Question

(a) Define atomic number and mass number. (b) Which of these can vary without changing the identity of the element?

Short answer

(a) The atomic number (Z) is the number of protons in an atom's nucleus and determines the element's chemical properties. The mass number (A) is the sum of protons and neutrons in the nucleus, representing the atomic mass in amu. (b) The mass number can vary without changing the element's identity since isotopes of the same element have different mass numbers but still maintain the same chemical properties and remain the same element.

Step-by-step solution

(a) Define Atomic Number and Mass Number

The atomic number (Z) of an element is the number of protons in the nucleus of an atom. It determines the element's chemical properties and its position in the periodic table. Every element has a unique atomic number. The mass number (A) of an atom is the sum of protons and neutrons in the nucleus. It represents the atomic mass of an atom, measured in atomic mass units (amu). The mass number is not unique for an element, as isotopes of the same element can have different mass numbers due to varying numbers of neutrons.

(b) Determine Which Characteristic Can Vary Without Changing the Identity of the Element

The identity of an element is determined by its atomic number because the atomic number defines the number of protons present in the atom, which in turn determines the element's chemical properties. Therefore, the mass number can vary without changing the identity of the element. Isotopes are atoms of the same element that have the same atomic number but different mass numbers because they have varying numbers of neutrons. These isotopes still maintain the same chemical properties and remain the same element, proving that the mass number can vary without changing the identity of an element.