Cambridge IGCSE Chemistry 0620

Atomic Structure

Essential Cambridge IGCSE atomic structure facts, formulas, diagrams and examples — followed by clearly separated higher-level atomic and quantum knowledge.

How to use this page: Study the Cambridge IGCSE section first. It contains the atomic-structure knowledge directly relevant to Cambridge IGCSE Chemistry. The second section introduces useful higher-level concepts and formulas and is clearly separated to avoid confusing them with the IGCSE requirements.

PART 1 • CAMBRIDGE IGCSE

Atomic Structure — Cambridge IGCSE Chemistry

Atomic structure, proton number, nucleon number, electronic configuration, ions and isotopes.

Figure 1

Simplified shell model: protons and neutrons are located in the nucleus, while electrons are shown in shells around the nucleus. This is a simplified educational representation of atomic structure.

IGCSE

1. Subatomic Particles

Atoms contain protons, neutrons and electrons.

Particle	Location	Relative charge	Relative mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0	1
Electron	Shells	−1	1/1836

Figure 2

Nuclear notation: A represents the nucleon (mass) number and Z represents the proton (atomic) number.

IGCSE

2. Atomic Number

The atomic number, also called the proton number, is the number of protons in the nucleus.

Z = number of protons

Carbon has 6 protons → Z = 6.

IGCSE

3. Mass Number

The mass number, also called the nucleon number, is the total number of protons and neutrons in the nucleus.

A = p + n

6 protons + 6 neutrons → A = 12.

IGCSE

4. Number of Neutrons

Subtract the proton number from the nucleon number.

n = A − Z

²³₁₁Na → 23 − 11 = 12 neutrons.

IGCSE

5. Neutral Atoms

A neutral atom has equal numbers of protons and electrons.

electrons = Z

Mg has Z = 12 → 12 protons and 12 electrons.

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6. Positive Ions — Cations

A positive ion forms when an atom loses one or more electrons.

electrons = Z − positive charge

Mg²⁺ → 12 − 2 = 10 electrons.

IGCSE

7. Negative Ions — Anions

A negative ion forms when an atom gains one or more electrons.

electrons = Z + |negative charge|

Cl⁻ → 17 + 1 = 18 electrons.

Figure 3

How Ions Form

2,8,1

Loses 1 electron

→

Na⁺

2,8

Positive ion • Cation

2,8,7

Gains 1 electron

→

Cl⁻

2,8,8

Negative ion • Anion

Key idea: ions form by gaining or losing electrons. The number of protons in the nucleus does not change.

Figure 4

Sodium: 11 electrons are arranged as 2,8,1. The single electron in the outer shell connects its electronic configuration with its position in Group I.

IGCSE

8. Electronic Configuration

For elements with proton numbers 1–20, you should be able to determine the electronic configuration of atoms and ions.

Na = 2,8,1

Ca = 2,8,8,2
Ca²⁺ = 2,8,8

IGCSE

9. Group and Outer Electrons

For elements in Groups I–VII, the group number corresponds to the number of electrons in the outer shell.

Group number = outer-shell electrons

Na = 2,8,1 → Group I.

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10. Period Number

The period number corresponds to the number of occupied electron shells.

Period number = occupied shells

Na = 2,8,1 → three occupied shells → Period 3.

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11. Noble Gases

Noble gases have a full outer electron shell. This electronic structure is associated with their low chemical reactivity.

Ne = 2,8 → full outer shell.

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12. Isotopes

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

Same protons • Different neutrons

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13. Isotope Symbols

Isotopes can be represented using their proton number and nucleon number.

¹²₆C ¹⁴₆C

Both are carbon because both have 6 protons.

Figure 5

Understanding Isotopes

¹²₆C

Carbon-12

6 protons
6 neutrons

¹⁴₆C

Carbon-14

6 protons
8 neutrons

Why are they isotopes? Both atoms contain 6 protons, so both are carbon. They contain different numbers of neutrons.

IGCSE Quick Memory Check

Z = number of protons.
A = protons + neutrons.
Neutrons = A − Z.
In a neutral atom: protons = electrons.
Positive ions form when electrons are lost.
Negative ions form when electrons are gained.
For Groups I–VII: group number corresponds to outer-shell electrons.
Period number corresponds to the number of occupied electron shells.
Isotopes have the same proton number but different numbers of neutrons.

PART 2 • BEYOND IGCSE

Further Atomic & Quantum Knowledge

Useful higher-level atomic, quantum and nuclear physics concepts. These are deliberately separated from the IGCSE section.

Important Scientific Correction

Do not confuse the number of orbitals with the maximum number of electrons in a subshell.

Number of orbitals in a subshell = 2l + 1

Maximum electrons = 2(2l + 1) = 4l + 2

Therefore: s = 2, p = 6, d = 10, f = 14 maximum electrons.

Figure 6

Simplified orbital shapes: the s subshell contains 1 orbital, p contains 3 orbitals and d contains 5 orbitals. Orbital diagrams represent regions associated with electron probability; they should not be interpreted as fixed planetary paths around the nucleus.

FURTHER STUDY

14. Maximum Shell Capacity

Maximum electrons = 2n²

Here n is the principal quantum number.

n = 1 → 2
n = 2 → 8
n = 3 → 18

FURTHER STUDY

15. Orbitals in a Shell

Number of orbitals = n²

n = 2 → 4 orbitals.

CORRECTED

16. Orbitals in a Subshell

Orbitals = 2l + 1

Subshell	l	Orbitals	Max e⁻
s	0	1	2
p	1	3	6
d	2	5	10
f	3	7	14

HYDROGEN-LIKE ONLY

17. Bohr Radius

r_n = n²a₀ / Z

This simple expression applies to hydrogen-like, one-electron atoms or ions.

HYDROGEN-LIKE ONLY

18. Bohr Energy

E_n = −13.6 Z² / n² eV

This expression applies to hydrogen-like one-electron species.

BOHR MODEL

19. Quantised Angular Momentum

mvr = nh / 2π

This is a condition from the historical Bohr model. It is not the modern quantum-mechanical description of an electron moving in a fixed orbit.

FURTHER STUDY

20. Photon Energy & Frequency

|ΔE| = hν

The magnitude of the energy difference determines the frequency of an absorbed or emitted photon.

FURTHER STUDY

21. Wavelength & Frequency

c = λν

λ = c / ν

For electromagnetic radiation in vacuum.

FURTHER STUDY

22. de Broglie Wavelength

λ = h / p

For a non-relativistic particle where p = mv:

λ = h / mv

HYDROGEN-LIKE

23. Rydberg Formula

1/λ = RZ²(1/n₁² − 1/n₂²)

For an emission transition where n₂ > n₁ in a hydrogen-like one-electron species.

FURTHER STUDY

24. Uncertainty Principle

Δx Δp ≥ ħ / 2

Δx Δp ≥ h / 4π

The expressions are equivalent because:

ħ = h / 2π

NUCLEAR PHYSICS

25. Mass Defect

Δm = Zm_p + Nm_n − M_nucleus

Here N is the number of neutrons (N = A − Z), and the expression compares the mass of free protons and neutrons with the actual mass of the nucleus.

NUCLEAR PHYSICS

26. Nuclear Binding Energy

E_b = Δmc²

The nuclear mass defect corresponds to the binding energy required to separate the nucleus into its constituent nucleons.

Cambridge IGCSE Exam Tip: Do not memorise the formulas in the Beyond IGCSE section as though they are all requirements of Cambridge IGCSE Chemistry 0620. Master the first section first: atomic structure, proton number, nucleon number, electronic configuration, ions and isotopes.

Final Formula Summary

Cambridge IGCSE

Z = number of protons
A = protons + neutrons
neutrons = A − Z
neutral atom: electrons = Z
cation: electrons = Z − positive charge
anion: electrons = Z + magnitude of negative charge

Further Knowledge

shell capacity = 2n²
orbitals in shell = n²
orbitals in subshell = 2l + 1
max electrons in subshell = 2(2l + 1)
|ΔE| = hν
c = λν
λ = h/p
ΔxΔp ≥ ħ/2
E_b = Δmc²

Academic Verification

Cambridge International Education: Cambridge IGCSE Chemistry (0620) syllabus. The IGCSE classification on this page follows the syllabus content concerning atoms, elements, compounds and isotopes.

OpenStax: used for supporting higher-level atomic-orbital, quantum-number and subshell concepts.

NIST: used as a reference for higher-level atomic spectroscopy and hydrogen-like atomic relationships.