Question

State the valency in each case and name the following elements or radicals given below. [The first two have been done for you]
1) $$K$$ [Ans: $$K^{+1}$$ - potassium] 21) $$OH$$
2) $$C{r}_2{O}_7$$ [Ans: $$C{r}_2{O_7}^{2-}$$ - dichromate] 22) $$O_2$$
3)$$Cl$$ 23) $$Zn{O}_2$$
4) $$Ni$$ 24) $$Si{O}_3$$
5) $$Cl{O}_3$$ 25) $$N{O}_3$$
6) $$C{O}_3$$ 26) $$S{O}_3$$
7) $$Ba$$ 27) $$S{O}_4$$
8) $$HC{O}_3$$ 28) $$P{O}_4$$
9) $$N{O}_2$$ 29) $$N$$
10) $$Na$$ 30) $$C$$
11) $$Br$$ 31) $$P{O}_3$$
12) $$Zn$$ 32) $$Al$$
13) $$Mg$$ 33) $$Ca$$
14) $$O$$ 34) $$H$$
15) $$Co$$ 35) $$Pb{O}_2$$
16) $$Cr{O}_4$$ 36) $$HS{O}_3$$
17) $$ClO$$ 37) $$Al{O}_2$$
18) $$Mn{O}_4$$ 38) $$Cr$$
19) $$Li$$ 39) $$HS{O}_4$$
20) $$I$$ 40) $$N{H}_4$$

Answer 1

Hint: Valency of any element is the number of electrons it has to lose or gain in order to become stable. If the atom has to lose electrons to become stable, then the valency will be positive and if the atom has to gain electrons to become stable, then the valency will be negative.

Complete answer:
Let us start from the third as the first two are already given.
3 - $$C{l}^{-1}$$ - Chlorine, it can also exhibit valency such as 1, 3, 5 and 7.
4 - $$N{i}^{+2}$$ - Nickel has a valency of positive two.
5 - $$Cl{O_3}^{-1}$$ - Chlorate has a valency of negative one.
6 - $$C{O_3}^{-2}$$ - Carbonate has a valency of negative two.
7 - $$B{a}^{+2}$$ - Barium has a valency of positive two.
8 - $$HC{O_3}^{+1}$$ - Bicarbonate has a valency of positive one.
9 - $$N{O_2}^{-1}$$ - Nitrite has a valency of negative one.
10 - $$N{a}^{+1}$$ - Sodium has a valency of positive one.
11 - $$B{r}^{-1}$$ - Bromine has a valency of negative one.
12 - $$Z{n}^{+2}$$ - Zinc has a valency of positive two.
13 - $$M{g}^{+2}$$ - Magnesium has a valency of positive two.
14 - $$O^{-2}$$ - Oxide has a valency of negative two.
15 - $$C{o}^{+2}$$ , $$C{o}^{+3}$$ - Cobalt has variable valency of positive two and three.
16 - $$Cr{O_4}^{-2}$$ - Chromate has a valency of negative two.
17 - $$Cl{O}^{-1}$$ - Hypochlorite has a valency of negative one.
18 - $$Mn{O_4}^{-1}$$ - Permanganate has a valency of negative one.
19 - $$L{i}^{+1}$$ - Lithium has a valency of positive one.
20 - $$I^{-1}$$ - Iodine has a valency of negative one.
21 - $$O{H}^{-1}$$ - Hydroxide has a valency of negative one.
22 - $$H_2{O_2}^{+2}$$ - Peroxide ion has valency of positive two.
23 - $$Zn{O_2}^{+2}$$ - Zincate has a valency of positive two.
24 - $$Si{O_3}^{-2}$$ - Silicate has a valency of negative two.
25 - $$N{O}^{-1}$$ - Nitrate ions have a valency of negative one.
26 - $$S{O_3}^{-2}$$ - Sulphite has a valency of negative two.
27 - $$S{O_4}^{+2}$$ - Sulphate has a valency of positive two.
28 - $$P{O}^{-3}$$ - Phosphate has a valency of negative three.
29 - $$N^{-3}$$ , $$N^{+5}$$ - Nitrogen has a valency of either positive five or negative three.
30 - $$C^{-4}$$ , $$C^{+4}$$ - Carbon has a valency of either positive four or negative four.
31 - $$P{O_3}^{-3}$$ - Phosphite has a valency of negative three.
32 - $$A{l}^{+3}$$ - Aluminium has a valency of positive three.
33 - $$C{a}^{+2}$$ - Calcium has a valency of positive two.
34 - $$H^{+1}$$ - Hydrogen has a valency of positive one.
35 - $$Pb{O_2}^{+2}$$ - Plumbite has a valency of positive two.
36 - $$HS{O_3}^{+1}$$ - Bisulphite has a valency of positive one.
37 - $$Al{O_2}^{-1}$$ - Meta aluminate has a valency of negative one.
38 - $$C{r}^{+3}$$ - Chromium has a valency of positive three.
39 - $$HS{O_4}^{-1}$$ - Bisulphate has a valency of negative one.
40 - $$N{H_4}^{+1}$$ - Ammonium has a valency of positive one.

Note: The knowledge of the names of these compounds and the valence electrons of each of those elements has to be known, without that we cannot get the correct answer. Also, most elements have more than one valence, here the most common valence is written.