46 - 50 Physics SBAs for the Primary FRCA
Question 46
If two atoms have the same atomic number but a different number of neutrons, which of the following is true?
- They are different elements
- They are isotopes
- They are inotropes
- The number of protons is the same
- The number of electrons is the same
Answer
- They are isotopes
An atom contains (we think) neutrons, protons and electrons. For practical purposes, in an uncharged atom (not an ion), we can assume the following:
- Electrons carry negative charge
- Protons carry positive charge and have mass
- Neutrons have no charge but have mass
- There are the same number of protons and electrons
Molecular weight = number of protons + number of neutrons
Atomic number = number of protons = number of electrons
If the atomic number is different, then they are atoms from two different elements.
If the atomic number is the same, but the molecular weight is different, then they are isotopes of the same element.
They might be inotropes as well, but that's a completely separate issue.