge on the ion — Li⁺ loses 1 electron, O²⁻ gains 2.

7 What Makes Isotopes Different

Isotopes are atoms of the same element with the same number of protons (and electrons) but different numbers of neutrons, giving them different nucleon (mass) numbers.

What the two boron atoms tell you:

Same number of protons → the same element, and the same chemical properties.
Different numbers of neutrons → different nucleon (mass) numbers.
Slightly different masses → melting point, boiling point and density differ.

	Identical between isotopes	Different between isotopes
Particles	Protons and electrons	Neutrons
Numbers	Proton (atomic) number	Nucleon (mass) number
Effect on properties	Same number of electrons, so the same valence electrons are gained or lost — chemical properties are identical.	Slightly different masses, which shows up in physical properties such as melting point, boiling point and density.

8 Relative Atomic Mass from Relative Abundance

The relative abundance of an isotope is the fraction of the element that exists as that particular atomic mass. The relative atomic mass shown in the Periodic Table is the average mass of the most common and stable isotopes, weighted by those abundances.

Relative atomic mass = (abundance of isotope 1 × its mass) + (abundance of isotope 2 × its mass) + …
Write each abundance as a decimal: 75% becomes 0.75.

Worked example — chlorine

Chlorine isotope	Relative abundance
Chlorine-35	75%
Chlorine-37	25%

Working
= (0.75 × 35) + (0.25 × 37)
= 26.25 + 9.25
= 35.5
The Periodic Table gives chlorine's relative atomic mass as 35.5, so this checks out.

Exam habit: sanity-check the answer before you write it down. The average must fall between the two isotope masses and must sit closer to the more abundant one. An answer of 37 for chlorine is impossible, and 35.5 leaning towards 35 is right because chlorine-35 is the more common isotope.

9 Put It Together — Exam-Style Question

A particle contains 13 protons, 14 neutrons and 13 electrons.

[2](a) State its proton (atomic) number and its nucleon (mass) number.

[1](b) Write its electronic configuration.

[1](c) Explain why the particle is electrically neutral.

[2](d) The atom loses three valence electrons. State the charge on the ion formed and write the half-equation.

[2](e) A second particle has 13 protons and 15 neutrons. State one way its chemical properties differ from the first particle, and give a reason.

Model answers.
(a) Proton number 13; nucleon number 13 + 14 = 27.
(b) 2.8.3 — fill the first shell, then the second, then the third.
(c) It has an equal number of protons and electrons (13 each), so the +1 and −1 charges cancel out. The neutrons are neutral and do not affect the charge.
(d) Charge 3+; half-equation Al → Al³⁺ + 3e⁻.
(e) They do not differ — it is an isotope of the same element, so it has the same number of electrons (13) and therefore the same valence electrons to gain or lose. Only its mass differs.

★ Chapter Concept Map

Atomic Structure — what the numbers tell you

An atomthe smallest particle of an element, keeping its chemical properties — the building block of all matter

IS MADE OF

Protons, neutrons, electronsproton +1, neutron 0, electron −1 and 1/1840 in mass — nucleus holds the nucleons, shells hold the electrons

Read the notation first: top number = nucleon (mass) number = protons + neutrons · bottom number = proton (atomic) number = protons · neutrons = top − bottom

Electronic configurationa.b.c.d — electrons in the 1st, 2nd, 3rd, 4th shell. First 20 elements: 2, 8, 8. Fill inner shells first.

ACROSS ALL
FOUR PARTS

Drawing the structurenucleus with proton and neutron numbers, each shell bigger than the last, electrons drawn singly, and always a key

Ionformed when an atom loses or gains valence electrons to reach a stable duplet or octet

CHARGE

Cation or anionmetals (Groups 1–13) lose electrons → more protons than electrons → positive. Non-metals (Groups 15–17) gain → more electrons → negative. Li → Li⁺ + e⁻ and O + 2e⁻ → O²⁻

Isotopessame protons, different neutrons → same chemical properties, different physical ones

AND MASS

Relative atomic massthe abundance-weighted average of the isotopes: (0.75 × 35) + (0.25 × 37) = 35.5 for chlorine

Two checks that catch most mistakes: electrons equal protons in a neutral atom, and a relative atomic mass must lie between the isotope masses, closer to the more abundant isotope.