A closer look at the atom

This balloon is filled with a gas that makes it float — helium. But what is helium? What is it made of? Helium gas is made up of tiny particles — helium atoms. Atoms are building blocks that make up the world.

They make up gases, liquids and solids — all the matter in the world! Atoms themselves are made up of different parts too! Atoms are often pictured as models that resemble tiny solar systems, with a nucleus at the centre and electrons orbiting around it. Let’s take a closer look at the nucleus first. The nucleus is made up of two different types of particles — protons and neutrons.

Protons and neutrons have a similar mass, but protons have a positive electric charge, while neutrons have no charge at all. The number of protons determines which element an atom is. For example, all helium atoms have two protons in their nucleus — that’s how we know they are helium atoms. Atoms with the same number of protons in the nucleus make up the same element. There can be anywhere from one to 118 protons in a nucleus, so there are 118 different elements.

The number of neutrons in the nucleus of atoms of an element can vary. Take these two atoms with two protons. Both are helium atoms, but one has two neutrons, while the other has one neutron. These two atoms are two different variants of helium. Variants of the same element, that differ by the number of neutrons, are known as isotopes.

Most elements have several isotopes. So, protons and neutrons are particles that make up the nucleus of an atom. They are the heaviest parts of the atom, and their combined mass is nearly all the mass of the atom. What about the electrons? In an atom, there are always as many electrons surrounding the nucleus as there are protons inside the nucleus.

Each electron has a negative electric charge. The positively charged protons and negatively charged electrons balance each other out. This makes the overall charge of the atom neutral. In our model it looks like the electrons are just a bit smaller than the protons and neutrons. But in reality, electrons are much smaller than the particles inside the nucleus.

Their mass is so small it hardly even counts in the mass of the atom! What’s more, electrons do not follow neat orbits like our model suggests. Instead they fly in all directions, forming an electron cloud. And lastly, electrons are much further away from the nucleus than it appears in the model. Picture the nucleus as a tiny lentil placed at the centrepoint of a football stadium.

The electrons whizzing around it wouldn’t even be inside the stadium! Most of the atom is just empty space. Electrons, especially those in the outermost layers, allow different atoms to react with each other. This way, individual atoms can combine with others to form molecules. So now we know: all matter in the world is made up of tiny atoms.

Atoms themselves are made up of even tinier particles: positively charged protons, negatively charged electrons, and neutrons that have no charge. Protons and neutrons are clustered in the nucleus at the centre of the atom. The number of protons in the nucleus determines what element the atom is. And the number of neutrons tells us what variant of an element — what isotope — the atom is. Electrons circle around the nucleus, and allow atoms to combine with other atoms and to form molecules.