Generally speaking (by chemists
anyway), this value
- the total energy
needed, for one mole
of a specific element
, to bring the electron
in the highest energy state
(though not necessarily the outermost
) far enough away from the atom that we can say that it is no longer a part of the atom (the atom becomes an ion
, hence ionization
This activity can be carried out in various ways although I believe the most common is by basically suscepting the element, in its gaseous state, to a strong electric field, where passing electrons collide with the atoms and transfer their kinetic energy into an electron on the atom, exciting it enough to effectively release it.
The value of the first ionization energy reflects how easy it is for a given element to give up its highest energy electron -- this effect is needed in ionic bonding as one atom will lose an electron to the other atom. The lower the first ionisation energy, the more likely the atom is to give up its electron to an electrophilic reagent (attracted to negative charges). Likewise, an element with a high first ionization energy will be more likely to be able to attract an electron from another element in an ionic bond. Typical first ionization is represented in the equation below:
Y(g) -> Y+(g) + e-
Finally, it's worth noting that ionization energies tend to increase along a period (with certain exceptions due to sub-shells), and decrease as you go down a group.
The energy will increase accross the periods because the electrons are approximately the same distance from the inner positively charged nucleus, but the nucleus becomes more positive as you move from left to right so there is greater attraction towards it.
As you go down a group, on the other hand, the electrons become further and further from the nucleus so there is less attraction, in addition, there are more electrons between them and the nucleus - and these electrons are trying to repel the outermost ones.