SPI Chemistry · Free notes

Atomic Structure and Periodic Classification: SPI Chemistry

Atomic Structure and Periodic Classification has 12 questions in the SPI bank, and one of them is from the 2022 paper. Five are EASY, seven are MODERATE and none is HARD. Almost every one needs a single fact or a short count of protons, neutrons or electrons. The marks are lost by mixing up a pair: isotopes and isobars, periods and groups, gaining and losing energy. These pages keep each pair apart.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

SPI home All formulas on one page

Subtopic notes

Formula & revision sheet

2 formulas · 4 reference tables · 13 gotchas across all subtopics — the exam-eve cheat-sheet

Structure of the Atom

Formulas (2)

Atomic number, mass number, isotopes and isobars · Mass number and neutrons $A = Z + N \qquad N = A - Z$
Electron shells, valence electrons and ions · Shell capacity and valency $\text{electrons in shell } n \le 2n^2 \qquad \text{valency} = 8 - \text{valence electrons (5 to 7 valence electrons)}$

Reference tables (1)

The particles of the atom and the models that found them5 rows

Discovery or model	Scientist	What it showed
Electron	J.J. Thomson	Cathode rays carry negative particles
Proton	E. Goldstein	Canal rays carry positive particles
Neutron	James Chadwick	A particle with no charge and a mass close to a proton's
Nucleus	Ernest Rutherford	Alpha-particle scattering from gold foil: a tiny, dense, positive centre The scattering experiment found the nucleus, not the proton or the neutron.
Fixed orbits	Niels Bohr	Electrons move in shells K, L, M, N without losing energy

Energy rises with distance from the nucleus: K is the lowest shell.

Watch out for (7)

Crediting Rutherford with the proton
→ The particles of the atom and the models that found them
Getting the energy direction backwards
→ The particles of the atom and the models that found them
Taking the mass number as the neutron count
→ Atomic number, mass number, isotopes and isobars
Thinking a heavier atom always has the larger atomic number
→ Atomic number, mass number, isotopes and isobars
Giving isobars the same chemistry
→ Atomic number, mass number, isotopes and isobars
Taking valency as the valence electrons every time
→ Electron shells, valence electrons and ions
Overfilling the M shell
→ Electron shells, valence electrons and ions

The Periodic Table and Its Trends

Reference tables (3)

Early attempts: Döbereiner, Newlands and Mendeleev4 rows

Attempt	Basis	Remembered for
Döbereiner's triads	Atomic mass, three at a time	Middle mass is the average of the other two
Newlands' octaves	Atomic mass	Every eighth element is like the first From hydrogen, counting it as the first, the eighth element is fluorine.
Mendeleev's table	Atomic mass and properties	Gaps left for gallium and germanium
Modern periodic table	Atomic number	7 periods and 18 groups

Newlands' pattern breaks down after calcium.

Periods, groups and shells in the modern table7 rows

Feature	Fact	Example
Periods	7 horizontal rows	Period 3 runs from sodium to argon
Groups	18 vertical columns	Group 18 holds the noble gases
Period number	Equals the number of shells	Sulphur (2, 8, 6) has 3 shells: period 3
Group number, groups 1 and 2	Equals the valence electrons	Calcium (2, 8, 8, 2): group 2
Group number, groups 13 to 18	Valence electrons plus 10	Phosphorus (2, 8, 5): group 15
Elements in each period	2, 8, 8, 18, 18, 32, 32	Period 1 holds only hydrogen and helium
Same group	Same valency, similar formulae	Group 1: $\mathrm{NaCl}$ and $\mathrm{KCl}$ A metal chloride $\mathrm{XCl_2}$ means valency 2: group 2.

Shells give the period, valence electrons give the group.

Trends across a period and down a group6 rows

Property	Across a period, left to right	Down a group
Number of shells	Stays the same	Increases
Valence electrons	Increase from 1 to 8	Stay the same
Atomic radius	Decreases	Increases Electrons are added across a period, but the radius still falls: the nucleus pulls harder.
Metallic character	Decreases	Increases
Non-metallic character	Increases	Decreases
Ease of losing electrons	Decreases	Increases

Every row of the down-a-group column is the reverse of the across-a-period column, except valence electrons.

Watch out for (6)

Counting the eighth element wrongly
→ Early attempts: Döbereiner, Newlands and Mendeleev
Saying Mendeleev used atomic number
→ Early attempts: Döbereiner, Newlands and Mendeleev
Swapping periods and groups
→ Periods, groups and shells in the modern table
Reading the period from the electron count
→ Periods, groups and shells in the modern table
Thinking more electrons means a bigger atom across a period
→ Trends across a period and down a group
Mixing up the direction of metallic character
→ Trends across a period and down a group

Question weightage by concept

6 concepts · 12 questions — where the marks actually sit, so you know what to drill first

Structure of the Atom5 questions · 42%

Concept	Qs	Share
The particles of the atom and the models that found them	2	17%
Atomic number, mass number, isotopes and isobars	2	17%
Electron shells, valence electrons and ions	1	8%

The Periodic Table and Its Trends7 questions · 58%

Concept	Qs	Share
Periods, groups and shells in the modern table	4	33%
Trends across a period and down a group	2	17%
Early attempts: Döbereiner, Newlands and Mendeleev	1	8%

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