Electronegativity refers to the ability of an atom in a covalent bond to pull electrons toward itself. Atoms have positively charged particles called protons and neutral particles called neutrons in their centers, with negatively charged particles called electrons orbiting their centers. In a covalent bond, two atoms share a pair of electrons, however when one atom is more electronegative than the other, that atom will attract shared electrons more strongly. In general, the electronegativity of atoms increases as you move up and to the right in the periodic table.
Electronegativity derives from two characteristics of atoms: their electron affinity and their ionization energy. If a gaseous atom has an electron added to it, energy will be released, and that energy is known as electron affinity. Ionization energy is the amount of energy required to remove the outermost electron from a gaseous atom. These two characteristics are taken together determine an atom's electronegativity. Nobel Prize-winning scientist Linus Pauling developed a numerical scale for rating the electronegativity of atoms, and according to this scale the halogens have the most electronegative atoms.
The most electronegative atoms, halogens such as fluorine and chlorine, tend to form pairs with less electronegative atoms, such as sodium and potassium. These pairs of atoms with great differences in electronegativity form ionic bonds, meaning instead of sharing electrons, one atom donates an electron to the other. The two atoms associate with each other through an ionic bond.
Nonpolar and Polar Covalent Bonds
When two atoms have the same electronegativity they will form a pure covalent bond. A simple example is a hydrogen gas molecule, in which two hydrogen atoms share an electron pair in a nonpolar bond.
More commonly two atoms with moderately different electronegativities are covalently bound, and in these instances electrons are not shared equally in the covalent bond, which is said to be polar. Some covalent bonds are strongly polar and others are weakly polar. The strength of polarity is determined by the difference in electronegativity between two atoms.
Significance of Polarity
The importance of polarity can be seen by examining the properties of water. Water is a polar molecule because oxygen is more electronegative than hydrogen, and shared electrons in the covalent bonds between oxygen and hydrogen are more strongly attracted to oxygen. Because of this polarity water molecules form hydrogen bonds with each other in which the hydrogen atoms of one water molecule are attracted to the oxygen atom of another water molecule.
The hydrogen bonding of water explain its many unique properties. Water has high surface tension, it floats when it freezes, and has a high boiling temperature compared to similarly sized molecules. Without these properties life on earth would not exist as we know it, and ultimately the properties of water are determined by the electronegativities of the two kinds of atoms that form it.
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