Elements are arranged in periods and groups on the periodic table. The elements are arranged according to increasing atomic number.
The atomic radius is a measure of the size of the atom.
Electronegativity is the tendency of atoms to attract electrons.
Across a period the ionisation energy and electronegativity increase. The atomic radius decreases across a period.
The groups on the periodic table are labelled from 1 to 18. Group 1 is known as the alkali metals, group 2 is known as the alkali earth metals, group 17 is known as the halogens and the group 18 is known as the noble gases. The elements in a group have similar properties.
The atomic radius and the density both increase down a group. The ionisation energy, electronegativity, and melting and boiling points all decrease down a group.