Which of the following statements is/are correct?
I. The number of valence electrons in carbon is 6.
II, The atoms `""_(18)^(40)X` and `""_(20)^(40)Y` represent a pair of isotopes.
III. Number of electrons present in the outermost shell of an atom determines the valency of the atom.
IV. C-14, N-15 and 0-16 are isotones.

Options

A. I and III only
B. II and III only
C. III and IV only
D. I, II, III and IV

Explanation

To determine which statements are correct, let's analyze each one step by step.

### Step 1: Analyze Statement I
**Statement I:** The number of valence electrons in carbon is 6.

- Carbon has an atomic number of 6, which means it has 6 electrons.
- The electron configuration of carbon is 1s² 2s² 2p².
- The outermost shell (the second shell) contains 4 electrons (2 in 2s and 2 in 2p).
- Therefore, the number of valence electrons in carbon is **4**, not 6.

**Conclusion for Statement I:** This statement is **incorrect**.

### Step 2: Analyze Statement II
**Statement II:** The atoms `""_(18)^(40)X` and `""_(20)^(40)Y` represent a pair of isotopes.

- Isotopes are defined as atoms of the same element (same atomic number) with different mass numbers.
- Here, `X` has an atomic number of 18 and `Y` has an atomic number of 20.
- Since they have different atomic numbers, they are not isotopes; they are different elements.

**Conclusion for Statement II:** This statement is **incorrect**.

### Step 3: Analyze Statement III
**Statement III:** The number of electrons present in the outermost shell of an atom determines the valency of the atom.

- The valency of an atom is indeed determined by the number of electrons in its outermost shell (valence shell).
- For example, carbon has 4 valence electrons, and its valency is 4.

**Conclusion for Statement III:** This statement is **correct**.

### Step 4: Analyze Statement IV
**Statement IV:** C-14, N-15, and O-16 are isotones.

- Isotones are defined as atoms that have the same number of neutrons but different atomic numbers.
- Let's calculate the number of neutrons in each:
- C-14: 14 (mass number) - 6 (atomic number) = 8 neutrons
- N-15: 15 (mass number) - 7 (atomic number) = 8 neutrons
- O-16: 16 (mass number) - 8 (atomic number) = 8 neutrons
- All three have 8 neutrons.

**Conclusion for Statement IV:** This statement is **correct**.

### Final Summary of Statements:
- Statement I: Incorrect
- Statement II: Incorrect
- Statement III: Correct
- Statement IV: Correct

### Final Answer:
The correct statements are **III and IV**.

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