Trends in ionisation energy

Question 1

Define first ionisation energy

Answer 1

The first ionisation energy of an atom is the minimum energy required to completely remove the most loosely bound electron from a neutral gaseous atom in its ground state

Question 2

What does the second ionisation energy of an element refer to?

Answer 2

It refers to the removal of a second electron from the positive ion formed when the first electron is removed

Question 3

How are ionisation energy values obtained?

Answer 3

They are obtained from the emission spectra of atoms

Question 4

What is the general patterns for ionisation energy values down and across the periodic table?

Answer 4

They decrease down the groups and increase along the periods in the periodic table

Question 5

What are the two reasons for the first ionisation energy values decrease down groups in the periodic table?

Answer 5

1. Increasing atomic radius:
   The atomic radius increases as you move down any group. The outermost electrons are further away from the attractive force of the nucleus. It then becomes easier to remove an electron from the outer energy level.
2. Screening effect of inner electrons:
   The nuclear charge increases down a group. This is cancelled out by the screening effect of the intervening energy levels of electrons.
   The outermost electrons are shielded from the attractive force of the nucleus and so are easier to remove.

Question 6

What are the two reasons for the first ionisation energy values increase across a period in the periodic table?

Answer 6

1. Increasing effective nuclear charge:
   The number of protons in the nucleus is increasing across a period. The attraction between the nucleus and the outer electrons is steadily increasing without any corresponding increase in screening by electrons in inner energy levels.
   The electrons are being held more firmly and require more energy to remove one of the electrons.
2. Decreasing atomic radius:
   The atomic radius decreases from left to right. An electron in the outermost level is becoming closer to the nucleus. Therefore, it is more difficult to remove the electron due to the increased attraction between it and the nucleus.

Question 7

What group of elements have the lowest ionisation energy?

Answer 7

The alkali metals

Question 8

Why are the alkali metals highly reactive?

Answer 8

They have the lowest ionisation energy. They tend to lose electrons readily.

Question 9

What group of elements have the highest ionisation energy?

Answer 9

The noble gases

Question 10

Why are the noble gases very unreactive?

Answer 10

They have very little tendency to lose electrons. They have the highest first ionisation energies.