Trends in the Modern Periodic Table

Valency : As you know, the valency of an element is determined by the number of valence electrons present in the termost shell of its atom.

Activity 5.6

1. How do you calculate the valency of an element from its electronic configuration?
2. What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?
3. Similarly find out the valencies of the first twenty elements.
4. How does the valency vary in a period on going from left to right?
5. How does the valency vary in going down a group?

Atomic size: The term atomic size refers to the radius of an atom. The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom. The atomic radius of hydrogen atom is 37 pm (picometre, 1 pm = 10^–12m). Let us study the variation of atomic size in a group and in a period.

Activity 5.7

1. Atomic radii of the elements of the second period are given below:

Period II elements : B Be O N Li C

Atomic radius (pm) : 88 111 66 74 152 77

1. Arrange them in decreasing order of their atomic radii.
2. Are the elements now arranged in the pattern of a period in the Periodic Table?
3. Which elements have the largest and the smallest atoms?
4. How does the atomic radius change as you go from left to right in a period?

You will see that the atomic radius decreases in moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

Activity 5.8

1. Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.

Group 1 Elements : Na Li Rb Cs K

Atomic Radius (pm) : 86 152 244 262 231

1. Name the elements which have the smallest and the largest atoms.
2. How does the atomic size vary as you go down a group?

You will see that the atomic size increases down the group. This is because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge.

Metallic and Non-metallic Properties

Activity 5.9

1. Examine elements of the third period and classify them as metals and non-metals.
2. On which side of the Periodic Table do you find the metals?
3. On which side of the Periodic Table do you find the non-metals?

As we can see, the metals like Na and Mg are towards the left-hand side of the Periodic Table while the non-metals like sulphur and chlorine are found on the right-hand side. In the middle, we have silicon, which is classified as a semi-metal or metalloid because it exhibits some properties of both metals and non-metals.

In the Modern Periodic Table, a zig-zag line separates metals from non-metals. The borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are intermediate in properties and are called metalloids or semi-metals.

As you have seen in Chapter 3, metals tend to lose electrons while forming bonds, that is, they are electropositive in nature.

Activity 5.10

1. How do you think the tendency to lose electrons will change in a group?
2. How will this tendency change in a period?

As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease. Down the group, the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus. Therefore, these can be lost easily. Hence metallic character decreases across a period and increases down a group.

Non-metals, on the other hand, are electronegative. They tend to form bonds by gaining electrons. Let us learn about the variation of this property.

Activity 5.11

1. How would the tendency to gain electrons change as you go from left to right across a period?
2. How would the tendency to gain electrons change as you go down a group?

As the trends in the electronegativity show, non-metals are found on the right-hand side of the Periodic Table towards the top.

These trends also help us to predict the nature of oxides formed by the elements because it is known to you that the oxides of metals are basic and that of non-metals are acidic in general.