Electronic configuration and position in the periodic table

Trends across and down the periodic table

The properties of elements change in a regular way across a period and down a group. This is why the periodic table is useful: from the position of an element we can predict how it behaves.

Property	Across a period (left to right)	Down a group
Atomic radius (size)	Decreases: more protons pull the shells in	Increases: more shells
Metallic character	Decreases	Increases
Ionisation energy (energy to remove an electron)	Increases	Decreases
Electronegativity (pull on shared electrons)	Increases	Decreases
Oxides	Basic → amphoteric → acidic	More basic

The atoms get smaller from left to right because the nuclear charge increases while the electrons are added to the same shell.

The period of an element is the number of shells that contain electrons. The group is the number of outer electrons (group 1 to group 8; the noble gases have a full shell). Elements in the same group have the same number of outer electrons, so they react in similar ways.

Group	Name	Outer electrons	Typical behaviour
1	Alkali metals (Li, Na, K)	1	Soft, very reactive; react with water to make hydrogen and an alkali; reactivity increases down the group
2	Alkaline earth metals (Mg, Ca)	2	Less reactive than group 1
7	Halogens (F, Cl, Br, I)	7	Reactive non-metals; reactivity decreases down the group
8	Noble gases (He, Ne, Ar)	8 (2 for He)	Unreactive, full outer shell

Try itPull of the nucleus across period 3

Choose an element from sodium (Z = 11) to argon (Z = 18). The ten inner electrons shield the nucleus, so the outer electrons feel a net positive charge about Z − 10.

Check yourself Going down group 1, the reactivity…

Check yourself An element has the electron arrangement 2,8,7. It is in…

Check yourself Across a period from left to right, atomic size…

Key terms

Ionisation energy: The energy needed to remove one electron from an atom.
Electronegativity: How strongly an atom attracts the electrons in a bond.
Alkali metals: The group 1 metals.
Halogens: The group 7 non-metals.

Summary

Across a period atomic size and metallic character decrease; ionisation energy and electronegativity increase.
Down a group atomic size and metallic character increase; ionisation energy decreases.
Period = number of shells; group = number of outer electrons.
Group 1 reactivity increases downwards; group 7 reactivity decreases downwards.