Trends in atomic radii

Question 1

Define the atomic radius (covalent radius)

Answer 1

The atomic radius (covalent radius) of an atom is defined as half the distance between the nuclei of two atoms of the same element that are joined together by a single covalent bond

Question 2

What principle states that it is impossible to know the position of an electron?

Answer 2

Heisenberg uncertainty principle

Question 3

Is it possible to measure exactly where the electron cloud of an aatom ends?

Answer 3

No it is not possible

Question 4

What is the bond length?

Answer 4

It is the distance between the nuclei of atoms

Question 5

How can the blond length be measured?

Answer 5

It may be measured by methods known as ‘X-ray diffraction’ and ‘electron diffraction’

Question 6

What can the atomic radius also be called?

Answer 6

covalent radius

Question 7

Define/explain the trend of atomic radius down the groups in the periodic table

Answer 7

The values of the atomic radius down the groups in the periodic table

Question 8

What is the trend of the atomic radius down the groups in the periodic table?

Answer 8

The values of the atomic radius increase down the groups in the periodic table

Question 9

Give the two reasons why the values of the atomic radius increase down the groups in the periodic table

Answer 9

1. The addition electrons are going into a new energy level which is further from the nucleus. Since the outer electrons are becoming further away from the nucleus, the atomic radius increases.
2. Screening effect of inner electrons. Even though the nuclear charge increases going down a group, the increase in the nuclear charge is lessened becasue of the screening effect .

Question 10

Explain what the screening effect is

Answer 10

The electrons in the inner energy level or levels help to screen or shield the outer electrons from the positive charge in the nucleus. Therefore the nuclear charge that increases is counteracted by the shielding effect of the inner energy levels of electrons.

Question 11

The values of the atomic radius do what across a period?

Answer 11

They decrease across a period

Question 12

Give the two reasons why the atomic radius decrease across a period

Answer 12

1. Increase in effective nuclear charge. The no. of electrons in the nucleus increases from left to right across any one period. This means that there is a greater attractive force on the outer electrons.
2. No increase in screening effect. When moving left to right on the periodic table, the extra electron which is being added is going into the same outer energy level.
   (no increase in screening effect caused by the electrons in the inner energy levels).
   (There is no additional energy level being added to counteract the increasing attracive force of the nucleus, then size of the atom shrinks/atomic radius decreases)

Question 13

How is the effective nuclear charge found?

Answer 13

The effective nuclear charge is obtained by subtracting the effect of the screening electrons from the effect of the positive charge of the nucleus.