C2.1 Structure and Bonding

C2.1.1 Forming Ions

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 = -

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C2.1.1 Ionic Compounds

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-

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C2.1.1 Giant Ionic Structures

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

(http://www.bbc.co.uk/staticarchive/79adeab969839ce1f6e868e80d6cccf378fd0820.gif)

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C2.1.1 Covalent Bonds

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

(http://www.bbc.co.uk/staticarchive/bdd7c0044c8cb249325cabd9eb190b204dcad6c6.gif)
Shared electron from one atom is a dot and the other electron is shown as a cross

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C2.1.1 Covalent Bonds in Macromolecules

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
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