Lesson Explainer: The Structure of Atoms Chemistry • First Year of Secondary School

In this explainer, we will learn how to describe the structure of the atom and what atoms are composed of and relate their size to everyday objects.

Humans have been wondering what the world is made of for thousands of years. Democritus was an ancient Greek philosopher who proposed that the world was made of indivisible atoms about two and a years ago. Scientists have been progressively refining this idea and have now reached a point where we have a clearer understanding of what an atom is and how an atom should be described.

Atoms can be considered to be fundamental units of chemical matter because they make up all of the chemical elements. Atoms are about one ten-thousandth of a millionth of a metre. This statement could be reworded to say that the average atom is about 0.0000000001 m or m wide. An atom is approximately 10‎ ‎000‎ ‎000 times smaller than a single grain of sand and approximately 1‎ ‎000‎ ‎000‎ ‎000 times smaller than a medium-sized orange.

The following image shows why orange-sized objects have to be magnified 100 million times before you can see any atoms. The image on the left shows an orange that is about 100 mm or 10 cm wide. The panels that follow show the effect of magnifying the two stones next to the orange and the cockroach crawling across them, again and again, until we can see single atoms.

By the time we reach the scale of the cockroach antennae, we will need to use units of micrometres, represented by μm. 1 μm is equivalent to m; thus, one micrometre is a millionth of a metre. At the scale of atoms and molecules, we need to use nanometres, represented by nm. One nanometre is a billionth of a metre and so is equivalent to m. This is very small indeed.

The atoms that make up the pollen on the antenna of a cockroach can be quite complex, and it is better to focus on simpler atoms such as hydrogen and helium if we want to understand the basic properties and parts of individual atoms. The following image shows the structure of hydrogen and helium atoms. The atoms both contain a positively charged nucleus. The nucleus is made up of some combination of neutrons and protons. The atoms also contain electrons that are found in the space outside of the positively charged nucleus.

Each electron has a negative electrostatic charge that exactly balances the positive electrostatic charge of one proton. This statement could be rewritten to say that protons have a relative electrostatic charge of and electrons have a relative electrostatic charge of . Neutrons are neutrally charged particles, and they always have an electrostatic charge value of 0. The atoms of any one element always have the same number of positively charged protons and negatively charged electrons. Because individual atoms contain an equal number of positively charged protons and negatively charged electrons, they are electrically neutral.

Protons and neutrons have essentially the same mass, and they are both much heavier than a single electron. Protons and neutrons have a relative mass of 1, and electrons have a relative mass of . The following table summarizes data about the subatomic particles that make up atoms.

Mass is distributed very unevenly in atoms. The atomic nucleus is just one small part of the atom, but it contains all of the heavy subatomic protons and neutrons. The rest of the atom is made up of incredibly light electrons.

Many atomic structure illustrations provide inaccurate information about the size of the nucleus. They give the false impression that the nucleus is just a few times smaller than an atom. The illustrations are designed to show the composition of an atom. They are used to show what the atom and its nucleus contain rather than to accurately represent the size of the nucleus and its subatomic particles. The following figure is a reasonably accurate representation of the atom. It uses a combination of arrows and numbers to compare the width of the nucleus with the width of the whole atom and its individual nucleons. It also compares the width of the electron with the width of the atom.

The diagram shows that the nucleus is roughly ten thousand times smaller than the atom. It also shows that protons are roughly ten times smaller than the nucleus or one hundred thousand times smaller than the whole atom. Electrons are shown to be about one thousand times smaller than a single proton, but this is nothing more than an estimated value. It is possible that electrons are even smaller than one-billionth of a billionth of a metre.

Let us consider a carbon atom and its nucleus. The comparison will help us to fully appreciate the incredibly small size of the atomic nucleus and its nucleons. It will also help to confirm the claim that the nucleus is orders of magnitudes smaller than the atom. The carbon atom has a radius of 70 pm, and its nucleus has a radius of 3 fm. These are incredibly small scales. A picometre (pm) is equivalent to m. In other words, one picometre is a thousand billion times smaller than a metre. A femtometre (fm) is even smaller. One femtometre is equivalent to m; this is a million billion times smaller than one metre. To put that into perspective, a femtometre is equal to 0.000000000000001 m. These values can be compared to show that the radius of the carbon atom is tens of thousands of times smaller than the radius of the whole carbon atom. They can be compared to show that nuclei are indeed tens of thousands of times smaller than whole atoms. The atomic nucleus is mind-bogglingly small, and it is amazingly made up of even smaller subatomic nucleons. The carbon atom nucleus has a radius of 3 fm, and protons have a radius that is much smaller than a single femtometre (1 fm).