Pearson Edexcel International GCSE Chemistry 4CH1: Atomic structure and the Periodic Table – Revision Notes

These notes condense sub-topics 1(c) Atomic structure and 1(d) The Periodic Table (points 1.14–1.24) of the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024). The qualification is untiered and none of these points has a “C” reference, so everything here can appear on Paper 1 and Paper 2. For full explanations and worked examples, use the atomic structure and Periodic Table study guide.

Other links: the 4CH1 Chemistry course hub, the printable 4CH1 checklist, the atomic structure and Periodic Table practice questions and the free 4CH1 all-topics diagnostic. The Principles of chemistry revision notes give a one-page view of the whole of Topic 1.

Definitions to learn word for word (1.14, 1.16)

Term	Definition
Atom	The smallest particle of an element that keeps the chemical properties of that element
Molecule	Two or more atoms chemically (covalently) bonded together
Atomic number	The number of protons in the nucleus of an atom
Mass number	The total number of protons and neutrons in the nucleus of an atom
Isotopes	Atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons)
Relative atomic mass (Ar)	The weighted mean mass of the atoms of an element compared with one-twelfth of the mass of an atom of carbon-12

A molecule can be an element (O₂, N₂, Cl₂) or a compound (H₂O, CO₂). Noble gases exist as single atoms, not molecules.

Sub-atomic particles (1.15)

Particle	Where	Relative mass	Relative charge
Proton	Nucleus	1	+1
Neutron	Nucleus	1	0
Electron	Shells around the nucleus	about 1/1840 (negligible)	−1

• Nearly all the mass is in the nucleus.
• Atoms are neutral: protons = electrons.

Method: counting particles

protons   = atomic number (bottom number)
neutrons  = mass number - atomic number
electrons = atomic number - charge on the ion
            (atom: charge 0;  2+ ion: subtract 2;  2- ion: add 2)

Reminder: ³²₁₆S²⁻ has 16 protons, 32 − 16 = 16 neutrons and 16 + 2 = 18 electrons.

Relative atomic mass from abundances (1.17)

Method in steps

1. Multiply each isotope’s mass number by its abundance.
2. Add the products.
3. Divide by the total abundance: 100 for percentages, or the sum of the parts for a ratio.
4. Round as the question asks (often 1 decimal place). No units.
5. Sense-check: Ar lies between the lightest and heaviest isotope, nearer the most abundant one.

Ar = sum of (mass number x abundance) / total abundance

Reminder: neon-20 and neon-22 occur in the ratio 9 : 1.

Ar = (20 x 9 + 22 x 1) / (9 + 1) = (180 + 22) / 10 = 202 / 10 = 20.2

The Periodic Table (1.18, 1.21)

• Arranged in order of increasing atomic number.
• Group = vertical column; similar chemical properties. Groups 1–7 and Group 0 (noble gases).
• Period = horizontal row.
• Metals: left-hand side and the centre block. Non-metals: right-hand side, beyond the stepped line. Hydrogen is a non-metal.
• Argon (atomic number 18) comes before potassium (19) even though its Ar is larger: the order is by atomic number.

Electronic configurations (1.19, 1.22)

Shell capacities for the first 20 elements: 2, 8, 8, then the fourth shell (K and Ca only).

Group	1	2	3	4	5	6	7	0
Period 2	Li 2,1	Be 2,2	B 2,3	C 2,4	N 2,5	O 2,6	F 2,7	Ne 2,8
Period 3	Na 2,8,1	Mg 2,8,2	Al 2,8,3	Si 2,8,4	P 2,8,5	S 2,8,6	Cl 2,8,7	Ar 2,8,8
Period 4	K 2,8,8,1	Ca 2,8,8,2

Period 1: H 1 and He 2.

Method: position ↔ configuration

number of shells          = period number
outer-shell electrons     = group number (Groups 1 to 7)
Group 0                   = full outer shell (He 2; Ne and Ar 8)
total electrons           = atomic number (neutral atom)

Reminder: Period 2, Group 5 → two shells, 5 outer electrons → 2,5 → nitrogen.

Classifying metals and non-metals (1.20)

Test	Metal	Non-metal
Electrical conductivity of the solid	Conducts	Does not conduct (exception: graphite)
Oxide	Basic: reacts with acids to form a salt and water; if soluble, gives an alkaline solution (pH above 7)	Acidic (generally): if soluble, gives an acidic solution (pH below 7)

Method: burn the element in oxygen; add the oxide to water with universal indicator, or test whether it reacts with an acid. Use both conductivity and oxide evidence before you decide.

Groups and noble gases (1.23, 1.24)

• Reactions involve outer-shell electrons. Same group → same number of outer electrons → similar chemical properties.
• Noble gases have a full outer shell, a stable arrangement. They have no tendency to lose, gain or share electrons, so they do not readily react.

Answer frames for “explain” questions

Each frame below shows the separate points a full answer needs. Adapt the element names to the question.

• Similar properties in a group (1.23): “Both atoms have [n] electrons in their outer shell. Chemical reactions involve the outer-shell electrons, so the elements react in a similar way.”
• Noble gas unreactivity (1.24): “The atom has a full outer shell of electrons. This is a stable arrangement, so it does not need to lose, gain or share electrons.”
• Ordering anomaly (1.18): “Elements are arranged by atomic number, not by relative atomic mass. [Element A] has the smaller atomic number, so it comes first and sits in the group with similar properties.”
• Isotopes reacting the same way (1.16): “Isotopes have the same number of protons, so the same number of electrons and the same electron arrangement.”
• Ar is not a whole number (1.17): “Ar is a weighted mean of the masses of the isotopes present, so it falls between their mass numbers.”

Must-know distinctions

• Atomic number vs mass number: protons only vs protons + neutrons.
• Mass number vs Ar: a whole number for one isotope vs a weighted mean for the element, usually not whole.
• Atom vs molecule: one particle of an element vs bonded atoms. Helium is atoms; oxygen gas is molecules.
• Group vs period: column (outer electrons) vs row (number of shells).
• Atom vs ion of the same isotope: same protons and neutrons; only the electrons differ.
• “Full outer shell” vs “8 outer electrons”: helium’s full outer shell has 2.

Quick self-test

1. State the relative charge and the position of a neutron.
2. Give the numbers of protons, neutrons and electrons in ¹⁹₉F⁻.
3. State what is meant by mass number.
4. Lithium is 7.5% lithium-6 and 92.5% lithium-7. Calculate Ar to 1 decimal place.
5. Bromine is 50.7% bromine-79 and 49.3% bromine-81. Calculate Ar to 1 decimal place.
6. Give the electronic configuration of phosphorus (atomic number 15) and state its group.
7. An atom has configuration 2,8,1. State its period and name the element.
8. Element J conducts when solid and its oxide reacts with dilute hydrochloric acid to form a salt and water. Is J a metal or a non-metal?
9. Explain why oxygen and sulfur have similar chemical properties.
10. Give the electronic configuration of argon and explain why argon does not readily react.

Answers

1. Charge 0; in the nucleus.
2. 9 protons; 19 − 9 = 10 neutrons; 9 + 1 = 10 electrons.
3. The total number of protons and neutrons in the nucleus of an atom.
4. (6 × 7.5 + 7 × 92.5) ÷ 100 = (45 + 647.5) ÷ 100 = 6.925 = 6.9.
5. (79 × 50.7 + 81 × 49.3) ÷ 100 = (4005.3 + 3993.3) ÷ 100 = 79.986 = 80.0.
6. 2,8,5; Group 5.
7. Period 3 (three shells); sodium (11 electrons).
8. Metal: it conducts and its oxide is basic.
9. Both have 6 electrons in the outer shell (O 2,6; S 2,8,6), and reactions involve outer-shell electrons.
10. 2,8,8. Its outer shell is full, which is stable, so it has no tendency to lose, gain or share electrons.

Where marks are usually lost

• Writing “the number of protons and electrons” for mass number.
• Leaving “same element” or “same number of protons” out of the isotope definition.
• Giving the electron a relative mass of 0, or a relative charge of “negative” with no number when the table asks for a value.
• Counting electrons in an ion the wrong way round: a positive ion has fewer electrons than protons.
• Dividing by 100 when abundances are a ratio such as 9 : 1.
• Giving Ar as a whole number, or with units, when 1 decimal place is asked for.
• Writing potassium as 2,8,9 or calcium as 2,8,10.
• Saying the Periodic Table is ordered by mass.
• Saying noble gases are unreactive because they have “8 electrons” or “a full shell of 8” without allowing for helium.
• Classifying from a single observation (for example “shiny, so a metal”) when the question gives oxide or conductivity data to use.

Official syllabus

Pearson Edexcel International GCSE in Chemistry (4CH1) specification, Issue 3, September 2024, published by Pearson Education Limited. Topic 1 Principles of chemistry, sub-topics (c) Atomic structure (points 1.14–1.17) and (d) The Periodic Table (points 1.18–1.24).