Quite amazingly the whole world is made up of atoms and molecules!
The primitive theory as postulated by Sir John Dalton was that atoms are the tiniest indivisible particles. However, this theory had many limitations and further studies found that an atom is actually made up of minute particles namely, protons, electrons and neutrons.
With the introduction of Rutherfordâ€™s atomic model, it was shown that at the centre of atom there is the positively charged proton and neutral charged neutron. The negatively charged electron revolves around the two in its orbit.
With further studies concrete atomic structure was established.
Now the problem arises with the periodic table and its elements. As each element has characteristic atoms and molecules, they also generate various numerical and problems sums that involves the concept of effective nuclear charge, a very important part of atomic structure.
For example, one such problem is-

1. Q) Rank the following elements by effective nuclear charge, zeff, for a valence electron from highest to lowest.

Sb, Sn, Sr, In and Rb.
To calculate its answer, we first need to know what the concept of effective nuclear charge is.
According to the definition- â€œeffective nuclear charge is the average nuclear charge felt by an individual electron in an atom. It is also taken into consideration the â€œshieldingâ€ effect of inner-shell electrons. The symbolic interpretation of effective nuclear is,
Zeff = Z â€“ S
Where, Z = number of protons in the nucleus
S = inner-shell electrons
To understand the concept a little better, let us take another example.

1. Q) What is the effective nuclear charge felt by an electron in the n=3 shell of sulfur?

Since, Zeff = Z â€“ S
Here, Z = 16 (sulfur as 16 protons in the nucleus)
Therefore, Zeff = 16 â€“ 10 = 6
Taking another question,

1. Q) What is the effective nuclear charge of the chlorine atom in the n=2 shell of an electron?

As we already know, Zeff = Z â€“ S
Here, Z= 17
And S= 2
Hence, Zeff = 17 â€“ 2 = 15
From the above examples, it is evident that Zeff < Z
Now, going back to the first question of ranking the elements, here, Zeff is the net positive charge and is given by: Zeff = Z â€“ S.
Zeff (Sb) = 51 â€“ 46 = 5+

Zeff (Sn) = 50 – 46 = 4+;

Zeff (Sr) = 38 â€“ 36 = 2+;

Zeff (In) = 49 â€“ 46 = 3+;

Zeff (Rb) = 37 â€“ 36 = 1+;

Therefore the rank will be:
Zeff (Sb) > Zeff (Sn) > Zeff (In) > Zeff (Sr) > Zeff (Rb)
Hence, students can easily Rank the following elements by effective nuclear charge, zeff, for a valence electron if they can get hold of the concepts easily. Atomic structure is a tough chapter to deal with as well as a crucial one. Hence clearing of the basic topic and sub-topics including effective nuclear charge, valence shell electrons etc are necessary. Help with the same is available with online sources; you just need to find the one for yourself.

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