Question

How can cobalt- $60$ and cobalt- $59$ be the same element but have different mass numbers?

Answer 1

When an atom has the same number of protons but has a different number of neutrons, it is said to be isotopes. Isotopes have the same atomic number but different atomic mass. For example- carbon- $12$ and carbon- $14$ . The different isotopes of the same element occupy the same position in the periodic table.

Complete answer:
Atomic number of an element is defined as the number of protons present in an element. It is denoted with a symbol $Z$ .
Atomic mass of an element is defined as the addition of protons and neutrons in an element. It is denoted with a symbol $A$.
An element having atomic number and atomic mass can be written as-
$_{Z}^{A}X$
Where, $X$ is an element
$Z$ is the atomic number
$A$ is the atomic mass
We can also write it as-
$_{p}^{n+p}X$
Where, $p$ is the proton
$n$ is the neutron
When we change the number of protons, it also changes the type of an element. But if we change the number of neutrons and do not change the number of protons then it is said to be the isotope of the same element.
In the case of cobalt- $60$ and cobalt- $59$ , the number of protons present is same that is $27$so therefore it is said to be the same element but the number of neutron present in cobalt- $59$ is $32$ and the number of neutron present in cobalt- $60$ is 33. Hence, it is said to be the isotope of cobalt.
Isotopes of Cobalt-
Cobalt- $60$
$_{27}^{33}Co$
Cobalt- $59$
$_{27}^{32}Co$

Note: The physical properties of all the isotopes are different. Physical properties include- mass, melting and boiling point and freezing point.
Isotopes that are radioactive are known as radioisotopes.