Question

the property which relates to the amount of energy required to remove an electron from the outermost shell of an isolated gaseous atom

Answer 1

Ionization energy is the quantity of energy that an isolated, gaseous atom in the ground electronic state must absorb to discharge an electron, resulting in a cation.

H(g)→H+(g)+e−(1.1)(1.1)H(g)→H+(g)+e−

This energy is usually expressed in kJ/mol, or the amount of energy it takes for all the atoms in a mole to lose one electron each.

When considering an initially neutral atom, expelling the first electron will require less energy than expelling the second, the second will require less energy than the third, and so on. Each successive electron requires more energy to be released. This is because after the first electron is lost, the overall charge of the atom becomes positive, and the negative forces of the electron will be attracted to the positive charge of the newly formed ion. The more electrons that are lost, the more positive this ion will be, the harder it is to separate the electrons from the atom. 

Answer 2

It's the ionization energy.....and this is quite low in case of metals i.e. group 1 and group 2 but is high for the nonmetals especially for group 17.
TRENDS IN PERIODIC TABLE:
⇒It decreases along a group.
⇒It increases across a period.