Atoms, elements and compounds

What does a chemical symbol represent?

One atom of an element.

What do chemical symbols have?

One or two letter.

How are two-letter chemical symbols represented?

The first letter is upper case

Examples of one-letter chemical symbols

H, O, C, K.

Examples of 2 letter chemical symbols

Mg, Al, He

How are names and symbols of elements in the periodic table displayed?

In the periodic table. Metals on the left and non-metals on the right.

What is an atom?

The smallest part of an element that can exist.

What is an element?

A substance made of one type of atom only.

What is the formula for most elements?

Their chemical symbol.

How do some non-metals exist as?

2 atoms joined together.

What elements exist as two-atom molecules?

I2, Br2, Cl2, O2, N2 and H2.

What is a molecule?

A collection of two or more atoms held together by chemical bonds

What is a compound?

A substance formed by the chemical union of two or more elements.

What does the formula of a compound show?

The symbols for the elements in the compound and the number of atoms of each element in a unit of the compound.

Examples of compounds

MgO, MgBr2, CO2, H2O.

What does a word equation show?

The names of the substances involved in a reaction.

Where are reactants and products in a word equation?

• Reactants on left of arrow.

• Products on right of arrow.

What does the arrow in a word equation mean?

Reacts to make.

What does a balanced equation have?

The same number of atoms of each element on both sides of the arrow.

How to balance equations?

• Write numbers to the left of the formulae where necessary.

• Do not change any formulae.

What is a formula?

A combination of symbols that indicates the chemical composition of a substance.