The two elements A and B have the same atomi mass but their atomic numbers are 19 and 20 respectively . A and B are

Options

A. isotopes
B. isobars
C. isomers
D. polymers

Answer

To solve the problem, we need to analyze the information given about the two elements A and B.

### Step-by-Step Solution:

1. **Identify Atomic Numbers and Mass Numbers**:
- Element A has an atomic number of 19.
- Element B has an atomic number of 20.
- Both elements have the same atomic mass (mass number).

2. **Understand Atomic Number and Mass Number**:
- The atomic number (Z) represents the number of protons in the nucleus of an atom.
- The mass number (A) is the total number of protons and neutrons in the nucleus.

3. **Determine the Relationship**:
- Since both elements have the same mass number but different atomic numbers, they must have different numbers of protons and neutrons.
- Element A (Z = 19) has 19 protons, while Element B (Z = 20) has 20 protons.

4. **Define Isobars**:
- Elements that have the same mass number but different atomic numbers are called isobars.
- In this case, A and B are isobars because they have the same mass number but different atomic numbers.

5. **Conclusion**:
- Therefore, the correct answer is that elements A and B are **isobars**.

### Final Answer:
A and B are **isobars**.