(a) Atomic number (Z) represents the number of protons in the nucleus of an atom. Mass number (A) represents the total number of protons and neutrons in the nucleus of an atom.

(b) The atomic number tells us the element to which the atom belongs. It is unique for each element. For example, all atoms with an atomic number of 6 are Carbon.

(c) The mass number tells us the total number of nucleons (protons and neutrons) in the nucleus of an atom. It is a whole number.

As defined above, an isotope is an atom of the same element with the same number of protons but a different number of neutrons. Therefore, isotopes of an element have the same chemical properties but different masses.

An example of an element with at least two naturally occurring isotopes is Hydrogen (H). Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H).

Their relative abundances are approximately:

Protium is present in approximately 99.985% abundance, Deuterium in about 0.015% and Tritium is extremely rare, occurring naturally in trace amounts.

Nuclear Notation:

The nuclear notation for an atom is written as: ^A_ZX, where:

• A = Mass number
• Z = Atomic number
• X = Chemical symbol of the element

In this case, the nuclear notation for element Y is: ³⁵₁₇Y

Number of Electrons:

In a neutral atom, the number of electrons is equal to the number of protons. Therefore, element Y has 17 electrons.

Name of Element Y:

The element with an atomic number of 17 is Chlorine (Cl).