[Unit 1.2] | Mass Number, Atomic Number & Isotopes

Question 1

Define Atomic Number.

Answer 1

The number of protons present within an atom. Within a neutrally charged atom it also determines the number of electrons present.

Question 2

Define Mass Number.

Answer 2

• The number of protons and neutrons in an atom.
• Electrons are not included due to their tiny mass.
• To calculate the number of neutrons in an atom, subtract the atomic number from the mass number.

Question 3

What is an isotope?

Answer 3

• An atom with the same number of protons and electrons but a different number of neutrons to what would be expected (and therefore a different mass number).
• Isotopes of elements are chemically similar as their electron configurations do not differ.

Question 4

Why can Carbon-14 be used to determine the age of carbon-based materials up to 60,000 years old?

Answer 4

• It is a radioactive / unstable isotope present in all organisms.
• Based on the number of undecayed nuclei present, the age can be determined using its half-life of 5730 years.