Explain the trend in the first ionisation energies of the group 1 elements

The general trend is a decrease in first ionisation energy, which is because as you descend the group the number of energy levels surrounding the positively charged nucleus increases. This means that the outermost electrons are held further away from the positive nucleus, and experience more shielding from the inner shells. This therefore means that the electrostatic attraction between the positive nucleus and negative outermost electron is weakened, so less energy is required for this attraction to be overcome and the electron to be liberated.