C2.1 Structure and Bonding

Chemical bonding involves either transferring or sharing electrons in the highest occupied energy levels of atoms in order to achieve the electronic structure of a noble gas. 

Noble gas structure: ions have the stable electronic structure of a noble gas from group 0, so have 8 electrons in their outer shell (except for helium which has 2) 

An ion forms when an atom transfers (either loses or gains) one or more electrons 

Positive ions: Metal and hydrogen atoms lose electrons to form positively charged ions. For example, sodium (2,8,1) loses its outer electron to form a sodium ion Na+ (2,8)

Negative ions: Non-metal atoms gain electrons to form negative ions. For example, a chlorine atom (2,8,7) gains one outer electron to form a chloride ion Cl- (2,8,8)

Patterns in the Periodic Table: The number of charges on a positive ion is the same as the group number of the element, and the number of charges on a negative ion is 8 minus the group number of the element

Group 1 = 1, Group 2 = 2+, Group 3 = 3+, Group 5 = 3-, Group 6 = 2-, Group 7 = -

Ionic compounds contain positive and negative ions, formed when atoms transfer electrons. Group 1 elements react with group 7 elements to produce ionic compunds.

Group 1 elements (alkali metals)

• Metals
• React with non-metals to form ionic compounds
• Produce ions with a 1+ charge
• e.g. lithium, sodium, potassium

Group 7 elements (halogens)

• Non-metals
• React with metal elements to form ionic compounds
• Produce ions with a 1- charge
• e.g. chlorine, bromine, iodine

Forming ionic compounds: e.g. sodium reacts with chlorine to form sodium chloride. The electron from the highest occupied energy level of Na is transferred to the highest occupied energy level of Cl, forming Na+ and Cl-

An ionic compound is a giant structure of ions:

• Ions held in a regular arrangement: lattice
• Ionic bonds
  • Strong electrostatic forces of attraction between oppositely charged ions
  • Act in all directions of the lattice and hold structure together

Covalent bonds form when atoms share electrons:

• Forms between two non-metal atoms
• Bonds between atoms are strong
• Shared electrons form full outer shells in both atoms
• Simple molecule e.g. H2, Cl2, O2, HCl, H2O, NH3 and CH4

Shared electron from one atom is a dot and the other electron is shown as a cross

Macromolecules have giant covalent structures and can be elements (like diamond) or compounds (like silica/silicon dioxide). Covalent bonds are usually represented by straight lines.

Diamond

• Single molecule containing carbon atoms
• Each carbon atom is covalently bonded to four other carbon atoms
• Has a giant covalent structure

Silica

• Each silicon atom is covalently bonded to four oxygen atoms
• Each oxygen atom is covalently bonded to two silicon atoms