Question

For many elements, the addition of one electron to the neutral energy is exothermic, and the potential energy is given in negative value. can you explain that? addition of an electron to an atom with a filled or half filled subshell may have a positive value of EA. why?

adding electrons to a noble gad always requires energy. why?

would you expect Na or Mg have the greater frist ionizaton energy? which should have second ionization energy ?

Answer 1

When you add an electron to a neutral atom it becomes negatively chraged anion.

A + e^- ------> A^-(g)

In most of the cases where the atom has less than octet electrons , the atom readily accepts the electron into its orbitals and the attraction between the extra electron and the nucleus overcomes the repulsons between the existing and incoming electrons. Thus energy is liberated duting the process.

This energy liberated is known as the electron affinity of the element. It is defined as the amount of enregy liberated when an eectron is added to a neutral isolated gaseous atom.

ii) When the configuration of atom is exactly half filled subshell or completely filled subshell, the configurations are quite stable.Thus by adding an extra electron they lose stability, thus they resist the aditionof electron. Hence the EA of those leement is positive.

For example Be has 1s2 2s2 condifuration. It has filled 2s subslevel. Thus it does not take an electron to become 1s2 2s2 2p1 which is of high energy. If you want to make it Be- ion by adding an electron you need to force (add energy to give the electron) that is why EA is positive.

Similary with noble gases which have completely filled s2p6 sublevels , adding an electron increases their energy due to the repulsions and the placement of extra electronin higher energy sublevel. Due to these reasons the electron need to be forced into the atom by supplying energy thus making the EA positive.

iii) Na has 3s1 and Mg has 3s2 as their valence configurations.

Na by losing this one electron becomes octet valence configuration, thus readily loses this electron by supply of small amout of energy. Thus Na has lower first I.E.

While Mg has completed sublevle so does not want to lose its stability by giving out an eletron. Thus has higher first I.E.

However Mg has lower second IE than Na as by losing the second electron it gains octet , while sodium already has octet.