Trends in the Periodic Table

In short: Properties change in patterns across the periodic table. Atoms get bigger down a group and smaller across a period, group 1 metals get more reactive down the group while halogens get less reactive, and electronegativity rises across a period and falls down a group.

The two big ideas

1. Across a period (left to right): the number of protons goes up, but the electrons are added to the same shell. The pull on the outer electrons gets stronger.
2. Down a group: a new shell is added each time. The outer electrons are further away, and the inner shells shield them from the nucleus. The pull gets weaker.

Atomic radius (size)

• Down a group: atoms get bigger, because there are more shells.
• Across a period: atoms get smaller, because more protons pull the same shell in tighter.

Sodium is bigger than chlorine, even though chlorine has more electrons.

Ionic radius

When an atom becomes an ion, its size changes.

• Positive ions are smaller than their atoms. $\mathrm{Na}$ loses its only outer electron, so its whole outer shell disappears. $\mathrm{N}{\mathrm{a}}^{+}$ is much smaller than $\mathrm{Na}$.
• Negative ions are bigger than their atoms. $\mathrm{Cl}$ gains an electron. The extra electron repels the others, and the same number of protons now holds more electrons, so the cloud spreads out. $\mathrm{C}{\mathrm{l}}^{-}$ is bigger than $\mathrm{Cl}$.

The octet rule

Atoms tend to gain, lose or share electrons until they have eight electrons in their outer shell (two for hydrogen and helium), like a noble gas. This is the octet rule. It explains why metals on the left lose electrons (they have only a few to give away) and non-metals on the right gain them (they are only a few short of eight).

Reactivity of metals (group 1)

Metals react by losing electrons. The easier it is to lose the outer electron, the more reactive the metal.

Down group 1, reactivity increases. Lithium fizzes gently on water. Sodium whizzes around and melts. Potassium bursts into a lilac flame. Caesium explodes. The outer electron is further away and easier to lose each time.

$$\text{lithium}+\text{water}\to \text{lithium hydroxide}+\text{hydrogen}$$

Reactivity of non-metals (group 17)

Non-metals react by gaining electrons. The stronger the pull on an incoming electron, the more reactive.

Down group 17, reactivity decreases. Fluorine is the most reactive element of all. Iodine is much calmer. In bigger atoms the incoming electron is further from the nucleus and feels a weaker pull.

A more reactive halogen displaces a less reactive one from its salt:

$$\text{chlorine}+\text{potassium bromide}\to \text{potassium chloride}+\text{bromine}$$

Metallic character

Metallic character is how much an element behaves like a metal.

• It increases down a group. Carbon at the top of group 14 is a non-metal, silicon and germanium are metalloids, tin and lead are metals.
• It decreases across a period. Period 3 goes from metals (Na, Mg, Al) to a metalloid (Si) to non-metals (P, S, Cl, Ar).

Other trends to know

Electronegativity is how strongly an atom pulls on shared electrons in a bond. Fluorine is the most electronegative element. Noble gases are usually left out because they rarely bond.

Ionisation energy is the energy needed to remove an electron from an atom. Atoms on the right of a period hold their electrons tightly, so their ionisation energy is high. Atoms lower down hold them loosely, so it is low.

Electron affinity is the energy released when an atom gains an electron. It shows how much an atom "wants" an extra electron. Halogens, one electron short of a full shell, have the highest electron affinities. Chlorine has a higher electron affinity than sulfur.

Quick summary

• Trends depend on the number of protons and on how far the outer electrons are from the nucleus.
• Atoms get bigger down a group and smaller across a period.
• Group 1 metals get more reactive down the group. Group 17 halogens get less reactive down the group.
• Metallic character increases down and decreases across.
• Electronegativity increases across and decreases down. Fluorine is the highest.

Quiz