​Ionic and Covalent Compounds CH5 by Owen Borville October 12, 2025

A compound is a substance composed of two or more elements combined in a specific ratio and held together by chemical bonds. Examples are water, sodium chloride, carbon dioxide, sodium bicarbonate, acetic acid, ammonia.

When atoms form compounds, it is their valence electrons that actually interact.

A Lewis dot diagram or symbol consists of the element's symbol with dots. Each dot represents a valence electron, as shown by the electron configuration. Ex. 1s2 2s2 2p1 There are three valence electrons in Boron's electron configuration.

​Therefore the Lewis dot symbol for boron includes its symbol, B, with three dots surrounding. Dots are positioned one in front, one on top, and one behind. The dots are not doubled until there are more than four dots, one in each direction.

For main group elements (non-transition metals), the number of dots should represent the number of electrons that are lost. For nonmetals in the second period, the number of unpaired dots is the number of bonds the atom can form.

Ions can also be represented by Lewis dot symbols. All valence electrons plus all electrons gained are shown. Electrons lost are not shown. The ionic charge is shown in parentheses or brackets.

Ionic bonding refers to the electrostatic attraction that holds oppositely charged ions together in an ionic compound. 

The resulting electrically neutral compound, sodium chloride, is represented with the chemical formula NaCl.

The chemical formula, or simply formula, of an ionic compound denotes the constituent elements and the ratio in which they combine.

A three-dimensional array of oppositely charged ions is called a lattice. Lattice energy is the amount of energy required to convert a mole of ionic solid to its constituent ions in the gas phase.

The formation of ionic bonds releases a large amount of energy. The magnitude of lattice energy is a measure of an ionic compound's stability. Lattice energy depends on the magnitudes of the charge and on the distance between them {(Q1 x Q2)/d^2]

A monatomic cation is named by adding the word ion to the name of the element. Ex. K+ is potassium ion.

A monatomic anion is named by changing the ending of the element's name to -ide. Ex. Cl- is chloride; O2- is oxide.

Some metals can form cations of more than one possible charge, such as:
​
Fe2+ ferrous ion Fe (II)
Fe3+ ferric ion Fe (III)
Mn2+ manganese (II) ion
Mn3+ manganese (III) ion
Mn4+ manganese (IV) ion

Ionic compounds are electronically neutral. In order for ionic compounds to be electronically neutral, the sum of the charges on the cation and anion in each formula must be zero.
Ex. Aluminum oxide = Al2O3
Al3+
O2-
Sum of the charges = 2(+3) + 3(-2) = 0

If the compound contains a metal and a nonmetal, the compound is most likely ionic. If the metal cation has only one possible charge, including alkali metal cations and alkaline earth metal cations such as Ag+, Al3+, Cd2+, Zn2+, the metal is named first, then -ide is added to the root of the nonmetal name.

If the metal cation has more than one possible charge, name the metal first, then specify the charge of the metal cation with Roman numerals in parentheses. Add -ide to the root of the nonmetal name.

To name ionic compounds, name the cation and don't use the word ion. Use a Roman numeral if the cation can have more then one charge. To name an anion, don't use the word ion. Ex. FeCl3 = iron (III) chloride

​When compounds form between elements with similar properties, electrons are not transferred from one element to another but instead are shared to give each atom a noble gas configuration. This idea is known as the Lewis Theory of Bonding, after Gilbert Lewis. Lewis theory depicts bond formation in H2 as:

This type of arrangement, where two atoms share a pair of electrons, in known as covalent bonding, and the shared pair of electrons constitutes a covalent bond.

A molecule is a combination of at least two atoms in a specific arrangement held together by chemical forces (chemical bonds). A molecule may be an element or a compound.

Different samples of a given compound always contain the same elements in the same ratio. This is known as the law of definite proportions.

If two elements can form two or more different compounds, the law of multiple proportions tells us that the ratio o f masses of one element that combine  with a fixed mass of the other element can be expressed in small whole numbers.

In addition to carbon dioxide, carbon also combines with oxygen to form carbon monoxide. The mass ratio of oxygen to carbon in carbon dioxide is 2.66:1 and the ratio of oxygen to carbon in carbon monoxide is 1:33:1. The ratio of two such mass ratios can be expressed as small whole numbers.

mass ratio of O to C in carbon dioxide = 2.66
mass ratio of O to C in carbon monoxide = 1.33 
2.66/1.33 = 2:1

Diatomic molecules contain two atoms and may be either heteronuclear or homonuclear.  Polyatomic molecules contain more than two atoms.

​A chemical formula denotes the composition of the substance. A molecular formula shows the exact number of atoms of each element in a molecule. Some elements have two or more distinct forms known as allotropes. For example, oxygen (O2) and ozone (O3) are allotropes of oxygen. A structural formal shows not only the elemental composition but also the general arrangements.

Molecular substances can also be represented using empirical formulas, the whole number ratios of elements. While the molecular formulas tell us the actual number of atoms (the true formula) the empirical formula gives the simplest formula.

Molecular formula = N2H4
Empirical formula = NH2

The molecular and empirical formulas are often the same, however.

Water has the same molecular and empirical formula, H2O. Methane also has the same molecular and empirical formula, CH4. Also carbon dioxide CO2, and propane C3H8, and ethyne C2H2.

Ethane, however, has its molecular formula C2H6, while its empirical formula is CH3. Acetylene, C2H2, can be simplified to CH in is empirical formula.

Naming molecular compounds: Binary molecular compounds are substances that consist of just two different elements. These are named by naming the first element that appears in the formula. Then, naming the second element that appears in the formula, changing its ending to -ide. Ex. HCl is named hydrogen chloride. HI is named hydrogen iodide.

Greek prefixes are used to signify the number of atoms of each element present: mono-(1), di-(2), tri-(3), tetra-(4), penta-(5), hexa-(6), hepta-(7), octa-(8), nona-(9), deca-(10).

CO carbon monoxide
CO2 carbon dioxide
SO2 sulfur dioxide
SO3 sulfur trioxide
NO2 nitrogen dioxide
N2O3 dinitrogen pentoxide
N2O4 dinitrogen tetroxide
CS carbon monosulfide
CS2 carbon disulfide

The prefix mono- is usually omitted for the first element. 

The names of molecular compounds containing hydrogen do not usually have the same guidelines for naming and use common names, like ammonia for NH3, water for H2O, hydrogen sulfide for H2S.

One definition of acid is a substance that produces hydrogen ions (H+) when dissolved in water. HCl is an example of a binary compound that is an acid when dissolved in water. To name these types of acids, remove the -gen ending from hydrogen. Change the  -ide ending on the second element to -ic. Hydrogen chloride => hydrochloric acid.

A compound must contain at least one ionizable hydrogen atom to be an acid when dissolved:
HF = Hydrogen fluoride = Hydrofluoric acid
HCl = Hydrogen chloride = Hydrochloric acid
HBr = Hydrogen bromide = Hydrobromic acid
HI = Hydrogen iodide = Hydroiodic acid
HCN = Hydrogen cyanide = Hydrocyanic acid

Organic Compounds contain carbon and hydrogen, and sometimes in combination with other atoms. Hydrocarbons contain only carbon and hydrogen. The simplest hydrocarbons are called alkanes.

Some examples of alkanes are methane (CH4), ethane (C2H6), propane (C3H8), Butane (C4H10), Pentane (C5H12), Hexane (C6H14), Heptane (C7H16), Octane (C8H18), Nonane (C9H20), Decane (C10H22).

Many organic compounds contain groups of atoms known as functional groups, which often determine a molecule's reactivity.:
Alcohol -OH
Aldehyde -CHO
Carboxylic acid -COOH
Amine -NH2

Polyatomic ions consist of a combination of two or more atoms. Formulas are determined following the same rule as for ionic compounds containing only monatomic ions: ions must combine in a ratio that gives a neutral formula overall:
Ex. Ca3(PO4)2 = 3(+2)+2(-3) = 0
Ca2+
​PO43-

Oxoanions are polyatomic anions that contain one or more oxygen atoms and one atom (the central atom) of another element.
(1) the ion with one more O atom than the -ate ion is called the per...ate ion. Thus, ClO3- is the chlorate ion, so ClO4- is the perchlorate ion.
(2) the ion with one less O atom than the -ate ion is called the -ite ion. Thus ClO2- is the chlorite ion.
(3) the ion with two fewer O atoms than the -ate ion is called the hypo...ite ion. Thus, ClO- is the hypochlorite ion.

perchlorate ClO4-
chlorate ClO3-
chlorite ClO2-
hypochlorite ClO-

nitrate NO3-
nitrite NO2-
​
phosphate (PO4)3-
phosphite (PO3)3-

sulfate (SO4)2-
sulfite (SO3)2-

Oxoacids, when dissolved in water, produce hydrogen ions and oxoanions.

An acid based on an -ate ion is called ...ic acid = HClO3 is chloric acid
An acid based on an -ite ion is called ....ous acid = HClO2 is chlorous acid
Prefixes in oxoanion names are retained in naming the oxoacids.
HClO4 is perchloric acid.
​HClO is hypochlorous acid.

Oxoacids can be monoprotic (one ionizable hydrogen) or polyprotic (more than one ionizable hydrogen). The names of anions indicate the number of the remaining hydrogens. 
H3PO4 phosphoric acid
H2PO4- dihydrogen phosphate ion
(HPO4)2- hydrogen phosphate ion
(PO4)3- phosphate ion

A hydrate is a compound that has a specific number of water molecules within its solid structure. For example, in its normal state copper (II) sulfate has five water molecules associated with it. Full systematic name: copper (II) sulfate pentahydrate: Cu(SO)4 . 5H2O.
Other hydrates are: BaCl2 * 2H2O
LiCl * H2O
MgSO4 * 7H2O
Sr(NO3)2 * 4H2O

When the water molecules are driven off by heating, the resulting compound, Cu(SO)4 is sometimes called anhydrous copper (II) sulfate. Anhydrous means the compound no longer has water molecules associated with it.
Cu(SO)4 is white in color.
​Cu(SO)4 * 5H2O is blue in color.

Common Inorganic Compounds with common and systematic names:
H2O water (dihydrogen monoxide)
H2SO4 sulfuric acid (dihydrogen sulfate)
NH3 ammonia (trihydrogen nitride)
CO2 dry ice (solid carbon dioxide)
NaCl salt (sodium chloride)
SiO2 silica (silicon dioxide)
N2O nitrous oxide (dinitrogen monoxide)
CaCO3 marble, chalk, limestone (calcium carbonate)
NaHCO3 baking soda (sodium hydrogen carbonate)
MgSO4 * 7H2O Epsom salt (magnesium sulfate heptahydrate)
Mg(OH)2 Milk of Magnesia (magnesium hydroxide)

The molecular mass is the mass in atomic mass units (amu) of an individual molecule. To calculate molecular mass, multiply the atomic mass for each element in a molecule by the number of atoms of that element and then total the masses. Ex. molecular mass of H2O = 2(atomic mass of H) + atomic mass of O = 2(1.008 amu) + 16.00 amu = 18.02 amu.

Because the atomic masses on the periodic table are average atomic masses, the result of such a determination is an average molecular mass, sometimes referred to as the molecular weight.

Although an ionic compound does not have a molecular mass, we can use its empirical formula to determine its formula mass (the mass of a formula unit) or the formula weight.
To calculate formula mass, multiply the atomic mass for each element in a formula unit by the number of atoms of that element and then total the masses.

Percent Composition: A list of the percent by mass of each element in a compound is known as the compound's percent composition by mass. 
Percent mass of an element = (n x atomic mass of element)/(molecular or formula mass of compound) x 100%
where n is the number of atoms of the element in a molecule or formula unit of the compound.

For a molecule of H2O2=
% H = (2 x 1.008 amu H)/(34.02 amu H2O2) x 100% = 5.926%
%O = (2 x 16.00 amu O)/(34.02 amu H2O2) x 100% = 94.06

The empirical formula of hydrogen peroxide (HO) could also have been used for the calculation. In this case, use the empirical formula mass, the mass in amu of one empirical formula, in place of the molecular formula. The empirical formula mass of H2O2 (the mass of HO) is 17.01 amu. 
%H = (1.008 amu H)/(17.01 amu HO) x 100% = 5.926 %
%O = (16.00 amu O)/(17.01) x 100 = 94.06%

The molar mass (M) of a substance is the mass in grams of one mole of the substance. The molar mass of an element is numerically equal to its atomic mass. 
Ex. 1 mol C = 12.01 g
One carbon atom = 12.01 amu
The molar mass of a compound is the sum of molar masses of the elements it contains:
1 mol H2O = 2 x 1.008 g + 16.00 g = 18.02 g
1 mol NaCl = 22.99 + 35.45g = 58.44 g

​Calculations of molar mass must specify which form of element is to be calculated, such as oxygen as a diatomic molecule (O2) or atomic oxygen (O).

Using the concepts of the mole and molar mass, the experimentally determined percent composition can be used to determine the empirical and/or molecular formula. The empirical formula gives only the ratio of atoms in a molecule, so there may be multiple compounds with the same empirical formula. If the approximate molar mass of the compound is known, the molecular formula can be determined by dividing the molar mass by the empirical formula mass. Then the empirical formula is multiplied by this number to obtain the molecular formula.