Particles in the atom and atomic radius
Cambridge A-Level Chemistry (9701) · Unit 1: Atomic structure · 8 flashcards
Particles in the atom and atomic radius is topic 1.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 1 — Atomic structure , alongside Isotopes, Electrons, energy levels and atomic orbitals and Ionisation energy. In one line: Protons: relative mass = 1, relative charge = +1; Neutrons: relative mass = 1, relative charge = 0; Electrons: relative mass = 1/1840, relative charge = -1.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 3 definitions, 4 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The relative masses and relative charges of protons, neutrons, and electrons
Protons: relative mass = 1, relative charge = +1; Neutrons: relative mass = 1, relative charge = 0; Electrons: relative mass = 1/1840, relative charge = -1.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- understand that atoms are mostly empty space surrounding a very small, dense nucleus that contains protons and neutrons; electrons are found in shells in the empty space around the nucleus
- identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses
- understand the terms atomic and proton number; mass and nucleon number
- describe the distribution of mass and charge within an atom
- describe the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field
- determine the numbers of protons, neutrons and electrons present in both atoms and ions given atomic or proton number, mass or nucleon number and charge
- state and explain qualitatively the variations in atomic radius and ionic radius across a period and down a group 1.2 Isotopes Learning outcomes
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Particles in the atom and atomic radius
- › Always define relative atomic mass as the weighted average mass of atoms of an element compared to 1/12th the mass of a carbon-12 atom.
- › Specify the exact atomic orbitals involved in hybridisation, such as the sp hybridised orbitals of nitrogen in the N2 molecule.
- › Look for the largest 'jump' in energy; a jump after four electrons indicates the fifth is coming from a closer, full inner shell.
- › Use the syllabus definition: nucleon number is the total number of protons and neutrons in the nucleus of an atom.
- › State that shielding is 'similar' across a period because electrons are added to the same main energy level, while nuclear charge increases.
Describe the distribution of mass and charge within an atom.
Atoms consist of a small, dense, positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons in shells. Most of the atom's mass is concentrated in the nucleus, while most of the volume is empty space.
What are the relative masses and relative charges of protons, neutrons, and electrons?
Protons: relative mass = 1, relative charge = +1; Neutrons: relative mass = 1, relative charge = 0; Electrons: relative mass = 1/1840, relative charge = -1.
Define atomic number and mass number.
Atomic number (proton number) is the number of protons in the nucleus of an atom. Mass number (nucleon number) is the total number of protons and neutrons in the nucleus of an atom.
How do beams of protons, neutrons, and electrons behave in an electric field?
Protons (positive charge) are deflected towards the negative plate. Electrons (negative charge) are deflected towards the positive plate and to a greater extent than protons due to lower mass. Neutrons (no charge) are not deflected.
Determine the number of protons, neutrons and electrons in the ion: ³⁹K⁺
Protons: 19 (atomic number of K), Neutrons: 20 (39 - 19), Electrons: 18 (19 - 1 due to +1 charge).
Explain the trend in atomic radius across a period.
Atomic radius decreases across a period due to increasing nuclear charge (more protons) which pulls the electrons closer to the nucleus, and little additional shielding.
Explain the trend in atomic radius down a group.
Atomic radius increases down a group because of the addition of electron shells. The outer electrons are further from the nucleus and are shielded by more inner electrons.
What is the difference between atomic radius and ionic radius?
Atomic radius refers to the size of a neutral atom, whereas ionic radius refers to the size of an ion (an atom that has gained or lost electrons). The ionic radius will differ based on whether an electron was lost (cation, smaller) or gained (anion, larger).
More Chemistry flashcards
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All Chemistry FlashcardsMore topics in Unit 1 — Atomic structure
Particles in the atom and atomic radius sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Particles in the atom and atomic radius deck
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