# Chemistry X | Periodic Classification of Elements | Trends in the Modern Periodic Table

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.

# Chemistry X | Periodic Classification of Elements | Position of Elements in the Modern Periodic Table

Position of Elements in the Modern Periodic Table

The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’. Let us see what decides the placing of an element in a certain group and period.

Activity 5.4

1. Look at the group 1 of the Modern Periodic Table, and name the elements present in it.
2. Write down the electronic configuration of the first three elements of group 1.
3. What similarity do you find in their electronic configurations?
4. How many valence electrons are present in these three elements?

You will find that all these elements contain the same number of valence electrons. Similarly, you will find that the elements present in any one group have the same number of valence electrons. For example, elements fluorine (F) and chlorine (Cl), belong to group 17, how many electrons do fluorine and chlorine have in their outermost shells? Hence, we can say that groups in the Periodic Table signify an identical outershell electronic configuration. On the other hand, the number of shells increases as we go down the group.

There is an anomaly when it comes to the position of hydrogen because it can be placed either in group 1 or group 17 in the first period. Can you say why?