AS Chemistry: Atomic Structure — Particles, Radius and Isotopes — Revision Notes

Condensed for revision. For the full explanation, use the Atomic Structure: Particles, Radius and Isotopes study guide, then test yourself with the practice questions. Next in Topic 1: the Atomic Structure and Ionisation Energy revision notes.

Syllabus: Cambridge International AS & A Level Chemistry 9701, 2025–2027, AS Level content: subtopics 1.1 Particles in the atom and atomic radius and 1.2 Isotopes.

The nuclear atom (1.1)

• An atom is mostly empty space around a very small, dense nucleus.
• The nucleus contains protons and neutrons (together called nucleons).
• Electrons are found in shells in the empty space around the nucleus.

Particle properties

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

Distribution of mass and charge

• Mass: almost all of the mass is in the nucleus, because protons and neutrons each have a relative mass of 1, compared with 1/1840 for an electron.
• Charge: the positive charge is concentrated in the tiny nucleus (the protons); the negative charge (the electrons) is spread through the large volume around it.
• A neutral atom has equal numbers of protons and electrons, so the charges cancel.

Atomic number and mass number

Term	Also called	Meaning
Atomic number, Z	proton number	number of protons in the nucleus (identifies the element)
Mass number, A	nucleon number	total number of protons + neutrons in the nucleus

number of neutrons  = mass number − atomic number
number of electrons = atomic number − charge on the ion

Beams in an electric field (1.1)

Beams of protons, neutrons and electrons, all moving at the same velocity, pass between a positive and a negative plate.

Beam	Direction	Size of deflection
Protons	curve towards the negative plate	small
Electrons	curve towards the positive plate (opposite way to protons)	much larger angle than protons
Neutrons	straight through, undeflected	none: no charge

Why the angles differ: at the same velocity, the deflection depends on charge ÷ mass.

proton:    charge ÷ mass = 1 ÷ 1        = 1
electron:  charge ÷ mass = 1 ÷ (1/1840) = 1840   (size of charge only)

Protons and electrons have charges of the same size, but the electron is far lighter, so it is deflected much more. The same rule compares ions: ¹H⁺ (1 ÷ 1 = 1) is deflected more than ⁴He²⁺ (2 ÷ 4 = 0.5), which is deflected by the same amount as ²H⁺ (1 ÷ 2 = 0.5).

Counting particles in atoms and ions (1.1)

Species	Protons	Neutrons	Electrons
²⁷₁₃Al	13	27 − 13 = 14	13
²⁴₁₂Mg²⁺	12	12	12 − 2 = 10
³²₁₆S²⁻	16	16	16 + 2 = 18
⁵⁶₂₆Fe³⁺	26	30	23
³⁷₁₇Cl⁻	17	20	18

A positive ion has lost electrons; a negative ion has gained electrons. The number of protons never changes when an ion forms.

Atomic radius and ionic radius (1.1)

Explain every trend with three factors: nuclear charge, number of shells (distance) and shielding by inner shells.

Trend	Atomic radius	Explanation
Across a period	decreases	nuclear charge increases; electrons are added to the same shell, so shielding is roughly constant; outer electrons are pulled closer
Down a group	increases	an extra shell each time; outer electrons are further from the nucleus and more shielded; this outweighs the increase in nuclear charge

Ions compared with their atoms:

• A cation is smaller than its atom: it usually loses its whole outer shell, and the remaining electrons are attracted by more protons than there are electrons.
• An anion is larger than its atom: extra electrons go into the same outer shell, increasing electron–electron repulsion, with the same nuclear charge.

Ionic radius across Period 3:

Ions	Electrons	Trend	Reason
Na⁺ > Mg²⁺ > Al³⁺	all 2,8 (isoelectronic)	decreases	same electron arrangement; nuclear charge rises from 11 to 13
P³⁻ > S²⁻ > Cl⁻	all 2,8,8 (isoelectronic)	decreases	same electron arrangement; nuclear charge rises from 15 to 17

The anions P³⁻, S²⁻ and Cl⁻ are all much larger than the cations Na⁺, Mg²⁺ and Al³⁺ because they have one more occupied shell.

Ionic radius down a group: increases (for example Li⁺ < Na⁺ < K⁺ and F⁻ < Cl⁻ < Br⁻), because each ion has one more shell than the one above it.

Isotopes (1.2)

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

Notation, for example ³⁵₁₇Cl: the mass (nucleon) number is written top left and the atomic (proton) number bottom left of the symbol.

Isotope	Protons	Neutrons	Electrons (atom)
³⁵₁₇Cl	17	18	17
³⁷₁₇Cl	17	20	17

Same chemical properties: isotopes of an element have the same number of electrons and the same electronic configuration. Chemical reactions involve the electrons, so the isotopes react in the same way.

Different physical properties (limited to mass and density): isotopes have different numbers of neutrons, so their atoms have different masses. Samples of the heavier isotope therefore have a greater density (for example, water made with ²H is denser than ordinary water).

Link forward (2.1 and 22.2): relative atomic mass, Ar, is the weighted mean mass of the isotopes. (Illustrative values: rounded abundances, relative isotopic masses taken as whole numbers.) Chlorine is 75% ³⁵Cl and 25% ³⁷Cl.

Ar = (35 × 75 + 37 × 25) ÷ 100
   = (2625 + 925) ÷ 100
   = 3550 ÷ 100 = 35.5

Exam traps

• Electrons are deflected towards the positive plate, protons towards the negative plate, and neutrons not at all. The sign of the charge decides which way; charge ÷ mass decides how much.
• The electron beam bends through a larger angle than the proton beam, not a smaller one.
• Mass number counts protons + neutrons, not electrons.
• For ions, change only the electron count: Mg²⁺ has 10 electrons, not 14.
• Across a period, shielding stays roughly the same; do not explain the decrease by “more shielding”.
• Down a group, nuclear charge does increase, but the extra shell and shielding win.
• Isotopes have the same chemical properties because they have the same electron configuration, not because they have the same mass.
• For 1.2, physical differences are limited to mass and density.

Self-test

1. State the relative charge and relative mass of a proton, a neutron and an electron.
2. Describe the distribution of mass and of charge in an atom.
3. Beams of protons, neutrons and electrons at the same velocity pass through an electric field. Describe and explain what happens to each beam.
4. Which is deflected more in an electric field at the same velocity, ¹H⁺ or ⁴He²⁺? Explain.
5. Give the numbers of protons, neutrons and electrons in ⁵²₂₄Cr³⁺.
6. Explain why atomic radius decreases across Period 3.
7. Explain why a sulfide ion, S²⁻, is larger than a sulfur atom.
8. Put Na⁺, Mg²⁺ and Al³⁺ in order of increasing ionic radius and explain the order.
9. Define isotopes and explain why ³⁵Cl and ³⁷Cl have the same chemical properties.
10. (Illustrative values.) Boron is 20% ¹⁰B and 80% ¹¹B. Calculate its relative atomic mass.

Answers:

1. Proton: +1, 1. Neutron: 0, 1. Electron: −1, 1/1840 (negligible).
2. Almost all the mass, and all the positive charge, is in the tiny nucleus (protons and neutrons); the negative charge (electrons) is spread through the large, mostly empty space around the nucleus.
3. Protons curve towards the negative plate; electrons curve towards the positive plate through a much larger angle, because they have a charge of the same size but a much smaller mass (larger charge ÷ mass); neutrons pass straight through because they are uncharged.
4. ¹H⁺: its charge ÷ mass is 1 ÷ 1 = 1, compared with 2 ÷ 4 = 0.5 for ⁴He²⁺.
5. Protons 24; neutrons 52 − 24 = 28; electrons 24 − 3 = 21.
6. Nuclear charge increases while electrons are added to the same shell, so shielding is roughly constant; the outer electrons are attracted more strongly and pulled closer to the nucleus.
7. S²⁻ has two extra electrons in the same outer shell with the same nuclear charge (16), so electron–electron repulsion increases and the electrons spread out further.
8. Al³⁺ < Mg²⁺ < Na⁺. All three have the arrangement 2,8, but the nuclear charge increases from 11 (Na) to 12 (Mg) to 13 (Al), so the electrons are pulled in more strongly.
9. Atoms of the same element with the same number of protons but different numbers of neutrons. Both have 17 electrons with the same electronic configuration, and chemical reactions involve the electrons, so they react in the same way.
10. Ar = (10 × 20 + 11 × 80) ÷ 100 = (200 + 880) ÷ 100 = 1080 ÷ 100 = 10.8

These are original notes written for revision. Isotope abundances marked illustrative are rounded, not data-book values. Check the full syllabus wording in the official 9701 syllabus.