One of the fundamental concepts you will learn about in chemistry is atoms. An atom is the smallest particle of a chemical element that can exist, and they make up everything around us.

Since atoms are so important for all topics in chemistry, they will be one of the first things you will learn about. In early highschool, you will also get introduced to The Periodic Table. This is such a useful (and iconic) tool that is widely used by all scientists 

The Atom

As mentioned in the introduction, atoms are the smallest part of a chemical element that cannot be broken down chemically. Everything you see in the universe is made of matter, which means everything is made of atoms.

Atoms are extremely small – so small that you can’t even see them under the most powerful microscope! More specifically, they have an average radius of about 0.1 nm.

Atoms have a distinct structure and are made of three subatomic particles: protons, neutrons, and electrons.

•      Protons are positively charged particles and have a relative atomic mass of one unit.

•       Neutrons do not have a charge, and they also have a relative atomic mass of one unit.

•      Electrons are negatively charged particles and have a relative atomic mass of zero (they don’t have a mass).

Niels Bohr was a Danish Physicist (born 1885) who made significant contributions to helping to understand atomic structure and quantum theory. To acknowledge this work, he won the Nobel Prize for Physics in 1922. He described the structure of atoms as follows:

•      The centre of an atom contains a nucleus. This nucleus consists of protons and neutrons and contains almost all the mass of the atom.

•       Surrounding the nucleus are circular orbits. Each orbit is a certain distance away from the nucleus and has a specific energy level.

•       Each orbit contains several electrons. The first shell can only hold two electrons, while the second and third shell can hold up to eight electrons. 

•       The electrons in the shell that is the furthest away from the nucleus are referred to as valence electrons.

•       99.9% of an atom is empty space since the electrons are so far away from the nucleus.

It is interesting to note that since nuclei are so difficult to split, atoms can last forever!

The Periodic Table

A chemical element is a chemical substance that cannot be broken down into other substances – they are made from atoms. Each of the atoms that create each element have a distinct number of subatomic particles, which affect the element’s physical and chemical properties.

There are many different chemical elements that are found in nature, which are summarised in The Periodic Table. Dmitri Mendeleev formulated this back in 1869, which is used greatly by scientists to this day. You will be also using it a lot throughout your studies of chemistry! This table organises the chemical elements into rows (called periods) and columns (called groups) – there are 118 elements altogether! 

As you can see in the diagram below, there are 18 groups and 7 periods.

• All the elements in the same group have the same number of valence electrons.

• The period number that an element is in tells you the number of shells that the electrons will occupy. 

The Periodic Table gives vital information about an element:

• The atomic number is equal to the number of protons in the atom’s nucleus.

• The mass number tells us the number of neutrons and protons in the nucleus. You can find the number of neutrons in the nucleus by subtracting the number of protons from the mass number. Since the mass number is usually a decimal, you can round up/down to find a whole number of neutrons – you can’t have half a neutron in a nucleus!

• The number of electrons in the atom is equal to the number of protons. This is because atoms have a neutral charge.

• Since it is tedious to constantly write out the full name of an element, they have each been given a chemical symbol. These symbols contain 1-2 letters from the Latin alphabet and the first letter is always capitalised.

• You can work out whether an element is a metal or non-metal by their position on the periodic table.

• The Periodic Table organises the elements in terms of increasing atomic number. The first element is Hydrogen, and the last element is Oganesson.
   

The above diagram shows what the ‘Calcium,’ panel looks like on the Periodic Table. Calcium is the most abundant mineral in the human body, and is used to regulate blood clotting, heart rhythms and nerve functions. It is also vital for bone growth.

Calcium has an atomic number of 20, which means it has 20 protons and 20 electrons. Since its mass number is 40, it must also have 20 neutrons (since 40-20=20).

How many shells are required for the electrons? Well, the first shell will have 2 electrons, and the second and third shell will have 8 electrons. This totals up to 18, which means there must be a fourth shell with 2 electrons in it. Additionally, by looking at a Periodic Table you can see that calcium is in the fourth period, meaning that the electrons occupy 4 shells.  

Isotopes

Isotopes of an element are atoms that have the same number of protons and electrons but different numbers of neutrons. As such, the mass of the two atoms differs, and each isotope will have different chemical properties.

For example, carbon has three isotopes, which are summarised in the table below. Each isotope has an atomic number of 6, but they each have an extra neutron.

Understanding atoms and the Periodic Table is one the very first things you will do in your studies of chemistry. It is quite a fascinating topic and everything you learn in future years will build up on this knowledge. You should spend time this year understanding these concepts, and hopefully you will gain a new appreciation for the elements around you!

If you would like to download some free notes summarising this topic, you can do so here.