2.1 Elements, Compounds and Mixtures

An element is a substance that cannot be further divided into simpler substances by chemical methods. All the atoms of an element contain the same number of protons.

Key characteristics of elements:

• Made up of just one type of atom
• Cannot be broken down into anything simpler by chemical reactions
• Each element has its own distinctive properties
• The atoms of any one element all have the same atomic number

There are now 118 known elements, but most of the known mass of the universe consists of just two elements:

• Hydrogen: 92% of the universe's mass
• Helium: 7% of the universe's mass
• All other elements: only 1% of the total

There are 94 elements found naturally on Earth, but just eight account for more than 98% of the mass of the Earth's crust.

Important fact: Two elements, silicon and oxygen, which are bound together in silicate rocks, make up almost three-quarters of the Earth's crust.

Only certain elements are able to form the complex compounds found in living things. For example, the human body contains:

• 65% oxygen
• 18% carbon
• 10% hydrogen
• 3% nitrogen
• 2% calcium
• 2% of other elements

Elements are essentially divided into two main categories:

1. Metals: A class of chemical elements (and alloys) that all have a characteristic shiny appearance and are good conductors of heat and electricity.

2. Non-metals: A class of chemical elements that are typically poor conductors of heat and electricity.

Many elements do not exist as separate individual atoms. Instead, they form diatomic molecules - molecules containing two atoms.

Examples of diatomic elements include:

• Hydrogen: $\mathrm{H}_2$
• Oxygen: $\mathrm{O}_2$

The only elements that are made up of individual atoms moving almost independently of each other are the noble gases (Group VIII of the Periodic Table). The noble gases are the elements whose electron arrangements are most stable, so their atoms do not combine with each other.