The closer an electron is to a nucleus, the more energy it takes to remove that electron from the atom. This trend is easy to understand: small atoms have few electrons that are close to the nucleus. The ionization energy is the energy required to remove the least tightly bound electron from an atom, producing a positive ion and a free electron:Ĭompare this figure to the earlier figure of Atomic Radii and note that in general, small size means large ionization energy. Next, we look at two energetic properties of atoms, the ionization energy ( IE) and the electron affinity ( EA). These orbitals are slowly shrinking in size because the nuclear charge is increasing as we go across a row, and increasing nuclear charge means increasing the force attracting electrons to the nucleus, making the orbitals contract. These atoms' sizes are governed by the size of their single highest-energy electron's orbital, which is 1s for H but 6s for Cs.Īs we go across a row (look at Li through Ne, for example), the size decreases, but rather slowly: we are adding electrons into orbitals with (for this row) the same principal quantum number. This clearly explains the trend in increasing size as we go down any one column: H is smaller than Li, etc., ending the first column with the biggest atom for which we have reliable data, Cs. The general trend is easy to spot and to understand: heavier atoms (those with large atomic numbers) have more electrons, and increasing the number of electrons means placing electrons into orbitals with ever-increasing principal quantum numberorbitals with ever-increasing size.
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