Which of the following is wrong statement?

We begin by recalling that a statement is called “wrong” when it contradicts an experimentally established or theoretically proved fact. One by-one, let us examine every option.

First, consider the colour of solid ozone. Experimental data show that when ozone is cooled to its solid form it appears violet-black. Hence the sentence “Ozone is violet-black in solid state” is in agreement with observation, so Option A is correct and therefore not the wrong statement we are looking for.

Next, let us check the magnetic behaviour of ozone. The molecular orbital picture of $$O_3$$ contains only paired electrons; there is no unpaired electron anywhere in the molecule. A substance that possesses only paired electrons is called diamagnetic. Thus the sentence “Ozone is diamagnetic gas” is factually correct, so Option B is also correct.

Now we analyse the geometry of the $$O_3$$ molecule. The central oxygen atom in ozone is $$sp^2$$ hybridised. The three $$sp^2$$ orbitals form two $$\sigma$$-bonds with the terminal oxygens and hold one lone pair, producing a trigonal planar arrangement of electron pairs. Because one vertex is occupied by a lone pair, the observable molecular shape becomes angular (bent) with an $$O\!-\!O\!-\!O$$ bond angle of about $$117^{\circ}$$. Consequently, the statement “$$O_3$$ molecule is bent” is true; Option D is also correct.

The only remaining option is the claim that “$$ONCl$$ and $$ONO^-$$ are isoelectronic.” Two species are called isoelectronic when they possess the same total number of electrons. Let us count the total electrons in each species.

For $$ONCl$$ (better written as $$NOCl$$, nitrosyl chloride) we add the atomic numbers of every atom:

$$\text{Total electrons in }ONCl = Z_O + Z_N + Z_{Cl} = 8 + 7 + 17 = 32$$

For $$ONO^-$$ we have two oxygens, one nitrogen, and an extra electron because of the negative charge:

$$\text{Total electrons in }ONO^- = 2Z_O + Z_N + 1 = 2(8) + 7 + 1 = 16 + 8 = 24$$

Hence $$ONCl$$ contains $$32$$ electrons whereas $$ONO^-$$ contains only $$24$$ electrons. The numbers are clearly different, so the two species are not isoelectronic. Therefore Option C makes a false claim and is the wrong statement in the list.

Since Options A, B and D are all correct while Option C is false, the wrong statement is given in Option C, which corresponds to number 3 in the question.

Hence, the correct answer is Option 3.