Some elements are found naturally as pairs of atoms rather than single atoms when they are in their elemental form. These are called diatomic elements, and there are seven common ones students must memorize: H2, N2, F2, O2, I2, Cl2, and Br2. The mnemonic Have No Fear Of Ice Cold Beer helps you remember them in order.
Knowing these elements matters because chemical formulas must be written correctly before equations can be balanced.
Understanding Chemistry: Seven diatomic elements
Atoms join in pairs when sharing electrons gives them a more stable outer electron arrangement. A covalent bond holds the two atoms together because both positively charged nuclei attract the shared electrons. The bonds are not all equally strong.
Nitrogen has a very strong triple bond, so nitrogen gas usually reacts slowly at room temperature. This is one reason most of the air does not readily burn or support burning. Oxygen has a double bond and reacts more easily than nitrogen in many situations.
The halogens have single bonds. Their molecules can split apart during reactions, allowing the atoms to form new bonds with other substances.
The word elemental is important in formula writing. It means the substance contains only one kind of element. Once an atom is part of a compound, its formula depends on the compound, not on the form of the pure element.
For example, chlorine occurs as Cl2 in a sample of chlorine gas. In sodium chloride, each formula unit contains one sodium atom joined with one chlorine atom, so chlorine is written Cl. Water contains hydrogen and oxygen, but it is not written as a pair of hydrogen molecules joined to an oxygen molecule.
The small numbers in a formula describe the number of atoms in one particle of that substance. They cannot be changed while balancing an equation.
These substances appear in familiar places, though some need careful handling. Oxygen is used in breathing equipment, welding, and hospital care. Nitrogen is used to make fertilizers and to provide an unreactive atmosphere for some food packages.
Hydrogen can be used as a fuel, although it ignites very easily. Chlorine compounds help disinfect swimming pools and drinking water, but chlorine gas itself is toxic. Bromine is a reddish brown liquid at room temperature and produces harmful fumes.
Iodine is a dark solid that can form a purple vapor when warmed. Their different states show that molecules made from two atoms can still have very different physical properties.
When balancing a chemical equation, first write the correct formula for every substance. Then count each type of atom on both sides. A coefficient placed in front of a formula changes the number of whole particles.
It does not alter the atoms inside one particle. For hydrogen burning in oxygen, the balanced statement is two hydrogen molecules plus one oxygen molecule produce two water molecules. This gives four hydrogen atoms and two oxygen atoms on each side.
A common error is to write a single H or O for the pure gases, which creates the wrong atom count from the start. It helps to separate two tasks in your mind.
Write correct formulas first. Balance the atom totals second.
Key Facts
- The seven diatomic elements are H2, N2, O2, F2, Cl2, Br2, and I2.
- Mnemonic: Have No Fear Of Ice Cold Beer = H, N, F, O, I, Cl, Br.
- A diatomic molecule contains two atoms bonded together, such as O2 or N2.
- Use diatomic formulas only when the element is alone in its elemental form.
- Combustion of hydrogen: 2H2 + O2 -> 2H2O.
- Chlorine is Cl2, not C2, because the symbol for chlorine is Cl.
Vocabulary
- Diatomic element
- An element that naturally exists as molecules made of two atoms when it is in its pure elemental form.
- Elemental form
- The form of a substance when it is not chemically combined with any other element.
- Molecule
- A group of two or more atoms held together by chemical bonds.
- Chemical formula
- A set of element symbols and subscripts that shows what atoms are present in a substance and how many of each there are.
- Subscript
- A small number written after a chemical symbol that tells how many atoms of that element are in one molecule or formula unit.
Common Mistakes to Avoid
- Writing H instead of H2 for elemental hydrogen is wrong because hydrogen is diatomic when it is not bonded to another element.
- Writing O instead of O2 in a reaction is wrong when oxygen gas is a reactant or product because elemental oxygen exists as O2.
- Thinking the C in the mnemonic means carbon is wrong because it stands for chlorine, whose chemical symbol is Cl.
- Changing diatomic elements inside compounds is wrong because the diatomic rule applies only to pure elements, not to atoms already bonded in compounds such as H2O or HCl.
Practice Questions
- 1 Write the correct formulas for the elemental forms of hydrogen, nitrogen, oxygen, chlorine, and bromine.
- 2 Balance the equation for hydrogen reacting with oxygen to form water: H2 + O2 -> H2O.
- 3 A student writes chlorine gas as C2 because the mnemonic includes Ice Cold. Explain the mistake and write the correct formula for chlorine gas.