CNC Illustration by Gwendolyn Say (@saydrawings)

1. The electron was the first subatomic particle discovered

Like how COVID has changed our world, something even smaller changed the way we see the world around a century ago. It was the discovery of the electron by Joseph John Thomson, a discovery deemed so revolutionary that it was declared “the new theory of matter” by The Straits Times in 1907.

Back then when there was no TikTok, scientists spent their time thinking about what electricity is. What flows in an electrical wire? To answer this question, scientists used the cathode ray tube to force electricity out of the wire and into a vacuum. In the vacuum, the electricity glowed like a ghostly ray. Not to FOMO, Thomson did the same but distinguished himself with two brilliant observations:

  1. A positively charged plate attracted the ray, suggesting that it contained negatively charged particles
  2. The attraction curved the path of the ray significantly, suggesting that the particles were extremely light

The particles were given the name electron. Since an electron is so much lighter than even the lightest atom, it was assumed to be a part of the atom. Scientists realised that it is a subatomic particle that makes up every atom!

Electrons are negatively charged subatomic particles.

2. An electron is so light that we ignore its mass

subatomic particle relative mass
electron very small
proton 1
neutron 1

If your plate of nasi lemak were a hydrogen atom, an electron would have the mass of a grain of rice. Remove the grain and you would barely notice a change in mass.

In the same vein, we ignore the mass of electrons as it is negligible compared to the mass of protons and neutrons.

3. An atom has as many electrons as protons

We know that atoms are electrically neutral. So the negative charges of the electrons must be cancelled out by the positive charges of the protons.

Mysteriously, nature has made it extremely convenient for us: an electron’s negative charge is the direct opposite of a proton’s positive charge. Therefore, the number of electrons in an atom is simply the number of protons.

In an atom: number of electrons = proton number

4. Electrons move in ways we know not

While the other subatomic particles are packed into a nucleus, electrons are spread across the vast empty space around the nucleus. Early scientists imagined that the tiny electrons orbit the nucleus like how the Earth orbits the Sun.

However, modern physics with the funky name of quantum mechanics has this theory called the uncertainty principle. It tells us that we cannot simultaneously pinpoint the position and the movement of an electron. Simply put, we can never know exactly how an electron moves, unlike how we can trace the movement of the Earth around the Sun.

5. But we can do a vibe check to know an electron’s energy level

The arrangement of electrons in a lithium atom

Scientists can measure the energy level of electrons using spectroscopy, which is a fancy name for a technique that analyses the colours of light.

From the measurement, we now know that electrons in an atom have different energy levels. Those with the least energy are usually found in the region of space closest to the nucleus. We can think of this region as the first shell around the nucleus.

In contrast, the outermost shell furthest away from the nucleus has the highest energy level. It is sometimes called the valence shell.

Electrons are arranged in shells. The first shell has the lowest energy level and is the closest to the nucleus.

6. Three is a crowd for the first shell

electron shell maximum number
first 2
second 8
third 8 and more

Electrons are lazy: they occupy the first shell with the lowest energy level if they can. For example, hydrogen atom has a single electron and it is found in the first shell. Its bigger counterpart, the helium atom, has 2 electrons and they are both found in the first shell.

But no crowding please! The first shell is the smallest and it can only hold up to 2 electrons. So in lithium, which has 3 electrons, the odd one out will be found in the second shell.

The second shell is bigger with enough room for 8 electrons. And the third shell is even more so. For potassium and calcium, the third shell can hold a maximum of 8 electrons. However, heavier atoms can hold a maximum of 18 electrons in the third shell.

Electrons prefer to occupy a lower energy shell, unless it is fully filled.

7. Electronic structure sounds cheem, but we draw it simply

The electronic structure of the first 11 electrons

The arrangement of electrons into shells with different energy levels is called the electronic structure. We can describe it by drawing a simplified diagram, showing the shells as circles and the electrons as dots.

This diagram elegantly sums up what we know about the electrons: filling up different shells outside the nucleus from the lowest energy level upward. But we have to be careful not to read too much into it. The circles do not suggest that the electrons orbit around the nucleus in a fixed path, for that is something we can never know.