Atomic Structure: Particles, Radius and Isotopes

This guide covers subtopics 1.1, Particles in the atom and atomic radius, and 1.2, Isotopes, from Topic 1 of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Both are AS Level content, assessed from the first year of the course.

Before studying this

This resource assumes you already know, from IGCSE or O Level, that atoms contain protons, neutrons and electrons; their relative charges and approximate relative masses; and what proton (atomic) number and nucleon (mass) number mean. Atomic Structure covers all of that in full, including a worked isotope-abundance calculation, and is the right place to start if any of it is unfamiliar.

What’s new at AS: quantitative reasoning about how atoms behave as charged particles (their behaviour in an electric field), and — the bigger step up — explaining atomic and ionic radius trends across a period and down a group in terms of nuclear charge and shielding, rather than simply describing them. Once you’ve worked through this page, Atomic Structure: Orbitals and Ionisation Energy continues Topic 1 into sub-shells, orbitals and ionisation energy.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 1

1.1 Particles in the atom and atomic radius — understanding that atoms are mostly empty space surrounding a very small, dense nucleus containing protons and neutrons, with electrons in shells in the space around it; describing protons, neutrons and electrons in terms of relative charge and relative mass; the terms atomic (proton) number and mass (nucleon) number; the distribution of mass and charge within an atom; the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field; determining the numbers of protons, neutrons and electrons in atoms and ions given atomic/proton number, mass/nucleon number and charge; stating and explaining qualitatively the variations in atomic radius and ionic radius across a period and down a group.

1.2 Isotopes — defining isotope in terms of numbers of protons and neutrons; the notation for isotopes (mass/nucleon number and atomic/proton number); stating and explaining why isotopes of the same element have the same chemical properties; stating and explaining why isotopes of the same element have different physical properties, limited to mass and density.

Relative charge and mass

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	−1	1/1840 (negligible)

Because an electron’s mass is negligible compared with a proton or neutron, almost all of an atom’s mass sits in the nucleus, while almost all of an atom’s volume is the empty space occupied by the electrons — the atom is, by volume, mostly empty space around a tiny, dense nucleus.

Behaviour in an electric field

A beam of protons, neutrons and electrons travelling at the same velocity through a uniform electric field separates into three distinct paths:

• Protons deflect towards the negative plate — they are positively charged and attracted to it.
• Electrons deflect towards the positive plate — attracted to it as negatively charged particles — and deflect through a much larger angle than the protons, because their mass is so much smaller for the same charge magnitude.
• Neutrons pass straight through, undeflected, because they carry no charge and an electric field exerts no force on them.

Atomic and ionic radius trends

Across a period (left to right), atomic radius decreases. Each successive element adds a proton to the nucleus (increasing nuclear charge) and an electron to the same outer shell — so shielding from inner shells stays roughly constant while the increasing nuclear charge pulls the outer electrons in more strongly, contracting the atom.

Down a group, atomic radius increases. Each successive element adds a complete new outer shell. The extra shielding from the additional inner shells, and the greater distance of the outer electrons from the nucleus, outweigh the simultaneous increase in nuclear charge — so the atom gets larger despite having more protons.

Ionic radius follows the same nuclear-charge-and-shielding logic, with one extra step: forming a cation by removing outer electrons (often losing a whole shell) makes the ion smaller than its parent atom, while forming an anion by adding electrons to the outer shell increases electron–electron repulsion and makes the ion larger than its parent atom.

Worked example. Explain why a sodium ion, Na⁺, is smaller than a sodium atom, Na, but a chloride ion, Cl⁻, is larger than a chlorine atom, Cl.

Sodium (2,8,1) loses its single outer-shell electron to form Na⁺ (2,8) — this removes the entire outer shell, so the ion has one fewer occupied shell than the atom, and the remaining electrons are held by the same nuclear charge with no extra shielding to oppose it. The ion is markedly smaller. Chlorine (2,8,7) gains one electron to form Cl⁻ (2,8,8) — no new shell is added, but the extra electron increases repulsion within the same outer shell, pushing the existing electrons slightly further apart. The ion is larger, but only modestly, since the nuclear charge and number of shells are unchanged.

Isotopes

Isotopes are atoms of the same element (same proton/atomic number) with different numbers of neutrons (and therefore different mass/nucleon numbers). The standard notation places the mass number as a superscript and the atomic number as a subscript before the element symbol, for example ³⁵₁₇Cl and ³⁷₁₇Cl for the two stable isotopes of chlorine.

Why isotopes share chemical properties: chemical behaviour is determined by electron arrangement, which is determined by proton number. Since isotopes of the same element have identical proton (and therefore electron) numbers, they have identical electron arrangements and react identically.

Why isotopes differ in physical properties: the extra neutrons add mass without changing the electron arrangement, so isotopes differ in mass and, correspondingly, in density — but not in properties that depend on electron behaviour.

Common mistakes

• Predicting deflection direction by mass instead of charge. All three particles are affected (or not) by the field according to their charge, not their mass — mass only affects how much a charged particle deflects, which is why electrons swing further than protons despite deflecting the same direction logic (opposite-to-charge-sign attraction) would suggest for any charged particle.
• Explaining the atomic radius trend across a period by “more shielding.” Shielding stays roughly constant across a period, because electrons are being added to the same shell — the decrease is driven by increasing nuclear charge, not changing shielding.
• Explaining the atomic radius trend down a group by “more nuclear charge.” Nuclear charge does increase down a group, but the dominant effect is the additional shell (distance and shielding) — that’s why the atom still gets bigger despite the larger nuclear charge.
• Assuming all isotopes are chemically distinguishable. They are not, by ordinary chemical means — only techniques sensitive to mass (such as mass spectrometry) or nuclear behaviour distinguish them.

Quick revision checklist

• Relative charge and mass of protons, neutrons and electrons
• Deflection behaviour of proton, neutron and electron beams in an electric field, and why the deflection angles differ
• Determining particle numbers from atomic/proton number, mass/nucleon number and charge
• Atomic radius: decreases across a period (nuclear charge), increases down a group (shielding/distance)
• Ionic radius: cations smaller than their parent atom, anions larger
• Isotope definition and notation; same chemical properties (same electron arrangement), different physical properties limited to mass and density

Related resources

• Atomic Structure — the IGCSE/O Level shell model this resource builds on
• Atomic Structure: Orbitals and Ionisation Energy — the next step in Topic 1: sub-shells, orbitals and ionisation energy
• Chemical Bonding: Electronegativity, Ionic and Metallic Bonds — nuclear charge and shielding applied to bonding
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.