2.2.3/4 - atomic masses

Question 1

What are isotopes?

Answer 1

Isotopes are atoms of the same element that have the same number of protons (and electrons) but a different number of neutrons. They have different masses.

Question 2

Why do different isotopes of the same element react in the same way?

Answer 2

• Although isotopes have different numbers of neutrons, they have the same number and arrangement of electrons.
• Only the electrons are involved in chemical reactions, so both isotopes will react in the same way.
• Neutrons make no difference to chemical reactivity.

Question 3

Define relative atomic mass, Ar

Answer 3

Relative atomic mass is the average mass per atom of the naturally occurring form of an element relative to 1/12th of the mass of carbon.

Question 4

Why is the carbon-12 isotope the international standard for the measurement of relative mass?

Answer 4

An atom of the carbon-12 isotope is made up of 6 protons, 6 neutrons and 6 electrons. The mass of an atom of carbon-12 is 12 atomic mass units, 12u. Carbon was also readily available for scientists to use.

Question 5

Define relative isotopic mass

Answer 5

Relative isotopic mass is the mass of an atom of an isotope relative to 1/12th of the mass of a Carbon-12 atom.

For an isotope, the relative isotopic mass is the same as the mass number.

Question 6

When suggesting that the relative isotopic mass of oxygen-16 is 16 and of sodium-23 is 23, what two assumptions have been made?

Answer 6

• the tiny contribution that electrons make to the mass of the atom has been neglected
• the masses of a proton and a neutron are both 1 atomic mass unit (1u)

Question 7

What does the contribution made by an isotope to the overall mass of the element depend on?

Answer 7

• the % abundance of the isotope
• the relative mass of the isotope

These contributions are combined to get the relative atomic mass of the element.

Question 8

What is the equation for calculating the relative atomic mass of an element from it’s isotopes?

Answer 8

{ abundances of isotopes x isotopic masses of isotopes

divided by 100

Question 9

Define relative molecular mass, Mr

Answer 9

Relative molecular mass is the sum of the relative atomic masses of the constituent atoms of a molecule relative to 1/12th of the mass of a carbon-12 atom.

This can be found by adding together the Ar’s of each atom making up a molecule.

Question 10

Why is the relative formula mass sometimes used rather than the relative molecular mass?

Answer 10

Compounds with giant structures do not exist as molecules, so finding the relative molecular mass would mean adding the masses of thousands of atoms together. Instead we use the relative formula mass, which can be found by adding the Ar’s of each atom making up a formula unit.

Question 11

What is mass spectrometry used for?

Answer 11

• to identify an unknown compound
• to find the relative abundance of each isotope of an element
• to determine structural information about molecules

Question 12

How does a mass spectrometer determine the mass of a molecule or isotope?

Answer 12

It measures the mass-to-charge ratio of ions by causing substances to become positive ions which are then passed through the mass spectrometer and separated according to their mass and charge. A computer within the mass spectrometer analyses the data on the ions present and produces a mass spectrum.

Question 13

Define empirical formula

Answer 13

The empirical formula is the formula showing the simplest whole number ratio of atoms of each element present in a compound.

Question 14

Define molecular formula

Answer 14

The molecular formula is the formula showing the actual number of atoms of each element in one molecule.

Question 15

Define stoichiometry

Answer 15

Stoichiometry is the molar relationship between the relative quantities of substances taking part in a reaction.