According to the atomic model, electrons orbit the atom's nucleus at specific levels, or shells. Electrons in the outermost shell, which are involved in chemical bonding, are known as valence electrons. Since atoms are more stable when their outer shell is filled, they tend to lose, gain, or share electrons to complete their outer shell.

Two common types of chemical bonds—ionic and covalent—differ in how the atoms achieve full outer shells. In an ionic bond, electrons transfer from one atom to the other. In a covalent bond, the atoms share electrons. Whether a bond is ionic or covalent depends on the electronegativities of the atoms involved.

Electronegativity is the measure of an atom's ability to attract electrons. Some atoms tend to lose electrons, while others are more likely to gain them. Elements with low electronegativity, such as metals, have outer shells that are almost empty; these elements give up electrons fairly easily. Elements with high electronegativity, such as nonmetals, have outer shells that are mostly full; these elements tend to hold on to their electrons. In general, elements on the left of the periodic table have low electronegativities, whereas elements on the right side of the table have high electronegativities. Ionic bonds form when the electronegativities of two atoms vary significantly; covalent bonds form when the electronegativities of the atoms are similar.

A single atom is held together by the attraction between the protons in its nucleus and its orbiting electrons. When two atoms approach each other, each nucleus also attracts the other atom's electrons. The sharing of valence electrons in covalent bonding is the result of both atoms "fighting" for the electrons, with neither one "winning." Consequently, the atoms are effectively bonded together.

If the atoms get too close, they repel each other because of proton-proton and electron-electron repulsion. A molecule is most stable when its electrostatic potential energy is at a minimum—in other words, when its atoms are located at a distance where the proton-electron attractions balance these repulsions. At this distance, the atoms can share their valence electrons and form a covalent bond.

Covalent bonds can be formed with one or more pairs of electrons. A single bond is the sharing of one electron pair. It is also common to have double bonds (two pairs of electrons shared between the atoms) and triple bonds (three pairs of electrons shared between the atoms).

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