3 Bonds and Compounds

What chemical bonds do

A chemical bond is an attractive interaction that holds atoms or ions together in a stable arrangement. Three common types are ionic, covalent, and metallic bonds.

Three types of bonding

In ionic bonding, electrons transfer from one atom to another, producing oppositely charged ions that attract. For example, sodium can lose an electron to form $\mathrm{Na^+}$, while chlorine gains one to form $\mathrm{Cl^-}$; their attraction holds solid sodium chloride together. Ionic bonding is common between metals and nonmetals, but this is a useful pattern rather than an exception-free rule.

In covalent bonding, atoms share pairs of electrons. Two hydrogen atoms share electrons to form $\mathrm{H_2}$. Covalent bonds are common between nonmetals. Unequal sharing is described as polar covalent; equal or nearly equal sharing is described as nonpolar covalent.

Metallic bonding involves attractions among many metal atoms and mobile, shared electrons. This type of bonding helps explain why metals conduct electricity and can be shaped without easily breaking apart.

A substance can contain more than one kind of bond. In potassium nitrate, $\mathrm{KNO_3}$, $\mathrm{K^+}$ ions attract $\mathrm{NO_3^-}$ ions through ionic bonding, while nitrogen and oxygen atoms within each nitrate ion are joined by covalent bonds.

Molecules and compounds

A molecule is a discrete group of atoms held together by covalent bonds. It may contain atoms of one element, as in $\mathrm{O_2}$, or atoms of several elements, as in $\mathrm{H_2O}$.

A compound is a substance made of atoms of at least two different elements chemically combined in fixed proportions. Water is both a molecule and a compound. Oxygen gas, $\mathrm{O_2}$, is a molecule but not a compound because it contains only one element.

Ionic solids such as sodium chloride do not consist of separate $\mathrm{NaCl}$ molecules. Instead, they form extended arrangements of positive and negative ions. The formula $\mathrm{NaCl}$ gives the simplest whole-number ratio of ions in the solid, called a formula unit.

Reading chemical formulas

A chemical formula uses element symbols and numbers to show which elements are present and how many atoms or ions are represented.

A subscript to the right of a symbol gives the number of those atoms. For example, $\mathrm{H_2O}$ has two hydrogen atoms and one oxygen atom; when a symbol has no subscript, its count is one.

Parentheses group atoms that occur together. In $\mathrm{Ca(NO_3)_2}$, the $2$ outside the parentheses means there are two nitrate groups, giving two nitrogen atoms and six oxygen atoms.

A coefficient in front of a formula counts whole particles or formula units. In $3\mathrm{H_2O}$, the coefficient $3$ means three water molecules, containing six hydrogen atoms and three oxygen atoms altogether. A coefficient multiplies every subscript in the formula.

Formulas for ionic and covalent substances

Formulas for ionic compounds are written so the total positive and negative charges balance. Magnesium forms $\mathrm{Mg^{2+}}$, and chlorine forms $\mathrm{Cl^-}$. Two chloride ions are needed to balance one magnesium ion, so the neutral formula is $\mathrm{MgCl_2}$. Its subscripts give the simplest whole-number ratio of ions; they do not show the charges themselves.

Covalent formulas show the actual numbers of atoms in each molecule. For example, $\mathrm{CO_2}$ represents one carbon atom bonded with two oxygen atoms. Different covalent compounds can contain the same elements in different ratios, so the formula must show those ratios.

Molecular and empirical formulas

A molecular formula gives the actual number of each type of atom in a molecule, while an empirical formula gives the simplest whole-number ratio. Glucose has molecular formula $\mathrm{C_6H_{12}O_6}$. Dividing all its subscripts by $6$ gives the empirical formula $\mathrm{CH_2O}$, with the ratio $1:2:1$.