IB Chemistry - Periodicity

Atomic Radius

distance from the centre of the atom to the furthermost electron of the atom

Atomic Radius trend across period

Atomic radius decreases across period from left to right. Nucleus’s positive charge is greater relative to charge of electrons in same shell, pulling electrons closer to nucleus and thus reducing atomic radius

Atomic Radius trend down group

Atomic radius increase down group from top to bottom. Each new period begins with new energy level which is farther from nucleus, increasing atomic radius.

Ionic Radius

distance from the centre of the ion to the furthermost electron of the ion

Ionic cations trend

Radii of cations (e.g. Mg+) are smaller than their parent. This is because there are more protons than electrons in the cation, so valence are more strongly attracted to nucleus.

Ionic anions trend

Radii of anions(e.g. Cl–) are larger than their parent. This is because the extra electron in the anion results in greater repulsion between valence electrons, thus larger radius.

Ionisation energy

the energy required to remove 1 mole of e- from 1 mole of gaseous atoms to form 1 mole of unipositive ions

Ionisation trend across period

IE increases across period from left to right. This is because since electrons are pulled closer to nucleus (see atomic radius), more difficult to remove them.

Ionisation trend down group

IE decreases down a group from top to bottom. This is because atomic radii increase down group, making easier to remove electron. Also because of shielding effect weakens forces between nucleus and outer electrons.

Electron affinity

the energy required to add1 mole of e- to 1 mole of gaseous atoms to form 1 mole of uninegative ions

Electron affinity trend across period

Increases across a period from left to right. Effective nuclear charge increase therefore there is stronger attraction for e- (becomes more negative. With some exceptions)

Electron affinity trend down group

Decreases down a group from top to bottom. Down a group valence e- further from nucleus therefore less attraction (becomes less negative. With some exceptions)

Electronegativity

The tendency of an atom in a covalent molecule to attract a pair of shared e- towards itself.

Electronegativity trend across period

Electronegativity increases across period from left to right. effective nuclear charge increases therefore bonding e- closer to nucleus and more strongly attracted to it

Electronegativity down group

Electronegativity decreases down a group from top to bottom. This is because atomic radii increase down group, making it easier to remove electron. Also because of shielding effect weakens forces between nucleus and outer electrons.