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 also equals the number of electrons in a neutral atom. The mass number (A) is the total number of protons and neutrons in the nucleus. (b) The mass number can vary without changing the element's identity, as changing the number of neutrons results in different isotopes of the same element, while the atomic number remains the same.

Step-by-step solution

Define Atomic Number

The atomic number, denoted by Z, is the number of protons in an atom's nucleus. It is also equal to the number of electrons orbiting the nucleus in a neutral atom. Atomic numbers are whole numbers and are used to identify elements in the periodic table.

Define Mass Number

The mass number, denoted by A, is the total number of protons and neutrons in an atom's nucleus. Unlike the atomic number, the mass number does not uniquely identify an element, as atoms of the same element can have different numbers of neutrons. Atoms with the same atomic number but different mass numbers are called isotopes.

Determine which of the two can vary without changing the element's identity

The atomic number determines which element an atom belongs to, as elements are defined by the number of protons in their nuclei. Changing the atomic number would result in a different element. On the other hand, mass number can vary without changing the identity of the element as it only changes the number of neutrons in the nucleus. When mass number changes while the atomic number remains the same, the atom is still the same element but exists as a different isotope. Thus, mass number can vary without changing the identity of the element.