State and explain the general trend in the first ionisation energy across a period.

First, let's check we understand the concept of the first ionisation energy. The term is defined as 'the energy required to remove a mole of electrons from a mole of gaseous atoms'. That is, for each atom in our mole's worth of gas, we want to remove a single electron to form a mole of ions, each with a single positive charge. The energy required to remove this electron from each will be greater for the elements in which there is a greater attraction between the nucleus and the outermost electron - since there is an attraction between the nucleus and electron in the first place, we need to put in energy in order to separate them.

Across a period, as we go from left to right, there is an increase in nuclear charge, as each element has one more proton than the last. Although the other electrons surrounding the nucleus shield the outer electrons from the positively charged nucleus, the shielding effect does not increase very quickly, since we are adding electrons to the same shell as we move along the period. Therefore, the attraction of the outermost electron by the nucleus will increase faster than the shielding effect. This means that removing this electron will require a greater input of energy, and so overall, we expect a general increase in the first ionisation energy across a period.