Define electronegativity - Leaving Cert Chemistry - Question b - 2006

Electronegativity generally increases across a period due to two main factors: a decrease in atomic radius and an increase in effective nuclear charge. As we move left to right across a period, the number of protons in the nucleus increases, which results in a stronger positive charge. Simultaneously, the atoms become smaller, which allows the nucleus to attract shared electrons more effectively, thereby increasing electronegativity.

Reactivity in Group I increases down the group primarily due to the increase in atomic size and the number of electron shells. With each subsequent element, an additional electron shell is added, making the outermost electron further from the nucleus. Although the effective nuclear charge remains the same, the outermost electron is held less tightly by the nucleus, allowing it to be removed more easily during reactions. This makes the alkali metals more reactive as you move down the group.