Question

why is Sodium atom bigger than the atoms of both Lithium and magnesium

Answer 1

sodium atom is more bigger than lithium and magnesium because on going from left to right in a group the atomic radius or atomic size of an element decreases

Answer 2

HEYA!

SODIUM atom is bigger than the atoms of LITHIUM and MAGNESIUM because-
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Bigger than MAGNESIUM-

Sodium is at the left most corner of the periodic table.

And we know that 'On moving from left to right in a periodic table , the size of atoms decreases '

REASON- As the atomic number increases the number of proton also increases. Due to large +ve charge on the nucleus , the electrons are pulled in more close to the nucleus and the size of atom decreases.

PS- Refer to the attachment for more information.
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Bigger than LITHIUM

We know that as we move downwards in a periodic table , the size of Atom increases.

REASON - The number of electron shells in the atom gradually increases due to which the size of atom increases.

In case of Sodium and lithium, sodium occurs in the lower shell so its size is bigger.

Hope this helps!