IBDP Chemistry - The Nuclear Atom

In this topic of IBDP Chemistry, we will learn about the nuclear atom, mass spectrometry and relative atomic mass.

The Nuclear Atom

Atoms contains protons, neutrons and electrons.

• The mass number (the larger number) contains the number of protons and neutrons.
• The atomic number (the smaller number) contains the number of protons only.
• To find the number of neutrons, subtract the atomic number from the mass number.

Isotopes are different atoms of the same element with different mass numbers (different number of neutrons)

• Isotopes have the same chemical properties (react in the same way) as they have the same number of electrons.
• But they have different physcial properties (e.g. different melting and boiling points) as they have different masses and move at different speeds.

Radioisotopes are radioactive isotopes of an element. Some radioisotopes you need to know for IB Chemistry include:

• ¹⁴C is used in radiocarbon dating to date fossils.
• ⁶⁰Co is used in radiotherapy to destroy cancerous cells with gamma rays.
• ¹³¹I and ¹²⁵I are used as medical tracers to generate pictures of the activity of the thyroid gland.

Mass Spectrometry

A mass spectrometer separates particles based on their mass. There are 5 steps in separating and identifying different particles using a mass spectrometer.

• Vaporisation - heat to turn the substance/sample into a gas
• Ionisation - bombarding the gaseous particles with high energy electrons to knock out an electron to form a positive ion
• Acceleration - accelerate the particles through the spectrometer using an electrical field.
• Deflection - deflecting the particles by passing them through a magnetic field.
• The amount of deflection depends on both the mass and charge of the ion.
• The smaller the charge, the greater the deflection.
• The lighter the ion, the greater the deflection.
• Detection - the abundance of each type of ion is detected based on their deflection and the place at which they reach the detector.

A mass spectrometer allows us to identify the abundances of different isotopes of a particular element, which can allow us to calculate the relative atomic mass of the element.

Relative Atomic Mass

The Relative Atomic Mass of an element is defined as the average mass of the naturally occuring isotopes of an element, relative to the mass of 1/12 of an atom of carbon-12.

In IB Chemistry, you will be expected to be able to calculate the relative atomic mass of an element (given the percentage abundance of each isotope) as well as the natural abundance of each isotopes of an element (given the relative atomic mass).

To calculate RAM: Multiply the mass and the abundance of each isotope, add them together and divide by 100.

To calculate the Natural Abundance of different isotopes of an element: Use the same equation as above, but use x to represent the percentage abundance of one isotope, and 100-x for the percentage abundance of the other isotope. This over 100 = the average atomic mass that they should give you. Then rearrange this to find x (i.e. the percentage abundance of the first isotope). Subtract x from 100 to find the abundance ofthe other isotope.

This is the end of this topic

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