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?

Activity 5.5

1. If you look at the long form of the Periodic Table, you will find that the elements Li, Be, B, C, N, O, F, and Ne are present in the second period. Write down their electronic configuration.
2. Do these elements also contain the same number of valence electrons?
3. Do they contain the same number of shells?

You will find that these elements do not have the same number of valence electrons, but they contain the same number of shells. You also observe that the number of valence shell electrons increases by one unit, as the atomic number increases by one unit on moving from left to right in a period.

Or we can say that atoms of different elements with the same number of occupied shells are placed in the same period. Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period of the Modern Periodic Table, since the electrons in the atoms of these elements are filled in K, L and M shells. Write the electronic configuration of these elements and confirm the above statement. Each period marks a new electronic shell getting filled.

How many elements are there in the first, second, third and fourth periods?

We can explain the number of elements in these periods based on how electrons are filled into various shells. You will study the details of this in higher classes. Recall that the maximum number of electrons that can be accommodated in a shell depends on the formula 2n² where ‘n’ is the number of the given shell from the nucleus.

For example,

K Shell – 2 × (1)² = 2, hence the first period has 2 elements.

L Shell – 2 × (2)² = 8, hence the second period has 8 elements.

M Shell – 2 × (3)² = 18, but the outermost shell can have only

8 electrons, so the third period also has only 8 elements.

The position of an element in the Periodic Table tells us about its chemical reactivity. As you have learnt, the valence electrons determine the kind and number of bonds formed by an element. Can you now say why Mendeléev’s choice of formulae of compounds as the basis for deciding the position of an element in his Table was a good one? How would this lead to elements with similar chemical properties being placed in the same group?