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Atomic Structures & Electronic Configurations

Basics such as charges and masses of subatomic particles will be found in the physics section. For more details go to our Physics section

Basic Definitions

An atom is the smallest unit which can exist on its own and take part in a chemical reaction. It is also neutral in charge

The atom is made out of 3 Subatomic particles:

1. Neutron

2. Proton

3. Electron

We will see the charges and the relative masses of these subatomic particles

Name Charge A.M.U
Neutron 0 1
Proton +e 1
Electron -e 1/1836
Note that the exact fraction must be written for the electron

You will need to know some basic terms of an atom

Let us first see the structure of an atom

structure of an atom with electrons and nucleus. A labelled atom


It contains both neutrons and protons of the atom and so it's positively charged. Why? This is because, neutrons are neutral and protons are positively charged. The nucleus is very small and highly densed and is at the center of the atom.


These are known to be fundamental particles which orbit around the nucleus of an atom in energy levels. If you need to know more go to the leptons section.

The Key

A period key to identify the atomic number and mass number of an atom

Atomic Number ( Z )

The total number of protons in the nucleus of an atom

The proton number of an atom identifies the element. This is a fundamental fact and this proves that isotopes are of the same element

Mass Number ( A )

The total number of protons and neutrons in the nucleus of an atom

It is the same as the nucleon number

Particles in the nucleus of an atom are called nucleons. So if there are 7 protons and 8 neutrons, it has 15 nucleons.


Is a special combination of the number of neutrons and protons in the nucleus

For example the Na - 23 and the Na - 24 Have two different nuclides

Deflection of Subatomic particles in Magnetic fields

deflections in a magnetic field

For you to predict its motion, you need to fleming left hand rule

Imagine the Proton is like the current and the Electron is like the opposite of current. The Neutron is uncharged and so it is undeflected

This is not required in the Chemistry Alevel Syllabus but, you can refer it more here under the Physics Section

Deflection of Subatomic Particles in Electric Fields

deflections in a electric field

Remember that the Electron is deflected towards the positive plate and the Proton is deflected towards negative plate. The Neutron is uncharged and so not deflected

There is another important point you need to remember!

The Electron is deflected by a greater angle than the proton

To identify the amount of deflection, we need to use the mass-charge ratio

By using the mass to charge ratio, the proton has 1836 times greater mass to charge ratio than the electron. So it means it is deflected less by 1836

In other words, if both are moving at the same initial speed. The proton will move 1836 times futher than the electron

In summary, comment about each subatomic particle and the difference in deflections

Electron Configurations

We need to know that electrons are held in energy levels called principal quantum shells

The energy level which is the most closest has the lowest energy

Energy levels are divided into subshells S, P, D or F

Each type of subshell contain different number of Orbitals

An orbital is the lowest category and it is a region of space which can be occupied by either 1 or 2 Electrons only

We will see the characteristics of each subshell

Subshell Shape Number of Orbitals
S Spherical 1
P Dumbell shape 3
D - 5
F - 7

You only need to know the shapes of the S and P Orbital

So for the first principal Quantum shell, there is only one S orbital. So the first energy level can hold only two electrons

The second principal Quantum shell has an S and a P Subshell which can carry an overall of 8 electrons

The 3rd principal Quantum shell has an S, P and D subshell which can carry an overall of 18 electrons

Configuration Syntax

When electron number increases (successive elements) the electrons are added from the lowest principle quantum shell to the highest

Here is the electronic configuration for the Helium atom which has 2 electrons


The general syntax is:

1S2 2S2 2P6 3S2 3P6

This is the electronic configuration of Argon

The power tells us how many electrons are in each the subshell

The 1S tells us it is the subshell S of the first principle Quantum shell

As you can see the electrons are filled in a specific order but, we will see some exceptions for more larger elements

For elements above argon, you might expect the 3d Subshell to be filled but, actually the 4S Subshell is filled first! Why?

This is because, the 4S subshell is at a lower energy level than the 3d Subshell even though the 4S is at a higher principal quantum shell. So elctrons are first filled in the 4S Subshell before the 3P subshell

Now we will see the electronic configurations of elements above Argon

K - 1S2 2S2 2P6 3S2 3P6 3D0 4S1
Ca - 1S2 2S2 2P6 3S2 3P6 3D0 4S2
Sc - 1S2 2S2 2P6 3S2 3P6 3D1 4S2
Ti - 1S2 2S2 2P6 3S2 3P6 3D2 4S2
The pattern of electronic configuration goes on

Note how we still write 3d. This is because when writing the electronic configuration it is better to use this format especially when you need to find the electronic configuration of ions

Some Harder Exceptions

The electronic configuration increases in the normal way but you need to know two elements which does not follow the normal rule

Chromium (Cr) - 24

Chromium has 24 electrons and it has a slightly different electronic configuration

If we follow the normal rule, the chromium must have a configuration of 1S2 2S2 2P6 3S2 3P6 3D4 4S2.

But this is not the case, an electron from the 4S subshell is carried to the 3D subshell. To reduce the repulsion forces between the electrons and make it more stable

So actually Chromium has an electronic configuration of:

1S2 2S2 2P6 3S2 3P6 3D5 4S1

So in this all orbitals in the 3D subshell are half filled!

The next element fills the electron from 4s again

There is another exception you need to know

Copper also follows a similar rule

Copper has 29 electrons and so the if we follow the normal rule it will be like this:

1S2 2S2 2P6 3S2 3P6 3D9 4S2

This is also not the case and so the electron in the 4S subshell fills the D Subshell

1S2 2S2 2P6 3S2 3P6 3D10 4S1

These are the main exceptions you need to know for the Cambridge Chemistry Alevels Syllabus

Patterns in the Periodic table

We will discuss how to identify the electronic configuration of the elements depending on its position in the periodic table

If an element is in the S block then it will end with an S , the period number will tell us the principle Quantum shell

If an element is in the P block then it will end with an P , the period number will tell us the principle Quantum shell.

Halogens always end with P6 , the period number will tell us the principle Quantum shell

Transition metals end with the D subshell

The group 5 elements always have an half filled P subshell of P3. This is an important rule to remember!

Hund's Law

When electrons are filled to a subshell, one electron is filled to each orbital first to reduce the repulsion forces between electrons and to make it stable

This is seen when we draw the boxes. This shows that only one electron is filled for each orbital instead of 2 being filled at once

Hunds law diagram of electrons

Electronic Configurations of Ions

For anions, it is easy because we need to add the electrons in the normal way. It will follow the same way as the larger element's electronic configurations

For example, S2- has 16+2 electrons. So it follow the same electronic configuration as chlorine

But for cations it is a bit different. Some might think that we need to remove the electrons from the Outer most shell. For example, some might think that we need to remove electrons first from the 3d subshell then the 4s

This is not the case and that's why we followed the normal way of writing the electronic configuration

For example, the Copper atom has a configuration:

1S2 2S2 2P6 3S2 3P6 3D10 4S1

The Cu+ ion will have the configuration:

1S2 2S2 2P6 3S2 3P6 3D10 4S0

The electronic configuration of Cu2+ is:

1S2 2S2 2P6 3S2 3P6 3D9 

You don't need to know why but, just remember this pattern

Quick Representations

Some questions ask you to just give the electronic configuration of an ion or an atom. In this case, we can use the short rule

Let us take an example:

The electronic configuration of Copper is:

[Ar] 3D10 4S1

You can replace part of it with the closest noble gas like this but, we don't recommend you to use this method.

If they ask you to find the full electronic configuration then you need to write the full thing!

End of Chapter Questions
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