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  1. Describe the structure of an atom in terms of electrons and a nucleus containing protons and neutrons.

We’ve learned that all substances in the world are made up of atoms. But what’s inside these atoms? Let’s take a look.

At the centre of the atom is the nucleus. The nucleus is made up of protons and neutrons. Protons and neutrons are collectively called nucleons. So, the nucleus contains nucleons!

Outside, the nucleus, particles called electrons around electron shells/energy levels (the ‘orbits’ on which they whiz around). Each shell contain a certain level of electrons depending on the element.

Protons, neutrons and electrons are called subatomic particles because they make up any atom.


  1. Describe the build-up of electrons in ‘shells’ and understand the significance of the noble gas electronic structures and of valency electrons.

The first shell is nearest to the nucleus and can only contain a maximum of two electrons. All of the following shells can hold a maximum of eight electrons.

The arrangement of the electrons in shells is called the electronic structure or electronic configuration. In a sodium atom, there are two electrons in the first shell, eight electrons in the second and one in the third. This electronic configuration can be written as 2,8,1.

The number of electrons in an atom’s outer shell are called valence electrons. Sodium, has 1 valence electron. Valence electrons are important , as it determines whether there will be a reaction between particular elements and if yes, what products will be formed as a result. Chemical reactions occur when atoms’ gain or lose electrons- this will be explained in the next topic.

This is a section of the periodic table, the table that contains all the elements that have been discovered by humans.

The last column is called group 0 and the elements in it are called noble gases. They have valence electrons that are the maximum number of electrons that can go into their shells (‘He’ has one shell and so has 2 valence electrons, and the rest of the gases in the column have two or more shells, so they have 8 valence electrons), meaning they have full outer shells of electrons. This electronic structure is called stable, because they cannot lose or gain electrons to take part in chemical reactions. Thus noble gases do not combine with other elements to form compounds.


  1. State the relative charges and approximate relative masses of protons, neutrons and electrons.

A proton (p) has a relative mass of 1 and a positive (+1) charge

A neutron (n) has a relative mass of 1 and has no charge

An electron (e) has a negligible (about 0.00054) relative mass and a negative (-1) charge.


Once again, look at the atom.  Most of the atom’s weight is in the nucleus, since electrons weigh hardly anything.

The no.of protons and electrons in an atom will always be equal, that means that the no. of positive and negative charges in an atom will be equal- thus, the atom has no overall charge.


  1. Define atomic (proton) number and mass (nucleon) number.

Atomic number or proton number is the number of protons in the nucleus (or the electrons since they’re equal in number!). Every atom of the same element has the same number. Hydrogen has one proton, so every atom of hydrogen will have a proton number of 1.

Mass number of nucleon number is the number of nucleons (no. of protons and neutrons) in the nucleus. Sine the electrons have next to no weight, only the protons and neutrons are considered when calculating the mass of the atom.

  1. Use atomic (proton) number and the simple structure of atoms to explain the basis of the Periodic Table, with special reference to the elements with atomic (proton) numbers 1 to 20.

In the periodic table, elements are arranged in order of atomic number.

The elements in each column have the same number of valence electrons too (check the periodic table below to see if it’s true!).

Here are the first 20 elements in the periodic table, that you need to learn, with their respective proton numbers and electronic configurations.


  1. Define isotope

We have learned that the protons, neutrons and electrons of atoms of the same element will be equal. Every sodium atom has 11 protons, 12 neutrons and 11 electrons. (To calculate no. of neutrons you can subtract the mass number (23) from the atomic number(11)). Isotopes are atoms of the same element with the same number of protons but different number of neutrons. Sodium has an isotope with 13 neutrons instead of the normal 12.



Notes submitted by Lintha

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8 thoughts on “C3.3 – Atomic Structure and the Periodic Table

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