Question

which of the following does NOT define an element? A.) elemental symbol B.) number of protons C.) mass number D.) atomic number

Answer 1

An elemental symbol is unique to each particular element and hence, rule out (A).

Every element can have only a unique number of protons and hence electrons in its neutral state. The atomic number of an element is the number of protons present in the nucleus of the atom. Since the number of protons is unique for each and every element, the atomic number is also a unique property of the element. Therefore, rule out (B) and (D).

The mass number of an element is the sum of the number of protons and neutrons present in the nucleus of the element. Every element has a unique number of protons, but the number of neutrons may vary in elements and hence, two or more elements can have the same mass number. Consider H₁ and He₂ where the subscripts denote the atomic numbers of the elements. H₁ has an isotope having mass number 3, i.e, ³H₁ (the superscript denotes the mass number). Similarly, He₂ has an isotope having mass number 2, i.e, ³He₂ (two protons and one neutron). These two elements have the same mass numbers, but offcourse, different elemental symbol, number of protons and hence, atomic numbers. Therefore, (C) is the right answer.