From my review book: \"Ionization energy is the energy required to remove an ele

Question

From my review book:

"Ionization energy is the energy required to remove an electron. Removing an electron always requires an input of heat, which makes it an endothermic process."

If Lithium (or Na, or K, or Mg, etc.) is more stable as a cation (Li+, Na, K+, Mg2+, etc.)....then why does it take energy or input of heat at all to remove an electron? Shouldn't Na --> Na+ release heat and energy into its surroundings, (i.e. dissipate the heat), as it is more energetically favorable as Na+ than Na?

From what I understand, more stability = lower energy, broadly speaking-- so why would any energy whatsoever be required for Li to move to Li+, if Lithium in its solid form is so reactive/unstable?

Thank you!

Explanation / Answer

In an atom an electron is tightly bounded by the attractive force of the nucleus. So energy is required to free an electron from the electrostatic force of the nucleus.

The energy required to remove an electron from the outermost shell of a neutral gaseous atom is known as Ionization Energy (IE). It is quantitatively expressed in symbols as

X + energy ? X+ + e?

where X is any atom or molecule capable of being ionized, X+ is that atom or molecule with an electron removed, and e? is the removed electron. This is an endothermic process. Generally, the closer the electrons are to the nucleus of the atom, the higher the atom's ionization energy.

Comparison of IEs of atoms in the periodic table reveals two patterns:

IEs generally increase as one moves from left to right within a given period.
IEs generally decrease as one moves down a given group.

Some factors affecting the ionization energy include:

Nuclear charge: the greater the magnitude of nuclear charge the more tightly the electrons are held by the nucleus and hence more will be ionization energy.
Number of electron shells: the greater the size of the atom less tightly the electrons are held by the nucleus and ionization energy will be less
Effective nuclear charge (Zeff): the greater the magnitude of electron shielding and penetration the less tightly the electrons are held by the nucleus, the lower the Zeff of the electron, and hence less will be the ionization energy.
Type of orbital ionized: the atom having a more stable electronic configuration has less tendency to lose electrons and consequently has high ionization energy.
Occupancy of the orbital matters: if the orbital is half or completely filled then it is harder to remove electrons. The excess energy that you provide during ionization is taken up by the expelled electron and not by the resultant ion. So that, energy of X+ < energy of X.

There is another term known as Electron Affinity (EA). It is defined as the energy released when an electron is added into the outermost shell of a neutral gaseous atom. It is an exothermic process. So IE is a positive term whereas EA is a negative term.

The energy of electron at infinite distance from the nucleus is taken as zero. Now when the electron moves closer towards the nucleus (it does so under the attractive force of the nucleus) its energy becomes negative.

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