Exercise: Predicting shapes of molecules Draw Lewis structure of the molecule. Identify electron clouds around the central atom, and how many are bonded. Use the Valence shell electron pair repulsion rule (and your table) to work out the shape (remember: electrons repel) =D use to models too Try: CH4 PCl3 COCl2 CS2 H2O

and Electronegativity Polarity and Electronegativity

Electronegativity When two non-metal atoms share an electron pair in a covalent bond, the pair of bonding electrons may or may not be shared equally. This depends on the electronegativity of the two atoms involved. Electronegativity describes how strongly the atoms of an element hold onto or attract electrons. Its magnitude increases from left to right across the periodic table (from groups 1 to 17, excluding the inert gases group 18) and going up any group.

Electronegativity Trend

Polarity The polarity of a covalent bond can be determined by looking at the electronegativities of the atoms sharing the electrons and determining if the sharing will occur evenly or unevenly.

If two atoms of equal electronegativity bond together Both atoms have the same tendency to attract the bonding pair of electrons, and so it will be found on average half way between the two atoms. To get a bond like this, A and B would usually have to be the same atom. You will find this sort of bond in, for example, H2 or Cl2 molecules.

What happens if B is slightly more electronegative than A? B will attract the electron pair rather more than A does. That means that the B end of the bond has more than its fair share of electron density and so becomes slightly negative. At the same time, the A end (rather short of electrons) becomes slightly positive. In the diagram, "" (read as "delta") means "slightly" - so δ+ means "slightly positive".

When two atoms of the same (or very similar) electronegativity combine, the bonding electrons are shared equally and we say the bond formed is a non-polar covalent bond When two atoms of different electronegativity are joined by a covalent bond then the electron pair will spend more time nearer to the more electronegative atom (the one with the greater electron attracting power). This results in the bond being a polar covalent bond.

Once again... POLAR COVALENT BONDS = uneven sharing NON-POLAR COVALENT = even sharing This is a pretty important point

The greater the difference in electronegativity between the two atoms, the more polar the covalent bond.

Oxygen is more electronegative than hydrogen. Electrons are negative, when they spend more time around a certain atom, it causes it to have a slightly negative charge. The absence of electrons gives a slight positive charge.

Dipoles This separation in charge is called a charge dipole and results in the molecule having a dipole moment. The more polar the bond is, the larger the dipole is. This dipole can become so large that we say the electrons are completely given away. δ+ δ- + - NON-POLAR POLAR IONIC

Dipoles Each bond in a molecule has a dipole if it is polar. Non-polar bonds have no dipole as there is even sharing. (but can have a temporary dipole)

Summary No electronegativity difference between two atoms leads to a pure non-polar covalent bond. A small electronegativity difference leads to a polar covalent bond. A large electronegativity difference leads to an ionic bond.