Module 2. Electrons, bonding and structure. 5

The noble gasses
An atom is the smallest particle of an element that retains its chemical properties.
Only six elements exist naturally as single unbounded atoms:

• helium, He
• neon, Ne
• argon, Ar
• krypton, Kr
• xenon, Xe
• radon, Rn 

In the atoms of a noble gas:

• all elements are paired with opposite signs
• the outer shell contains 2 electrons (for He) or 8 electrons (all other noble gasses) 

An outer shell of eight electrons is extremely stable. It is this stability that makes the noble gasses so unreactive. The 8 electrons in the outer shell of a noble gas are made up of 2 in the s-orbital and 2 each in the 3p-orbitals. 

Chemical bonds are classified into 3 main types: ionic; convalent; and metallic. A compound is a substance formed from 2 or more chemically bonded elements in a fixed ratio, usually shown by a chemical formula. 

Ionic bonding

• In general, ionic bonding occurs in compounds consisting of a metal and a non-metal
• If we imagine a bond forming between atoms, electrons are transferred from the metal atom to the non-metal atom to form oppositely charged ions that attract e.g. NaCl, Mgo, Fe2O3

Convalent bonding

• Convalent bonding occurs in compound consisting of 2 non-metals
• If we imagine a bond forming between atoms, electrons are shared between the atoms. e.g. O2, H2, H2O, and C (diamond and graphite)

Metallic bonding

• Metallic bonding occurs in metals. Electrons are shared between all the atoms. e.g. all metals - iron, zinc, aluminimum; alloys - brass (copper and zinc), bronze (copper and tin)