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05 Secondary 3 Chemistry

Chapter 02: Atomic Structure

Secondary Chemistry Revision: Atomic Structure

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Atomic Structure is the most fundamental topic to master if you want to master Chemistry!


Let's dive straight in!


The table below shows the properties and location where the subatomic particles are found. It is important to know the properties of the subatomic particles and their location.


Particle Charge Relative Mass Location
Proton + 1 1 Nucleus of Atom
Neutron 0 1
Electron - 1 1/1840 Electron Shell / Orbital

What you see in the periodic table for an element will be denoted as:

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where a is the proton (atomic) number, b is the mass (nucleon) number and X is the chemical symbol of the element.

Proton (Atomic) Number is the number of protons the element has in the nucleus of an atom.

Mass (Nucleon) Number is defined as the total number of protons and neutrons in the nucleus of an atom.

As an atom is electrically neutral (having an overall charge of zero), the number of electrons must be equal to the number of protons in an atom.

An element only has one proton number. The number of protons gives the identity of the element. Therefore, to identify the elements of a particle (atom or ion), look at the number of protons the particle has.

A common mistake is mixing up between proton number and mass number. Therefore, when referring to the periodic table, check the legend too to see you are looking at the right number. The periodic table is your best friend in chemistry examinations apart from your calculator.

The protons and neutrons are found in the nucleus of the atom, which is surrounded by electron shell(s) containing electrons. An example of a lithium atom is shown below:

atomic structure

Question: Which statement about an atom is incorrect?

  1. Atoms of an element can have different nucleon (mass) number.
  2. If a proton is added to the nucleus of the atoms of an element, the element changes into a different element.
  3. The nuclei of all atoms contain both protons and neutrons.
  4. The proton number is always less than or equal to the nucleon (mass) number.

Explanations of the statements

  • True. Some elements have isotopes (atoms of the same element with the same proton number but different mass number).
  • True. An element only has one proton number. If the number of protons in the atom changes, the identity of the atom changes as well, according to the proton number.
  • False. Refer to the periodic table that hydrogen has a mass number of 1, and a proton number of 1. Thus, its mass number is due only that 1 proton, and 0 neutrons.
  • True, as shown in the periodic table. Except for hydrogen where the proton number is equal to its mass number, the rest of the elements has their proton number lower than their respective mass number.


Isotopes are atoms of the same element carrying the same proton number but a different mass number. Thus, they have a different number of neutrons in the nucleus of the isotopes.

For example, carbon is made up of isotopes of carbon-12, carbon-13 and carbon-14. The relative atomic mass of carbon in the periodic table is the average mass of all the carbon isotopes according to their relative abundance in nature.

Question: Do isotopes of the same element have different reactions?

The answer is no, they have the same reactions. This is because reactivity is dependent on the number of valence electrons. For isotopes, they have the same proton number, thus they have the same number of electrons, and hence, the same chemical reactivity.

Question: Deuterium,
, is an isotope of hydrogen. Which statement about deuterium is false?

  1. Its diatomic molecules diffuse at a faster rate than that of hydrogen.
  2. It has a higher density than hydrogen.
  3. It has the same number of protons but a different number of neutrons as hydrogen.
  4. It undergoes the same chemical reactions as hydrogen.

Explanation of the statements

  • False. Deuterium is heavier than hydrogen. As the speed of diffusion decreases with the increasing mass of gas, deuterium molecules diffuse slower than hydrogen.
  • True. As density equals mass/volume, for the same volume of deuterium and hydrogen, the density of deuterium is higher than hydrogen as it is heavier.
  • True. The definition of isotopes are atoms of the same element with the same proton number but a different mass number, and thus a different number of neutrons.
  • True, isotopes undergo the same chemical reactions.

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