Ionization energy or nuclear power: The amount of energy required to remove outermost loosely bound electron of an isolated gaseous atom is called ionization energy. It increases in a period but decreases in a group.
Electronegativity: The amount of energy required for an atom to attract outer electrons is called electronegativity. It increases left to right in a period but decreases top to bottom in a group.
Duplet state: The state of an atom in which the outermost shell of the atom has the capacity to fill only two electrons is called duplet state. For example: the state of helium atom is called duplet.