SPI Chemistry · Atomic Structure and Periodic Classification

The Periodic Table and Its Trends

The modern periodic table puts the elements in order of atomic number, in 7 periods and 18 groups, so that size and metallic character change in a pattern you can predict.

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Why this matters

Seven questions in the bank, three EASY and four MODERATE, one of them from the 2022 paper: Newlands' octaves, the number of periods, which elements share a period, the shell that period 2 fills, the group of an element from its chloride, and the trends in size and metallic character.

Concept 1 of 3: Early attempts: Döbereiner, Newlands and Mendeleev

Before the modern table, chemists tried to sort elements by atomic mass and found patterns that worked only part of the way. Each attempt is remembered for one idea and one weakness.

Definition

Döbereiner's triads: groups of three similar elements in which the middle one's atomic mass is about the average of the other two (Li 7, Na 23, K 39).
Newlands' law of octaves: in order of atomic mass, every eighth element is like the first, as the eighth note of music repeats the first. Count the starting element as the first. It worked only up to calcium.
Mendeleev arranged 63 elements by atomic mass and their properties, and left gaps for elements not yet found. Eka-aluminium and eka-silicon were later found as gallium and germanium.
Modern periodic law (Moseley): properties repeat with atomic number, not atomic mass.

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.

Practice this conceptself-check · 5 quick reps

The same idea in a practice question:

Practice questionModerate

Example 1 · Atomic Structure and Periodic Classification · The Periodic Table and Its Trends

Out of the following ____ is the 8th element after Hydrogen in Newlands Octave.

Counting the eighth element wrongly

In Newlands' octaves the starting element counts as the first. From lithium the order is Li, Be, B, C, N, O, F, Na, so sodium is the eighth, not fluorine.

Saying Mendeleev used atomic number

Mendeleev arranged the elements by atomic mass. The modern periodic table, based on atomic number, came later from Moseley's work.

Concept 2 of 3: Periods, groups and shells in the modern table

Write an element's electronic configuration and you can place it. The number of shells gives the period. The number of valence electrons gives the group, and elements in one group share their valency.

Definition

A period is a horizontal row: there are 7. A group is a vertical column: there are 18.
Period number = number of shells. All elements of period 2 fill the K and L shells, so their outer shell is L.
Groups 1 and 2: group number = valence electrons. Groups 13 to 18: group number = valence electrons + 10.
Elements in one group have the same valence electrons and valency, so their compounds have the same formula: $\mathrm{MgCl_2}$ and $\mathrm{CaCl_2}$ .

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.

Practice this conceptself-check · 5 quick reps

The same idea in a practice question:

Practice questionModerate

Example 2 · Atomic Structure and Periodic Classification · The Periodic Table and Its Trends

Which of the given elements A, B, C, D and E with atomic number 2, 3, 7, 10 and 30 respectively belong to the same period?

Swapping periods and groups

The modern table has 7 periods (rows) and 18 groups (columns). An option of 18 for the number of periods is the count of groups.

Reading the period from the electron count

The period is the number of shells, not the number of electrons. Magnesium, 2, 8, 2, has three shells and is in period 3, even though its outer shell has 2 electrons.

Drill 3 more on periods, groups and shells in the modern table

Concept 3 of 3: Trends across a period and down a group

Across a period the shells stay the same but the nucleus gets more positive, so it pulls the electrons in tighter: atoms shrink and hold their electrons harder. Down a group a new shell is added each time: atoms grow and let go of electrons more easily.

Definition

Across a period (left to right): the number of shells stays the same, nuclear charge rises, atomic radius decreases, metallic character decreases.
Down a group: a shell is added each step, atomic radius increases, metallic character increases.
Metallic character is the ease of losing electrons. So the most metallic elements sit at the bottom left, and the most non-metallic at the top right (leaving out the noble gases).

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.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

SPI · 2022 · SPI Entrance Examination 2022 · Q94Easy

Example 3 · Atomic Structure and Periodic Classification · The Periodic Table and Its Trends

Within a period of the periodic table, while going from left to right, the outermost shell____ and atomic radius____.

Thinking more electrons means a bigger atom across a period

Across a period electrons go into the same shell while the nuclear charge rises, so the atom gets smaller. The radius grows only down a group, where a new shell is added.

Mixing up the direction of metallic character

Metallic character falls from left to right and rises from top to bottom. Among elements of one period, the one furthest left is the most metallic.

Drill 1 more on trends across a period and down a group

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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

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