From the elements $^{39}_{19}\mathrm{A}$, $^{28}_{14}\mathrm{B}$, $^{16}_{8}\mathrm{C}$ and $^{40}_{18}\mathrm{D}$ identify: (a) the most electro positive element. (b) a noble gas. (c) a metalloid. (d) an element which will gain 2 electrons to attain nearest noble gas configuration. (e) formula of compound formed between A and C. (f) elements belonging to same period.

Based on the atomic numbers (subscripts) of the given elements, we first identify their electronic configurations:

• $^{39}_{19}\text{A}$: Atomic number $Z = 19$. Electronic configuration = $2, 8, 8, 1$ (Potassium, $\text{K}$)
• $^{28}_{14}\text{B}$: Atomic number $Z = 14$. Electronic configuration = $2, 8, 4$ (Silicon, $\text{Si}$)
• $^{16}_{8}\text{C}$: Atomic number $Z = 8$. Electronic configuration = $2, 6$ (Oxygen, $\text{O}$)
• $^{40}_{18}\text{D}$: Atomic number $Z = 18$. Electronic configuration = $2, 8, 8$ (Argon, \text{Ar})

(a) The most electropositive element:

$\text{A}$ ($^{39}_{19}\text{A}$) is the most electropositive element because it has only $1$ valence electron in its outermost shell, which it can lose very easily to attain a stable octet.

(b) A noble gas:

$\text{D}$ ($^{40}_{18}\text{D}$) is a noble gas because its outermost shell is completely filled (octet configuration: $2, 8, 8$).

(c) A metalloid:

$\text{B}$ ($^{28}_{14}\text{B}$) is a metalloid (Silicon) because it has $4$ valence electrons and exhibits properties intermediate between metals and non-metals.

(d) Element which will gain 2 electrons to attain nearest noble gas configuration:

$\text{C}$ ($^{16}_{8}\text{C}$) has $6$ valence electrons ($2, 6$). It needs $2$ more electrons to complete its octet and attain the stable configuration of Neon ($2, 8$).

(e) Formula of compound formed between A and C:

• Element $\text{A}$ has a valency of $+1$ (loses $1$ electron: $\text{A}^+$).
• Element $\text{C}$ has a valency of $-2$ (gains $2$ electrons: $\text{C}^{2-}$).
• By criss-cross method, the chemical formula is $\text{A}_2\text{C}$ (which represents Potassium Oxide, $\text{K}_2\text{O}$).

(f) Elements belonging to the same period:

$\text{B}$ and $\text{D}$ belong to the same period (Period 3) because both have electrons distributed across three shells ($K$, $L$, and $M$).