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Cambridge IGCSE Chemistry 0620: Atomic Structure Notes

Every atom has a nucleus of protons and neutrons, surrounded by electrons in shells. These notes explain proton number, nucleon number, electronic configuration, isotopes and ions for the Cambridge IGCSE Chemistry (0620) syllabus, with worked examples, an interactive atom builder, practice questions and exam tips.

Cambridge 0620Atomic structureFirst 20 elements

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Written by Ms. Humaira Asim, a science teacher with 15+ years of teaching experience. Written for the Cambridge IGCSE Chemistry 0620 syllabus (2026–2028).

Core or Extended? Some content, such as calculating relative atomic mass from isotope abundances, is Extended-tier. Check your own syllabus to see which parts apply to you.

Elements, compounds and mixtures

An element is a substance made of only one type of atom, such as oxygen or iron. A compound is a substance made of two or more elements chemically bonded together in a fixed ratio, such as water (H₂O). A mixture contains substances that are not chemically joined, such as air or salt water, and can be separated by physical methods like filtration or distillation.

The structure of the atom

An atom has a tiny, dense, positively charged nucleus containing protons and neutrons. Negatively charged electrons move around the nucleus in shells (energy levels). Most of an atom is empty space.

Particle	Relative charge	Relative mass	Where it is found
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	about 1/1840 (negligible)	Shells around the nucleus

An atom has no overall charge because the number of protons (+) equals the number of electrons (−).

Proton number and nucleon number

The proton number (atomic number, Z) is the number of protons in the nucleus. It identifies the element. The nucleon number (mass number, A) is the total number of protons and neutrons.

neutrons = nucleon number − proton numberFor a neutral atom: protons = electrons = proton number.

Worked example 1: particles in a sodium atom

How many protons, neutrons and electrons are in ²³₁₁Na?

Protons = proton number = 11
Electrons = protons (neutral atom) = 11
Neutrons = 23 − 11 = 12

11 protons, 12 neutrons, 11 electrons

Electronic configuration

Electrons fill the shell closest to the nucleus first. For the first 20 elements, the first shell holds up to 2 electrons, the second up to 8, and the third up to 8 before the fourth shell starts to fill. We write the arrangement as numbers separated by commas, for example sodium is 2,8,1.

The number of outer-shell electrons equals the group number (Groups I to VII).
The number of occupied shells equals the period number.
Noble gases (Group VIII or 0) have a full outer shell, so they are very unreactive.

Proton number	Element	Electronic configuration	Group	Period
1	Hydrogen (H)	1	I	1
2	Helium (He)	2	VIII (0)	1
3	Lithium (Li)	2,1	I	2
4	Beryllium (Be)	2,2	II	2
5	Boron (B)	2,3	III	2
6	Carbon (C)	2,4	IV	2
7	Nitrogen (N)	2,5	V	2
8	Oxygen (O)	2,6	VI	2
9	Fluorine (F)	2,7	VII	2
10	Neon (Ne)	2,8	VIII (0)	2
11	Sodium (Na)	2,8,1	I	3
12	Magnesium (Mg)	2,8,2	II	3
13	Aluminium (Al)	2,8,3	III	3
14	Silicon (Si)	2,8,4	IV	3
15	Phosphorus (P)	2,8,5	V	3
16	Sulfur (S)	2,8,6	VI	3
17	Chlorine (Cl)	2,8,7	VII	3
18	Argon (Ar)	2,8,8	VIII (0)	3
19	Potassium (K)	2,8,8,1	I	4
20	Calcium (Ca)	2,8,8,2	II	4

Worked example 2: electronic configuration of phosphorus

Phosphorus has proton number 15. Give its electronic configuration, group and period.

15 electrons: 2 in the first shell, 8 in the second, 5 left over
Configuration: 2,8,5
5 outer electrons means Group V; three shells means Period 3

2,8,5, Group V, Period 3

Isotopes

Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. They have different nucleon numbers. For example, chlorine has isotopes ³⁵₁₇Cl and ³⁷₁₇Cl.

Isotopes have the same chemical properties, because they have the same number of electrons and the same outer-shell arrangement.
They have slightly different physical properties, such as mass and density.
Some isotopes are radioactive. Examples of uses include cobalt-60 in cancer treatment and radioactive tracers to detect leaks in pipes.

A_r = (mass 1 × % 1 + mass 2 × % 2) ÷ 100The relative atomic mass is the weighted average mass of the isotopes of an element.

Worked example 3: relative atomic mass of chlorine

Chlorine is 75% ³⁵₁₇Cl and 25% ³⁷₁₇Cl. Calculate its A_r.

(35 × 75) + (37 × 25) = 2625 + 925 = 3550
3550 ÷ 100

A_r = 35.5

Ions

An ion is an atom or group of atoms that has gained or lost electrons, so it has an overall charge. Atoms lose or gain electrons to get a full outer shell. A metal atom usually loses electrons to form a positive ion. A non-metal atom usually gains electrons to form a negative ion. The number of protons never changes.

Worked example 4: particles in an aluminium ion

How many protons, neutrons and electrons are in ²⁷₁₃Al³⁺, and what is its electronic configuration?

Protons = 13 and neutrons = 27 − 13 = 14
The 3+ charge means 3 electrons have been lost: 13 − 3 = 10 electrons
10 electrons fill shells as 2,8

13 protons, 14 neutrons, 10 electrons, configuration 2,8

Common mistakes with atomic structure

Mixing up proton number and nucleon number. The smaller number identifies the element. The larger number includes the neutrons.
Using A_r as the number of neutrons. Neutrons = nucleon number − proton number.
Changing the protons in an ion. Only the electrons change. The nucleus stays the same.
Putting too many electrons in a shell. For the first 20 elements the limits are 2, then 8, then 8.
Saying isotopes have different chemical properties. They have the same chemical properties because they have the same electrons.
Forgetting units and symbols when writing nucleon notation. Write the nucleon number above and the proton number below the symbol.

Exam tip: In an isotope question, always say “same number of protons, different number of neutrons.” In a chemical-properties question, always link the answer to the number of outer electrons.

Interactive atom builder

Choose any of the first 20 elements, then add a charge to see an ion. The diagram and the numbers update as you choose. Neutrons are shown for the most common isotope.

Element Charge

Element
Protons
Neutrons
Electrons
Configuration
Group
Period

Practice questions with answers

Try each question first, then open the answer to check your working.

Q1. How many protons, neutrons and electrons are in ³¹₁₅P?

Protons = 15, electrons = 15, neutrons = 31 − 15 = 16. 15 protons, 16 neutrons, 15 electrons

Q2. Chlorine has proton number 17. Write its electronic configuration and give its group and period.

17 electrons are arranged 2,8,7. 2,8,7, Group VII, Period 3

Q3. Explain why isotopes of the same element have the same chemical properties.

They have the same number of electrons, so the same outer-shell electron arrangement. Chemical reactions involve the outer electrons.

Q4. How many electrons are in a Mg²⁺ ion, and what is its electronic configuration? (Proton number of Mg = 12)

Mg has 12 electrons. It loses 2, leaving 10. 10 electrons, configuration 2,8

Q5. (Extended) Neon is 90% ²⁰₁₀Ne and 10% ²²₁₀Ne. Calculate its relative atomic mass.

(20 × 90 + 22 × 10) ÷ 100 = (1800 + 220) ÷ 100 = 20.2

Q6. An atom has the electronic configuration 2,8,3. Which element is it, and which group and period is it in?

Total electrons = 13, so the proton number is 13, which is aluminium. Aluminium, Group III, Period 3

Q7. (Extended) Boron is 20% ¹⁰₅B and 80% ¹¹₅B. Calculate its relative atomic mass.

(10 × 20 + 11 × 80) ÷ 100 = (200 + 880) ÷ 100 = 10.8

Frequently asked questions

What is the difference between proton number and nucleon number?

The proton number is the number of protons and identifies the element. The nucleon number is the number of protons plus neutrons.

How many electrons can each shell hold?

For the first 20 elements: up to 2 in the first shell, up to 8 in the second, and up to 8 in the third.

How do you work out the group and period from the electronic configuration?

The number of outer-shell electrons is the group number, and the number of shells with electrons is the period number.

What is an isotope?

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

Why are noble gases unreactive?

They have a full outer shell of electrons, so they do not need to gain, lose or share electrons.

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