Chemical Symbols and Formulae

Section 1 of 7Symbols of elements

One symbol per element

• One symbol per element Each of the 118 elements has its own symbol, shown on the periodic table, such as C for carbon and Na for sodium.
• The first letter is always a capital and any second letter is lower case, so sodium is Na, never NA.

Reading the periodic table box

• Reading the periodic table box The symbol sits with two numbers.
  • The larger is the mass number and the smaller is the atomic number, so carbon is shown as 12 and 6.

Section 2 of 7Formulae of elements

Element formulae

Metals

• Metals A metal is written simply as its symbol, for example Mg or Fe, because its giant structure is represented by the simplest ratio.

Noble gases

• Noble gases Group 0 elements exist as separate atoms, so the formula is the same as the symbol, for example He.

Diatomic elements

• Diatomic elements Seven non-metals exist as molecules of two atoms: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
• These must always be written with the subscript 2 in equations.

Other non-metals

• Other non-metals Carbon, sulfur and phosphorus are written as C, S and P in simple equations.

Section 3 of 7Reading a molecular formula

1

Look at each capital letter. Each one starts a new element symbol.

2

A subscript number after a symbol tells you how many atoms of that element are present. No number means one atom.

3

A number applies only to the symbol directly in front of it, so H₂SO₄ has 2 hydrogen, 1 sulfur and 4 oxygen atoms.

4

Add the numbers for the total number of atoms in the molecule: H₂SO₄ has 7 atoms.

Section 4 of 7Common simple molecules

Substance	Formula	Atoms in one molecule
Hydrogen	H2	2 hydrogen
Water	H2O	2 hydrogen, 1 oxygen
Carbon dioxide	CO2	1 carbon, 2 oxygen
Methane	CH4	1 carbon, 4 hydrogen
Ammonia	NH3	1 nitrogen, 3 hydrogen

Section 5 of 7Formulae of ionic compounds

Metal first

• Metal first Ionic compounds contain a metal and a non-metal, and the metal is written first, as in NaCl.

Empirical formula

• Empirical formula An ionic compound is a giant lattice of millions of ions, so its formula shows only the simplest whole-number ratio of ions.

Exceptions

• Exceptions A few ionic compounds contain no metal.
  • Ammonium salts such as NH₄Cl contain the positive ammonium ion.

Properties differ

• Properties differ A compound has very different properties from the elements it is made from.
  • Sodium chloride is a harmless crystal, unlike reactive sodium or poisonous chlorine.

Section 6 of 7Key terms

Term	Meaning
Element	A substance made of atoms that all have the same number of protons.
Compound	A pure substance of two or more elements chemically combined in a fixed ratio.
Diatomic molecule	A molecule made of two atoms, such as O2.
Subscript	The small number after a symbol showing how many atoms of that element are present.
Empirical formula	The simplest whole-number ratio of atoms or ions in a compound.

Section 7 of 7Exam technique

1

Writing oxygen as O in an equation. It must be O₂, as must the other six diatomic elements.

2

Writing symbols with the wrong case, such as CO for cobalt (Co) or NA for sodium.

3

Changing a subscript to balance an equation. Subscripts are fixed by the compound's identity.

4

Thinking H₂O has the atoms joined in a straight line. The two hydrogen atoms are each bonded to the oxygen.