MAKING ORDER OUT OF CHAOS – THE MODERN PERIODIC TABLE
In 1913, Henry Moseley showed that the atomic number (symbolised as Z) of an element is a more fundamental property than its atomic mass. Accordingly, Mendeléev’s Periodic Law was modified and the atomic number was adopted as the basis of Modern Periodic Table and the Modern Periodic Law can be stated as follows:
‘Properties of elements are a periodic function of their atomic number.’
Let us recall that the atomic number gives us the number of protons in the nucleus of an atom and this number increases by one in going from one element to the next. Elements, when arranged in order of increasing atomic number, lead us to the classification known as the Modern Periodic Table (Table 5.6). Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number.
Activity 5.3
How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
How were the positions of isotopes of various elements decided in the Modern Periodic Table?
Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
Where do you think should hydrogen be placed in the Modern Periodic Table?
As we can see, the Modern Periodic Table takes care of three limitations of Mendléev’s Periodic Table. The anomalous position of hydrogen can be discussed after we see what are the bases on which the position of an element in the Modern Periodic Table depends.
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
Look at group 1 of the Modern Periodic Table, and name the elements present in it.
Write down the electronic configuration of the first three elements of group 1.
What similarity do you find in their electronic configurations?
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 outer - shell 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
If you look at the above Modern 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 configurations.
Do these elements also contain the same number of valence electrons?
Do they contain the same number of shells?
You will find that these elements of the second period 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 x (1)2 = 2, hence the first period has 2 elements.
L Shell – 2 x (2)2 = 8, hence the second period has 8 elements. The third, fourth, fifth, sixth, and seventh periods have 8, 18, 18, 32, and 32 elements respectively.
The reason for this you will study in higher classes.
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?