Periodicities of Some Atomic Properties

This section summarizes the variation, across the periods and down the groups of the Periodic Table, of (i) the ionization energies, (ii) the electron attachment energies (electron affinities), (iii) the atomic sizes and (iv) the electronegativity coefficients of the elements. 

The first ionization energy of an atom is the minimum energy required to convert one mole of the gaseous atom (in its ground - lowest energy - electronic state) into one mole of its gaseous unipositive ion  

Take care to note the definition of electron affinity given in the text. 

In some older texts the term is defined as the energy released upon attachment of an electron to a neutral atom. Using this definition the values given in this text would have the opposite sign. 

The electron attachment energy or electron affinity is defined as the change in internal energy (i.e. A U) that occurs when one mole of gaseous atoms of an element are converted by electron attachment to give one mole of gaseous negative ions  

---