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# Arrange the following as per the instruction in the brackets -A.${\text{He, Ar, Ne}}$ [increasing order of number of shells]B.${\text{Na, Li, K}}$ [increasing ionization energy]C.${\text{F, Cl, Br}}$ [increasing electronegativity]D.${\text{Na, K, Li}}$ [increasing atomic size]

Last updated date: 12th Sep 2024
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Hint: (A) The number of shells increases as we move down the group.
(B) The ionization energy decreases as we move down the group.
(C) The electronegativity decreases down the group
(D) Atomic size also increases down the group.

We need to calculate the order for increasing the number of shells. As we move down the group, each new shell adds up. ${\text{He, Ne , Ar}}$ has atomic number 2, 10 and 18. They belong to the same group that is group number 18. Hence the increasing order for the number of shells will be ${\text{He < Ne < Ar}}$.
Ionization energy is defined as the amount of energy required to remove the last electron from the gaseous atom. As we move down the group atomic size increases and shielding effect also increases. Due to this the attraction between the nucleus and last electron decreases, hence it is easier to remove the electron. ${\text{Li, Na, K}}$ Belongs to the same group that is group number 1 and hence down the group ionization energy will decrease. Lithium will have the highest ionization energy and potassium will have the least ionization energy. The increasing order of ionization energy will be ${\text{K}} < {\text{ Na < }}\,{\text{Li}}$ .
Electronegativity is defined as the tendency of an element to hold its electrons. As we move down the group atomic size increases and shielding effect also increases. Due to this the attraction between the nucleus and last electron decreases. Hence the holding tendency of the nucleus for its electrons will be less. Hence electronegativity decreases down the group. The increasing order is ${\text{Br < Cl < F}}$.
Due to the addition of extra shell shielding effect increases. The decrease of nuclear charge reaching the valence shell electron due to presence of inner shell electrons is known as shielding. Down the group each new shell adds up and shielding effect increases, both these factors contributing to the increasing atomic size down the group. Hence the increasing order is ${\text{Li < Na < K}}$.
${\text{He, Ne , Ar}}$are also known as Noble gases because they almost do not react with any element, there are certain exceptions though. ${\text{Li, Na, K}}$ are known as alkali metals, because their oxides and hydroxide give basic solutions with water. ${\text{F, Cl, Br}}$ Are known as halogens because they are salt producing elements, halo means salt and gen means producing.