Ionisation energy

Evidence for the model of an atom can be found from ionisation energy. This is the energy required to remove one mole of electrons from  one mole of atoms in their gaseous ground state (kJ mol-1). (Essentially measures how hard it is to remove an electron from an atom/ion).

Atoms and ions will have different first ionisation energies because each will have a different nuclear attraction to their outershell electrons. They do so because of atomic radius, nuclear charge and electron shielding. 

Atomic radius (a.r.) is half the distance between the nuclei of two of the same atoms joined together by a single covalent bond. As a.r. gets smaller then nuclear attraction increases.

Nuclear charge (n.c.) is essentially the number of protons in the nucleus. As n.c. gets larger then nuclear attraction increases.

Electron shielding (e.s.) is the repulsion between inner shells of electrons and the outershell.The fewer the number of shells then the larger the nuclear attraction.

A higher a.r. and e.s. will offset a higher n.c.

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