The number of protons, neutrons and electrons in four elements 1, 2, 3 and 4 are as follows:
(1) p = 6, n = 6, e = 6
(2) p = 6, n = 7, e = 6
(3) p = 18, n = 22, e = 18
(4) p = 19, n = 21, e = 19
In these

Options

A. element 1 and 2 are isotopes and element 3 and 4 are isobars
B. element 1 and 2 are isotopes but element 3 and 4 are not isobars
C. element 1 and 2 are isobars and element 3 and 4 are isotopes
D. element 1 and 2 are isobars but element 3 and 4 are not isotopes.

Explanation

To solve the problem, we need to identify the relationships between the four elements based on their protons, neutrons, and electrons. We will categorize them as isotopes or isobars.

### Step-by-Step Solution:

1. **Identify the Elements:**
- For Element 1:
- Protons (p) = 6, Neutrons (n) = 6, Electrons (e) = 6
- For Element 2:
- Protons (p) = 6, Neutrons (n) = 7, Electrons (e) = 6
- For Element 3:
- Protons (p) = 18, Neutrons (n) = 22, Electrons (e) = 18
- For Element 4:
- Protons (p) = 19, Neutrons (n) = 21, Electrons (e) = 19

2. **Determine Atomic Numbers:**
- The atomic number (Z) is equal to the number of protons (p).
- Therefore:
- Element 1: Z = 6
- Element 2: Z = 6
- Element 3: Z = 18
- Element 4: Z = 19

3. **Identify Isotopes:**
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
- Elements 1 and 2 both have 6 protons (Z = 6), but different numbers of neutrons (6 and 7 respectively).
- Thus, Elements 1 and 2 are isotopes of Carbon (C).

4. **Identify Isobars:**
- Isobars are atoms of different elements that have the same mass number (A), which is the sum of protons and neutrons.
- For Element 3:
- Mass number (A) = p + n = 18 + 22 = 40
- For Element 4:
- Mass number (A) = p + n = 19 + 21 = 40
- Since both Elements 3 and 4 have the same mass number (40) but different atomic numbers (18 and 19), they are isobars.

5. **Conclusion:**
- Elements 1 and 2 are isotopes of Carbon.
- Elements 3 and 4 are isobars with the same mass number but different atomic numbers.

### Final Answer:
- Elements 1 and 2 are **isotopes**.
- Elements 3 and 4 are **isobars**.