Examples:

H represents one atom of hydrogen

C represents one atom of carbon

Mg represents one atom of magnesium

We write chemical formulas for compounds. A compound will be either ionic or molecular. If molecular, the atoms are held together by sharing electrons. Atoms held together by sharing electrons are said to be covalently bonded. A group of covalently bonded atoms is called a molecule.

For molecular compounds, we can write a

Sometimes it is necessary to have more than one atom of an element, or more than one molecule of a compound. In these cases, we can write a number in front of the element symbol or chemical formula to indicate the number of atoms or molecules involved.

2Al represents 2 atoms of aluminum

4 Mg represents 4 atoms of magnesium

5H

3C

The numbers that appear in front of the symbols and formulas above are called

Many elements (for example, aluminum and magnesium, as shown above) are

There are a few elements, however, that exist as molecules. There are 7 elements that exist as diatomic molecules. That is, the fundamental particles that make up a sample of these elements are pairs of atoms, rather than individual atoms.

Oxygen exists as O

Nitrogen exists as N

Hydrogen exists as H

Fluorine exists as F

Chlorine exists as Cl

Bromine exists as Br

Iodine exists as I

There are also a few elements that exist as molecules that have more than 2 atoms:

The form of oxygen called ozone exists as O

The form of phosphorous called white phosphorous exists as P

Sulfur is normally encountered as S

Note that 2N and N

You can count the number of atoms of each element in a chemical formula by multiplying the coefficient by the subscript, recognizing that those coefficients and subscripts that are not shown are equal to 1. For example, if we write 2NH

For N we have 2 x 1 = 2

For H we have 2 x 3 = 6

Thus, in 2 molecules of NH

The compounds considered so far have all been molecular compounds. Ionic compounds do not exist as molecules. Rather than having atoms being joined together by sharing electrons, one or more atoms lose electrons (thereby becoming positive ions) and these electrons are then gained by one or more other atoms (thereby becoming negative ions). Since opposite charges attract one another, the positive and negative ions are held together to form (at room temperature, and even well above) a solid crystal.

Ionic compounds always form in such a way that the total positive charge is equal to the total negative charge. In this way, the ionic crystal as a whole is electrically neutral. Because there are no discrete molecules in such compounds, all we can do in expressing the chemical formula of these substances is to express the ratio of positive ions to negative ions. Consider ordinary table salt as an example. Chemically, we know it as sodium chloride, NaCl. Sodium forms the ion Na

We take the formula unit of an ionic compound to represent the smallest possible amount of that compound, in the same way that we take a molecule to represent the smallest possible amount of a molecular compound. Thus the smallest quantity of sodium chloride that we can have -- and still recognize it as sodium chloride -- is one sodium ion and one chloride ion.

Often, it is no more difficult to count the atoms in ionic formulas than it is in molecular formulas. Consider the following examples:

MgCl

For Mg we have 1 x 1 = 1 (1 Mg

For Cl we have 1 x 2 = 2 (2 Cl

2Al

For Al we have 2 x 2 = 4 (4 Al

For O we have 2 x 3 = 6 (6 O

It becomes slightly more difficult to count the atoms in ionic formulas when they contain polyatomic ions that occur more than once in the formula. In these cases, the subscrips within the polyatomic ion must be multiplied by the subscript that appears outside the parentheses. If a coefficient is present, it must also be multiplied, meaning sometimes there will be three numbers to multiply together to calculate the number of atoms of a particular element. Consider the following examples:

(NH

For N we have 1 x 2 = 2 (multiplying subscript of N by subscript of group)

For H we have 4 x 2 = 8 (multiplying subscript of H by subscript of group)

For C we have 1 (coefficient of formula, subscript of C and subscript of group are all 1)

For O we have 3 (subscript of O is 3, coefficient of formula and subscript of group are 1)

If you have trouble understanding the above, it may help to recall that (NH

A still more complicated case is when an element appears in more than one place in the formula. For example, lets count the atoms in 5(NH

For N we have 5 * 1 * 2 = 10

(multiplying coefficeint, subscript within group, and subscript outside of group)

For H we have 5 * 4 * 2 + 5 * 1 * 1 = 40 + 5 = 45

(for each group, multiplying coefficient, subscript in group and subscript outside of group)

For P we have 5 * 1 * 1 = 5

(Multiplying coefficeint, subscript within group, and subscript outside group)

For O we have 5 * 4 * 1 = 20

(Multiplying coefficient, subscript within group, and subscript outside group)

If you have trouble understanding the above, it may help to recall that 5(NH