Oxidation Number

To determine the correct order of N-compounds in decreasing order of oxidation states, we need to calculate the oxidation state of nitrogen in each compound provided:

1. HNO₃: The oxidation state of hydrogen is +1 and oxygen is -2. Let the oxidation state of nitrogen be x .
   The formula for calculating oxidation states is: +1 + x + 3(-2) = 0 .
   Solving: x - 5 = 0
   Thus, x = +5 .
2. NO: Oxygen has an oxidation state of -2. Let the oxidation state of nitrogen be x .
   The formula is: x - 2 = 0 .
   Solving: x = +2 .
3. N₂: In its elemental form, the oxidation state of nitrogen is 0 .
4. NH₄Cl: Hydrogen is +1 and since there are four hydrogens, it contributes +4. Chlorine is -1.
   Let the oxidation state of nitrogen be x .
   The formula for calculating oxidation states is: x + 4(1) - 1 = 0 .
   Solving: x + 3 = 0
   Thus, x = -3 .

Using the calculations above, we can arrange the oxidation states in decreasing order: +5, +2, 0, -3. Therefore, the decreasing order of oxidation states for the compounds is:

HNO₃, NO, N₂, NH₄Cl.

Thus, the correct option is HNO₃, NO, N₂, NH₄Cl.

The question asks which statement is not true for halogens, and the correct statement is, "All but fluorine show positive oxidation states."

1. All form monobasic oxyacids.
   • Halogens like chlorine, bromine, and iodine form monobasic oxyacids such as hypochlorous acid (HOCl), hypobromous acid (HOBr), and hypoiodous acid (HOI).
   • These acids contain one ionizable hydrogen (hence, monobasic).
2. All but fluorine show positive oxidation states.
   • This statement is not true because fluorine always exhibits a -1 oxidation state due to its high electronegativity and does not show positive oxidation states.
   • Other halogens (Cl, Br, I) can exhibit positive oxidation states in their compounds, e.g., Cl in HClO₄.
3. All are oxidizing agents.
   • Halogens are known to be strong oxidizing agents. They can receive electrons from other elements or compounds, thereby oxidizing them.
4. Chlorine has the highest electron-gain enthalpy.
   • This is true as chlorine is highly electronegative, and it has a high tendency to gain an electron, making its electron-gain enthalpy the highest among the halogens.

In conclusion, among the given options, the statement "All but fluorine show positive oxidation states." is not true as fluorine does not show positive oxidation states.