Question

Which of the following statement(s) is\are not correct?

• A. $\text{An element of a substance contains only one kind of atom}$
• B. $\text{A mixture is not always heterogeneous}$
• C. $\text{Elements can exist as atoms or molecules but compounds exist only as molecules}$
• D. $\text{Atoms of same element have always same atomic mass number}$

Answer 1

The correct option is D $\text{Atoms of same element have always same atomic mass number}$

Atoms of the same element can have different mass numbers and they are known as isotopes.

For example,

Hydrogen have $3$ isotopes.

$H_1^1,D_1^2,T_1^3$

Hence, option $D$ is an incorrect statement.