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3.1.1Chemical Reactions

Chemical Symbols and Formulae — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 2 of 20

📚 OCR Gateway GCSE Combined Science A J250 - Chemistry Hub / Topical Revision
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Part 1 of 5Symbols 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.

Part 2 of 5Formulae 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.

Part 3 of 5Reading 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.

Part 4 of 5Common simple molecules

SubstanceFormulaAtoms in one molecule
HydrogenH22 hydrogen
WaterH2O2 hydrogen, 1 oxygen
Carbon dioxideCO21 carbon, 2 oxygen
MethaneCH41 carbon, 4 hydrogen
AmmoniaNH31 nitrogen, 3 hydrogen

Part 5 of 5Formulae 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.

Key terms

TermMeaning
ElementA substance made of atoms that all have the same number of protons.
CompoundA pure substance of two or more elements chemically combined in a fixed ratio.
Diatomic moleculeA molecule made of two atoms, such as O2.
SubscriptThe small number after a symbol showing how many atoms of that element are present.
Empirical formulaThe simplest whole-number ratio of atoms or ions in a compound.

Avoid these mistakes

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.

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