Mass Number & Isotopes (Oxford AQA International A Level Chemistry)

The Composition of an Atom

Atomic number

• Atoms consist of protons, neutrons and electrons

• The atomic number (or proton number) is the number of protons in the nucleus of an atom

  • The symbol for this number is Z

• It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table

• The proton number is unique to each element, so no two elements have the same number of protons

• Electrons can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction

Mass number

• The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom

  • The symbol for this number is A

• The atomic number and mass number for every element is on the periodic table

Atomic number & Mass number diagram

Determining the number of protons, neutrons and electrons

• To calculate the number of protons, neutrons and electrons in an atom:

  • Number of protons= the atomic number

  • Number of neutrons= mass number - atomic number

  • Number of electrons= the atomic number

• To calculate the number of protons, neutrons and electrons in an ion:

  • Number of protons= the atomic number

  • Number of neutrons= mass number - atomic number

  • Number of electrons

    • The charge on the ion indicates the number of electrons that have been lost or gained

    • E.g. If the charge of an ion is 2+, two electrons have been lost but if it is 1- it has gained one

What are isotopes?

• Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons

• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number

  • E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively

  Isotopes of hydrogen

Time of Flight Mass Spectrometry

• Mass spectrometry is a powerful analytical technique

  • It is used to determine the relative atomic mass, A_r, of an element, based on the abundance and mass of each of its isotopes

  • Knowing the relative atomic mass allows the element to be identified

  • It is also used to find the relative molecular mass of molecules

• There are several types of mass spectrometer but they all work in the same way

  • The sample is ionised

  • It is then accelerated through the mass spectrum

  • The ions are separated based on the ratio of their charge to their mass

How does TOF mass spectrometry work?

• Time of flight mass spectrometry is a common form of mass spectrometry

• There are 4 key stages:

  • Ionisation

  • Acceleration

  • Ion drift

  • Detection

Ionisation

• The sample is dissolved in a volatile solvent

• The solvent is injected into the mass spectrometer using a fine hollow needle to create a mist

• The needle is attached to a high voltage power supply, so as the sample is injected, the particles are ionised by losing electrons

X (g) → X⁺ (g) + e^-

• Most of the positive ions will have a 1+ charge as it is difficult to remove further electrons

• The solvent evaporates until the mist contains only positively charged ions

Ionisation diagram

Acceleration

• The positively charged ions are attracted towards a negatively charged plate

• They accelerate towards it using an electric field

• This ensures all of the positive ions have the same kinetic energy

• Since all the positive ions will have the same kinetic energy, their velocity will depend on their mass

  • Lighter ions will move faster and heavier ions will move slower

Ion Drift

• The positively charged ions will pass through a hole in the negatively charged plate and move into a flight tube

• This is where the name 'Time of Flight' comes from

• The time of flight of each ion in this tube depends on their velocity

Detection

• When the ions have passed through the flight tube they hit a negatively charged 'detector' plate

• As they hit this electric plate, they gain an electron

• This gaining of an electron causes a current to be produced

  • The size of the current is proportional to the abundance of those ions hitting the plate and gaining an electron

• Lighter ions will reach the detector first followed by heavier ions

• The signal from the detector is transferred to a computer, which produces the mass spectrum

TOF spectrometry process

Interpreting Mass Spectra

• Mass spectroscopy can be used to find the relative abundance of isotopes experimentally

• The relative abundance of an isotope is the proportion of one particular isotope in a mixture of isotopes found in nature

  • For example, the relative abundance of Cl-35 and Cl-37 is 75% and 25% respectively

  • This means that in nature, 75% of the chlorine atoms is the Cl-35 isotope and 25% is the Cl-37 isotope

• The heights of the peaks in mass spectroscopy show the abundance of each isotope present

• The horizontal axis gives the mass to charge ratio, m/z

• For ions with a single charge, the m/z ratio and mass number are the same

The mass spectrum of boron

• Looking at the mass spectrum of boron:

  • There are two peaks so there are two isotopes of boron

  • One isotope has a mass number of 10, and the other 11

  • The relative abundance of B-10 is 19.9%

  • The relative abundance of B-11 is 80.1 %

• The data from mass spectrometry can be used to determine the relative atomic mass of an element

  1. For each isotope multiply the relative abundance by the m/z ratio

  2. Add these values together and divide by 100