State and explain the general trend in first ionisation energy across Period 3

As you go across period 3 there is a general increase in the trend of the first ionisation energy. This is because the atoms have a greater charge on the nucleus (this means they have more protons). However, they all have the same amount of shielding - this reflects the decrease in attraction between the protons in the nucleus and the electrons being removed. This is because the electrons being removed are from the same shell. The electron being removed from each atom is the outer electron, there is a stronger attraction between the nucleus and this electron, hence the increase in first ionisation energy.