(b) Which of these can vary without changing the identity of the element?

Key Concepts

Atomic Number

The atomic number of an element is defined as the number of protons in its nucleus. This number is unique to each element and determines its identity. Changing the atomic number would result in a different element, so it cannot vary without changing the identity of the element.

Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This means that while their atomic number (and thus their identity as an element) remains the same, their mass and some physical properties can vary. Isotopes illustrate how variations in neutron count do not alter the fundamental identity of the element.

Chemical Properties

Chemical properties refer to the characteristics of a substance that become evident during a chemical reaction. These properties can vary based on the conditions or the presence of other substances, but they do not change the identity of the element itself. For example, the reactivity of an element can change under different conditions, yet it remains the same element.