Electronegativity and electron pairs.

What is a chemical bond?

Covalent bonds.

Polarised Bonds

Ionic Bonds

Spectrum of Bonding

Predicting Types of Bond

 

Most compounds form bonds with pairs of electrons.

·        The position of the pair can affect its chemistry.

Definition: The electronegativity of an element is the relative ability of each of its atoms to attract the electron of a covalent bond towards itself.

·        A very electronegative atom will attract electrons away from the influence of a less electronegative atom.

What is a chemical bond?

  • Electrons are negatively charged, do they repel, but they can pair up.
  • If a pair of electrons exists between the nuclei of 2 atoms
  • Nuclei repel each other – but attract the electrons, holding the whole thing together

Covalent bonds

  • Two atoms A and B are similar in electronegativity.
  • Each provides 1 electron to make the bond
  • Attraction will be the same due to similar electronegativity
So electrons stay roughly halfway between them.

Polarised bonds

  • B is slightly more electronegative than A
  • B can draw the electrons towards itself
  • B will have more than its fair share of electrons
      • Will gain electron density and so be slightly negatively charged
      • A will be slightly positive
  • Represent this change in charge by using d+ and d-

Ionic Bonds

  • B is much more electronegative than A
  • The electron pair is totally associated with B
B has effectively ‘removed’ an electron from A, forming a negative ion

Bonds are very rarely 100% ionic or 100% covalent. Most contain character of one or the other, but remain sufficiently biased to enable classification as ionic or covalent.

Spectrum of Bonding

Roughly

  • A difference of >2.0 = Ionic Character
  • A difference of<1.0 = Covalent Character

Predicting Types of Bond

We can use these rules to help to predict the type of bond.

·        Only 2 kinds of elements

1.                  Metal

2.                  Non-metal

·        So there should only be 3 types of bonding.

Metal            « Non-Metal              Large difference in electronegativity

Metal            « Metal                                    Small difference in electronegativity

Non-Metal  « Non-Metal                        Small difference in electronegativity

So we can use these slightly simplified rules, and match them up with the types of bonding. Generally:

Metal            « Non-Metal  = Ionic Bonding

Metal            « Metal    =  Metallic Bonding

Non-Metal  « Non-Metal     = Covalent bonding