R-AECHE-16

To determine whether something is polar or non-polar, you have to take into account the following:

• relative electronegativity of composite atoms
• shape of molecule

Looking at symmetry of charges, not the symmetry of molecules.

Dipole is cancelled.

Dipole refers to the slightly negative/positive areas of a molecule

Periodic Trends:

• Electronegativity increases across a period because:

1. The atomic radius decreases.
2. The charge of the nucleus increases without significant extra shielding. New electrons do not contribute much to shielding because they are added to the same principal energy level across the period.

• Electronegativity decreases down a period because:

1. The atomic radius increases.
2. Although the charge on the nucleus increases, shielding also increases significantly. This is because electrons added down the group fill new principle electron levels.

Ionisation Energy:

• Upon applying energy, solids will turn into liquids, which will turn into gas.
• This is called vaporisation.
• If energy is continuously applied, a gas becomes an ion.
• However, there are two processes for the two types of ion.
• For the cation process, energy is absorbed.
• For the anion process, energy is released.
• Ionisation energy is:
  • Energy required to remove one mole of electrons from one mole of gaseous atoms.

e.g. $X_{g} \rightarrow X^+_{g}+\bar e^-$

The first ionisation energy of an element is the energy required to remove one mole of electrons from one mole of gaseous atoms.

The first ionisation energy is therefore a measure of the strength of the attraction between the outermost electron and the nucleus.

Effective nuclear charge = Nuclear charge - shielding.

Going across a period, nuclear charge increases.

When atomic radius decreases, electrons become closer and closer to the nucleus. Thus, it would be difficult to remove electrons from an atom.

There is a general increase in the first ionisation energies across periods.

Across period 3, the proton number increases but the amount of shielding does not change significantly. The effective nuclear charge increases.

The greater attraction between the nucleus and the outermost electrons mean

Outer electrons are closer to the nucleus, thus they will experience stronger nuclear attraction, and thus will require more energy to remove from an atom.

Always mention more/less energy required!!!!

Bonding

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