What is the basic structure of an atom according to the Cambridge IGCSE Chemistry syllabus?

In the Cambridge IGCSE Chemistry syllabus, an atom is described as consisting of a central nucleus containing protons and neutrons, surrounded by electrons in shells. The nucleus is positively charged due to protons, while electrons are negatively charged and orbit the nucleus in defined energy levels or shells.

How do you determine the number of protons, neutrons, and electrons in an atom?

The number of protons in an atom is equal to its atomic number, which is unique for each element and found on the Periodic Table. The number of electrons in a neutral atom is equal to the number of protons. Neutrons can be calculated by subtracting the atomic number from the atomic mass number (rounded to the nearest whole number), which is also provided on the Periodic Table.

What is the significance of the Periodic Table in understanding elements and compounds?

The Periodic Table is a crucial tool in Chemistry as it organizes elements based on increasing atomic number and similar chemical properties. It helps predict the behavior of elements, their compounds, and the trends in their properties, such as electronegativity, ionization energy, and atomic radius, which are essential for understanding chemical reactions and bonding.

How are elements classified in the Periodic Table according to the Cambridge IGCSE syllabus?

In the Cambridge IGCSE syllabus, elements in the Periodic Table are classified into metals, non-metals, and metalloids based on their physical and chemical properties. They are also grouped into periods (horizontal rows) and groups (vertical columns), with elements in the same group having similar properties due to the same number of electrons in their outer shell.

What is the role of electron configuration in determining the chemical properties of an element?

Electron configuration describes the distribution of electrons in an atom's shells and subshells. It plays a vital role in determining an element's chemical properties, such as reactivity and bonding behavior. Elements with similar electron configurations, particularly in their outermost shells, exhibit similar chemical properties, which is why they are grouped together in the Periodic Table.

Why is "Confusing $Z$ with $A$" a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Two numbers, similar position. How to avoid it: Z at BOTTOM = protons. A at TOP = total in nucleus. Source: 0620/42 — recurring

Why is "Putting more than 8 in the second shell" a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Forgetting shell capacity. How to avoid it: Simple model: shell 1 holds 2, shell 2 holds 8, shell 3 holds 8 (for elements up to Ca).

Why is "Predicting $\text{Al}^{3-}$ instead of $\text{Al}^{3+}$" a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Forgetting metals lose electrons. How to avoid it: Metals (Groups I-III) LOSE electrons → POSITIVE ions. Non-metals (V-VII) GAIN electrons → NEGATIVE ions.

Why is "Counting all electrons instead of outer shell only for Group" a common mistake in Atomic Structure and the Periodic Table?

Why it happens: Confusing total electron count with outer count. How to avoid it: Group = number of OUTER-SHELL electrons. Total electrons = Z.

Atoms, Elements and CompoundsCambridge IGCSE Chemistry (0620)

Atomic Structure and the Periodic Table

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Atomic Structure and the Periodic Table — Cambridge IGCSE 0620 Chemistry Extended (2026)

Protons, neutrons and electrons; proton number, nucleon number; electronic configurations; how electronic structure links to position on the Periodic Table.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

1.4 — State the relative charges and approximate masses of protons, neutrons and electrons.
1.4 — Use proton and nucleon numbers to deduce the number of electrons, protons and neutrons.
1.4 — Describe the build-up of electrons in shells (2, 8, 8, 2).
1.4 — Relate the electronic configuration of an element to its position on the Periodic Table.

Subatomic particles and notation

Three particles: proton (+1), neutron (0), electron (-1). Use $^{A}_{Z}\text{X}$ to identify a nuclide.

The three subatomic particles.

Particle	Charge	Relative mass	Location
Proton	$+1$	$1$	Nucleus
Neutron	$0$	$1$	Nucleus
Electron	$-1$	$\sim 1/1836$	Shells around nucleus

Atomic notation. $^{A}_{Z}\text{X}$

$X$ : element symbol.
$Z$ : proton number (atomic number) = number of protons. Defines the element.
$A$ : nucleon number (mass number) = protons + neutrons.

Number of each particle in a neutral atom.

Protons: $Z$ .
Neutrons: $A - Z$ .
Electrons: $Z$ (matches protons in a neutral atom).

Worked. $^{27}_{13}\text{Al}$ (aluminium-27).

Protons: $13$ .
Neutrons: $27 - 13 = 14$ .
Electrons (neutral): $13$ .

Ion. An atom that has gained or lost electrons.

Cation ( $+$ ): lost electrons.
Anion ( $-$ ): gained electrons.

Worked. Mg²⁺ ion. Magnesium has $Z = 12$ .

Protons: $12$ .
Electrons: $12 - 2 = 10$ (lost 2 electrons to form $2+$ ).

Proton +1, neutron 0, electron -1.
$^{A}_{Z}\text{X}$ : $Z$ protons, $A$ nucleons.
Neutrons = $A - Z$ .
Electrons (neutral atom) = $Z$ .
Ion: $\pm$ charge from gain/loss of electrons.

Electronic configurations (2, 8, 8, 2)

Electrons fill shells from inside out: 2 in first, 8 in second, 8 in third (for elements 1-20).

Shell capacities (for the first 20 elements).

Shell 1: max 2 electrons.
Shell 2: max 8.
Shell 3: max 8 (for IGCSE; technically 18 in full theory).
Shell 4: starts filling after shell 3 has 8.

Filling order. Fill the lowest shell first, then move outwards.

Worked. Sodium ( $Z = 11$ ).

Configuration: $2, 8, 1$ .
Shell 1: 2. Shell 2: 8. Shell 3: 1 outer electron.

Worked. Chlorine ( $Z = 17$ ).

Configuration: $2, 8, 7$ .

Worked. Calcium ( $Z = 20$ ).

Configuration: $2, 8, 8, 2$ .

Outer electrons (valence) determine reactivity. Group 1 (1 outer e⁻) and Group 17 (7 outer e⁻) are highly reactive. Group 18 (8 outer e⁻ — full shell) are inert (noble gases).

Drawing electronic structure. Cambridge accepts dot diagrams (concentric circles with × or • marks) — show the right number of electrons in each shell.

2, 8, 8, 2 capacity for shells 1-4 (first 20 elements).
Fill lowest shell first.
Outer-shell electrons = chemistry / reactivity.
Sodium: 2,8,1. Chlorine: 2,8,7.

Linking electronic structure to the Periodic Table

Period = number of shells. Group = number of outer electrons.

Period number = number of OCCUPIED shells.

Period 1: 1 shell (H, He).
Period 2: 2 shells (Li to Ne).
Period 3: 3 shells (Na to Ar).
etc.

Group number (for the main groups, 1-2 and 13-18) = number of OUTER-SHELL electrons.

Group 1 (alkali metals): 1 outer e⁻ (Li, Na, K).
Group 17 (halogens): 7 outer e⁻ (F, Cl, Br).
Group 18 (noble gases): full outer shell (He: 2; Ne: 8; Ar: 8).

Why the Periodic Table is the SHAPE it is. Elements in the same group have the SAME outer-shell configuration — that's why they have similar chemical properties. Sodium and potassium both have 1 outer electron → both react vigorously with water.

Worked. A neutral element has electronic configuration $2, 8, 6$ .

Period 3 (3 shells).
Group 16 (6 outer electrons).
Element: sulfur ( $Z = 16$ ).

Cambridge tip. Most properties (atomic radius, ionisation energy, reactivity) follow trends DOWN groups and ACROSS periods. See the Periodic Trends study note for details.

Period number = shells occupied.
Group number = outer-shell electrons.
Same group → same outer config → similar chemistry.
Configuration $2, 8, 6$ : Period 3, Group 16.

How it’s examined

Atomic structure appears every Paper 2 (4-5 marks: identify particles, write electronic configuration) and every Paper 4 (5-7 marks: link configuration to Periodic Table position, ions). Examiner reports flag confusing $A$ with $Z$ and forgetting that ions have a different number of electrons from the neutral atom.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (1.4); 0620/42 Oct/Nov 2024 — Q3 (electronic configurations); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Step-by-step worked examples — Atomic Structure and the Periodic Table

Step-by-step solutions to past-paper-style questions on atomic structure and the periodic table, written exactly the way a tutor would explain them at the board.

1Find subatomic particles in an atom
Core• nuclide
Question
How many protons, neutrons and electrons are in $^{23}_{11}\text{Na}$ (neutral atom)?
Step-by-step solution
1. Step 1
  Bottom number = proton number = $11$ .
2. Step 2
  Top number $= 23$ = nucleons. Neutrons $= 23 - 11 = 12$ .
3. Step 3
  Neutral → electrons $=$ protons $= 11$ .
Answer
$11$ protons, $12$ neutrons, $11$ electrons.
2Write electronic configuration
Core• Adapted from 0620/22 May/Jun 2024 Q3• configuration
Question
Write the electronic configuration of phosphorus, $Z = 15$ .
Step-by-step solution
1. Step 1
  Fill shells: $2$ , $8$ , then put the remaining $5$ in the third shell.
  $2,8,5$
Answer
$2,8,5$
3Group and Period from configuration
Extended• periodic table
Question
An element has electronic configuration $2,8,7$ . State its group and period.
Step-by-step solution
1. Step 1
  Number of outer-shell electrons → Group ( $7$ → Group VII).
2. Step 2
  Number of occupied shells → Period ( $3$ → Period 3).
Answer
Group VII, Period 3 (chlorine).
4Predict ion charge from group
Extended• ions
Question
Aluminium is in Group III. Write the formula and charge of the ion it forms.
Step-by-step solution
1. Step 1
  Group III → loses 3 electrons to reach a stable noble-gas configuration.
2. Step 2
  Ion: $\text{Al}^{3+}$ .
Answer
$\text{Al}^{3+}$

Key Formulae — Atomic Structure and the Periodic Table

The formulae you need to memorise for atomic structure and the periodic table on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Nuclide notation
$^{A}_{Z}\text{X}$
$A$
nucleon (mass) number
$Z$
proton (atomic) number
When to use
Specifying a particular isotope.
Shell-filling order
$2,\ 8,\ 8,\ \ldots\ \text{(simple model up to Ca)}$
When to use
Building electronic configurations of the first 20 elements.

Key Definitions and Keywords — Atomic Structure and the Periodic Table

Definitions to memorise and the exact keywords mark schemes credit for atomic structure and the periodic table answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Proton (atomic) number $Z$
Examiner keyword
Number of protons in a nucleus. Defines the element.
Nucleon (mass) number $A$
Examiner keyword
Total number of protons + neutrons in a nucleus.
Electronic configuration
Examiner keyword
How electrons are arranged in shells around the nucleus, e.g. $2,8,7$ for Cl.
Group
Examiner keyword
A vertical column in the Periodic Table. Members have the same number of outer-shell electrons.
Period
Examiner keyword
A horizontal row in the Periodic Table. Members have the same number of occupied electron shells.

Common Mistakes and Misconceptions — Atomic Structure and the Periodic Table

The traps other students keep falling into on atomic structure and the periodic table questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Confusing $Z$ with $A$
0620/42 — recurring
Why it happens
Two numbers, similar position.
How to avoid it
$Z$ at BOTTOM = protons. $A$ at TOP = total in nucleus.
✕Putting more than 8 in the second shell
Why it happens
Forgetting shell capacity.
How to avoid it
Simple model: shell 1 holds 2, shell 2 holds 8, shell 3 holds 8 (for elements up to Ca).
✕Predicting $\text{Al}^{3-}$ instead of $\text{Al}^{3+}$
Why it happens
Forgetting metals lose electrons.
How to avoid it
Metals (Groups I-III) LOSE electrons → POSITIVE ions. Non-metals (V-VII) GAIN electrons → NEGATIVE ions.
✕Counting all electrons instead of outer shell only for Group
Why it happens
Confusing total electron count with outer count.
How to avoid it
Group = number of OUTER-SHELL electrons. Total electrons = $Z$ .

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

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Atomic Structure and the Periodic Table — frequently asked questions

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Atomic Structure and the Periodic Table

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Find subatomic particles in an atom

2Write electronic configuration

3Group and Period from configuration

4Predict ion charge from group

Nuclide notation

Shell-filling order

Proton (atomic) number ZZZ

Nucleon (mass) number AAA

Electronic configuration

Group

Period

✕Confusing ZZZ with AAA

✕Putting more than 8 in the second shell

✕Predicting Al3−\text{Al}^{3-}Al3− instead of Al3+\text{Al}^{3+}Al3+

✕Counting all electrons instead of outer shell only for Group

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Atomic Structure and the Periodic Table — frequently asked questions

Proton (atomic) number $Z$

Nucleon (mass) number $A$

✕Confusing $Z$ with $A$

✕Predicting $\text{Al}^{3-}$ instead of $\text{Al}^{3+}$