Why Do Atoms Bond?

Difference Between Stability and Neutral Electrical Charge

Close up view of ball and stick molecular model
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Atoms form chemical bonds in order to make their outer electron shells more stable. The type of chemical bond maximizes the stability of the atoms that form it. An ionic bond, where one atom essentially donates an electron to another, forms when one atom becomes stable by losing its outer electrons and the other atoms become stable (usually by filling its valence shell) by gaining the electrons. Covalent bonds form when sharing atoms results in the highest stability.

Other types of bonds besides ionic and covalent chemical bonds exist, too.

Bonds and Valence Electrons

The very first electron shell only holds two electrons, A hydrogen atom (atomic number 1) has one proton and a lone electron, so it can readily share its electron with the outer shell of another atom. A helium atom (atomic number 2), has two protons and two electrons. The two electrons complete its outer electron shell (the only electron shell it has), plus the atom is electrically neutral this way. This makes helium stable and unlikely to form a chemical bond.

Past hydrogen and helium, it's easiest to apply the octet rule to predict whether two atoms will form bonds and how many bonds they will form. Most atoms need 8 electrons to complete their outer shell. So, an atom that has 2 outer electrons will often form a chemical bond with an atom that lacks two electrons to be "complete".

For example, a sodium atom has one lone electron in its outer shell.

A chlorine atom, in contrast, is short one electron to fill its outer shell. Sodium readily donates its outer electron (forming the Na+ ion, since it then has one more proton than it has electrons), while chlorine readily accepts a donated electrons (making the Cl- ion, since chlorine is stable when it has one more electron than it has protons).

Sodium and chlorine form an ionic bond with each other, to form table salt or sodium chloride.

A Note About Electrical Charge

You may be confused about whether the stability of an atom is related to its electrical charge. An atom that gains or loses an electron to form an ion is more stable than a neutral atom if the ion gets a full electron shell by forming the ion.

Because opposite charged ions attract each other, these atoms will readily form chemical bonds with each other.

Predicting Bonds Between Atoms

You can use the periodic table to make several predictions about whether atoms will form bonds and what type of bonds they might form with each other. On the far right-hand side of the periodic table is the group of elements called the noble gases. Atoms of these elements (e.g., helium, krypton, neon) have full outer electron shells. These atoms are stable and very rarely form bonds with other atoms.

One of the best ways to predict whether atoms will bond with each other and what type of bonds they will form is to compare the electronegativity values of the atoms. Electronegativity is a measure of the attraction an atom has to electrons in a chemical bond.

A large difference between electronegativity values between atoms indicates one atom is attracted to electrons, while the other is able to accept electrons.

These atoms usually form ionic bonds with each other. This type of bond forms between a metal atom and a nonmetal atom.

If the electronegativity values between two atoms are comparable, they may still form chemical bonds to increase the stability of their valence electron shell. These atoms usually form covalent bonds.

You can look up electronegativity values for each atom to compare them and decide whether an atom will form a bond or not. Electronegativity is a periodic table trend, so you can make general predictions without looking up specific values. Electronegativity increases as you move from left to right across the periodic table (except for the noble gases). It decreases as you move down a column or group of the table. Atoms on the left-hand side of the table readily form ionic bonds with atoms on the right side (again, except the noble gases).

Atoms in the middle of the table often form metallic or covalent bonds with each other.