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How does atomic size vary in the periodic table?

To understand that how do the sizes of atoms vary in ; let’s revise the core concepts: Atomic Radius: It’s the measure of the size of atom from its center to its outer most electronic shells. The Atomic size depends on the following two factors:

  1. : The no. of protons in an atom is called as its atomic number. For example: The Carbon has 6 protons in its nucleus, so its atomic number is 6. We also know that no. of electrons and protons are same in an atom i.e. there are 6 electrons and 6 protons in Carbon. And we can also understand that the charge of protons is compensated well by the equal no. of electrons.

  2. No. of Electronic shells: The number of principal energy levels in an atom that can be thought as orbits in which the electrons revolve around the nucleus. The no. of filled Electronic shells depends upon the atomic no. of the respective atom.

We know, the atomic number increases by 1 unit in the periodically arranged . For example: The first 6 elements:

Element: Hydrogen – Helium – Lithium – Beryllium – Boron – Carbon Atomic no.: 1 -----------------2------------3-------------4--------------5------------6

It can be said, “As the atomic number increase, the increase of atomic size should take place.”

Now, the basic understanding is attained. Let’s focus on the interesting phenomenon about the variation of atomic sizes in periodic table that is:

“Atomic size along the period decreases while down the group it increases.” Let it be explained one by one.

Atomic Size Along The Period Decreases:

Periods are the rows of the periodic table. The empirically measured atomic sizes of 2nd period’s elements are:

Element: Lithium – Beryllium – Boron – Carbon – Nitrogen – Oxygen – Fluorine

Empirically Measured Atomic size: 145 – 105—85 – 70 – 65 – 60 – 50

This is surprising that though the atomic number is increasing but the Atomic size is decreasing, which shows that the above explanation is contradictory. No, it’s not, because here the concept of no change in Electronic shells has to be considered.

Let’s compare Boron and Fluorine.

Atomic number of boron is 5 and its electronic configuration is: 1s2, 2s2, 2p1

Whereas the atomic number of fluorine is 9 and its electronic configuration is: 1s2, 2s2, 2p5

Though in fluorine the number of number of electrons are increasing along the increase of proton number but shell number is not increased due to which strong nuclear pull is practiced in the fluorine atom and resulting in contraction in the atomic size. Therefore, we say the atomic size of boron is larger than the atomic size of fluorine.

Atomic Sizes Increase Down The Groups:

This is very simple to understand because in down the group the atomic number and no. of electronic shells both increase and increase in atomic sizes can be justified.

Let’s take the example of Halogens in group 17. Elements = Empirically measured Atomic Sizes F = 50 Cl = 100 Br = 115 I = 140

Let’s quickly compare Cl with I:

Atomic number of Cl is 17 and its electronic configuration is: [Ne] 3s2 3p5

Atomic number of Iodine is 53 and its electronic configuration is: [Kr] 4d10 5s2 5p5

It’s clearly illustrated that the atomic number is increased and as well as increase in electronic shells had caused the nuclear pull to b decreased on the outer most shells. Therefore, the expansion in the atomic size has occurred. The decrease in nuclear pull is also assisted by the shielding effect of electrons of lower electronic shells.

Point to ponder: Can you relate the why does element with large atomic size release its outermost electrons easily?

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