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What you are referring to is the energy of ionization. This is the energy required to remove an outer (valence) electron from an atom to produce a positively charged cation. The lower the energy of ionization, the easier the electron is to remove and the easier the ion forms. Energy of ionization tends to parallel opposite the atomic radius in terms of trends across the periodic table. When moving down a group of elements (top to bottom), the atomic radius increases. This means that the distance between the nucleus and the outermost electron increases. As this distance increases, the less electrostatic interaction there is between the positively charged nucleus and the negatively charged electron. And the less interaction there is between the two, the easier the electron is to remove from the atom. So that is why the further away the outer electron is from the nucleus (larger atomic radius), the easier it is to remove to form an ion (smaller energy of ionization).
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