ionisation energy

X_(g) $\rightarrow$ X⁺ _(g) + e^- - first ionisation energy- the energy needed to remove one mole of electrons from one mole of atoms in their gaseous state

X⁺_(g) $\rightarrow$ X²⁺ _(g) + e^- - second ionisation energy

factors that influence ionisation energy

• the negative electrons are attracted to the positive nucleus- this attraction keeps the electrons held in their shells

• when an ion is formed, energy is supplied to remove an electron(s)- the electrons in the outer shell are lost first, they experience the smallest amount of nuclear attraction so require the smallest ionisation energy

• amount of protons- more protons = more attraction between the nucleus and the outer electrons

ionization energy is largely influenced by atomic radius

• atomic radius is influenced by 2 other factors:

• nuclear charge

• electron sheilding

factors influencing ionisation energy: atomic radii

• atomic radii decrease across period, down a group atomic radius increases

• across a period: number of protons increases, down a group shells increase

• as atomic radii increases the force of attraction between the positive nucleus and outer electrons decrease

first ionisation energy going down a group

• decreases as you go down the group because:

atomic radius increases- outer electron is further from the nucleus

more shielding between the nucleus and the other electrons

ionisation energy as you go across a period

• atomic radius decreases

• outer electrons are more attracted to the nucleus

• so ionisation energy increases across a period

• shielding- similar

• when electrons repel each other, it takes less energy to remove one of the electrons than if the electrons were in separate orbitals