Electronegativity
Electronegativity can be defined as the numerical measure of the ability of an atom to attract bonding electrons. Below is a table of the electronegativity values of some elements as they appear in the periodic table.
What happens when atoms with different electronegativity come together? Lets try and explain with a few animations below.
When non-metal atoms with identical electronegativity bond, electrons are shared evenly.

When non-metal atoms with slight variation in electronegativity bond, electrons are shared unevenly. This causes small charges to appear on opposite sides of the bond.

Which atom on the right has the greatest electronegativity?
When atoms with low electronegativity (metals) bond with atoms with high electronegativity (non-metals), electrons are transferred from one atom to another. Charged atoms (ions) are formed.

Electronegativity is the ability to attract electrons to an atom. This attraction is electrostatic in nature. It is the attraction between the nucleus (positive) and the valence electrons that is crucial in determining the electronegativity of the atom. Electrostatic forces depend on the size of the charges and the distance between them.

Look at the trend in atomic size across a period and down a group. Offer an explanation as to why electronegativity increases across a period but decreases down a group.