The Standard Model Part 2: Enter the Atom

Visual of an atom. Source: ArsTechnica

It’s commonly known that everything is made of atoms, but what does that really mean? What are atoms made of? How small are they?

In part one, (which I recommend you read before this) we went as small as we could go into your chosen object, and classified all particles into two groups: hadrons and leptons. But what is the next thing we would see if we zoomed out from your object? The answer is protons and neutrons. We learned that quarks make up protons and neutrons, but what are they?

In 1920, Earnest Rutherford, a New Zealand physicist, suggested the name “proton” for the positively charged particles of the atom. He also hypothesized there to be a neutral particle residing within the nucleus, which British physicist James Chadwick, a student of Rutherford’s, was able to prove in 1932.

Protons and neutrons are particles found within the nuclei, or center, of atoms. The quarks within protons and neutrons are held together by the strong interaction. The strong interaction is another one of the fundamental forces. It is carried by gluons, which are massless force-carrying particles. More later in the article.

For many years it was believed that protons and neutrons were elementary particles. It wasn’t until 1968 that the quark was discovered and disproved that idea. Today, as mentioned above, quarks are thought to be indivisible.

Quarks in protons and neutrons. Source: Hyper Physics


  • 1 down quark
  • 2 up quarks


  • 2 down quarks
  • 1 up quark

A down quark has a charge of -1/3, while an up quark has a charge of +2/3. Protons have a positive charge because they have 1 down quark and 2 up quarks, meaning the charge is -1/3 + 2/3 + 2/3, resulting in the overall charge being +1. Neutrons have a neutral charge because they contain 2 down quarks and 1 up quark, making the charge -1/3 + -1/3 + 2/3, resulting in the charge being 0.

Though protons and neutrons differ in charge, they have approximately the same mass. When a proton is added to an atom, a new element is created, whereas when a neutron is added, an isotope is created. An isotope is a heavier version of the given atom.

The amount of protons residing in an atom is unique to each element. For example, hydrogen atoms have one proton, helium atoms have two, lithium atoms have three, and so on. The chemical behavior of an element is determined by the amount of protons, and the atomic number of an element corresponds with the number of protons in that atom. All elements proven to exist are displayed on the Periodic Table of Elements in order of atomic number. Does this ring a bell from high school chemistry? Perhaps it at least sounds vaguely familiar…

The Periodic Table of Elements. Source: Science Notes

So, where are the protons and neutrons in your object? They are contained within the nucleus. In 1911 the nucleus was proven to exist by Ernest Rutherford. The nucleus is the dense, positively charged, central core of an atom, and it is responsible for most of an atom’s mass. We cannot conceptualize how dense the nucleus truly is. To put things into perspective, if you were able to put together enough nuclei to fill a standard matchbox with the dimensions of 4 cm by 5 cm by 1.5 cm., the matchbox would have a mass of almost 7 billion tons.

How is the nucleus held together? Electromagnetism carried by photons binds together the negatively charged electrons outside the nucleus with the positively charged protons inside the nucleus. This is because they have opposite charges. However, only protons and neutrons reside in the nucleus. Neutrons have a neutral charge, which means the positive charges of the protons should be pushing themselves away from each other.

This is a very interesting dilemma. The electromagnetic force does create repulsion between like charges, so it should make protons repel each other and blow the nucleus apart. However, two of the four fundamental forces, the strong interaction and the weak interaction, are also active within atomic nuclei.

The electromagnetic force dominates at macroscopic distances, while the strong and weak forces work within the subatomic world, and in this case, within the nucleus. The strong and weak forces don’t have effects at distances larger than a few femtometers. To put this into perspective, one femtometer is a millionth of a billionth of a meter, which is 100,000,000,000 (100 billion) times smaller than the width of a hair.

Additionally, the strong force is 137 times more powerful than the electromagnetic force, and binds protons and neutrons together within the nucleus. The strong force overpowers the electromagnetic repulsion between protons. The weak force plays a less but still vital role in nuclear processes. More about the four fundamental forces later.

We’ve covered what’s inside the atomic nuclei, and what’s inside protons and neutrons, but what else is in an atom? In 1897, British physicist, Joseph John (J.J.) Thomson discovered the electron. Electrons carry a negative charge and are therefore electrically attracted to the positively charged protons. Electrons are extremely lightweight and the electron cloud which they orbit within has a radius 10,000 times greater than the nucleus. The pathways in which they move around the nucleus are referred to as orbitals. This idea was proposed by Austrian physicist, Erwin Schrödinger in the 1920s. Today, this model is called the quantum model or the electron cloud model.

Today, the electron is understood to be an elementary particle, unable to be broken into smaller pieces. It was the first particle in the Standard Model to be discovered.

Visuals of electrons in atoms. Source: Northwestern University

Illustrations of particles in general can be misleading, especially those of electrons. They often portray particles as spherical and having definite locations, whereas this is not true.

The most accurate description for what is outside the nucleus where electrons are located is a cloud of probability. In fact, electrons don’t really “orbit” the nucleus in the way that planets orbit their host star, so the term orbital is very misleading. Instead, orbital refers to the most probable region where an electron is to be found. Instead of saying that electrons move in pathways, it’s more correct to refer to electrons moving within a surrounding fog around the nucleus. Electrons do not exist in any one place. Rather, they have a certain probability of being in one location as opposed to another. The illustration above shows this by light and dark shading: darker means more probable.

It makes sense that if a particle exists in space, we should be able to point to where it is; it’s difficult to grasp the idea of not being able to know where exactly an electron is at a given moment. However, due to wave-particle duality and the Heisenberg Uncertainty Principle, we can never know multiple things about a particle simultaneously. Not being able to determine the exact position and momentum of an electron at the same time can be accredited to the Uncertainty Principle.

Due to the wave-like nature of a particle, the electron is spread out over space. This simply means that there is not a precise location that the electron occupies. Instead, it occupies a range of positions. The momentum cannot be precisely known for the same reason. A particle consists of a packet of waves, each of which has a unique momentum. It can be said that a particle has a range of momentum.

“When people say things like: ‘the electron does not have a definite position,’ they are not positing something without evidence, they are trying to communicate something that is empirically supported but also very hard to articulate without mathematics.” -Forbes

Protons and neutrons have nearly the same mass while electrons are extremely tiny, over 1,800 times smaller than either a proton or a neutron. By means of comparison, if we say that a neutron had a mass of 1, then the relative masses of a proton and electron would be:

Neutron = 1
Proton = 0.99862349
Electron = 0.00054386734

Protons and neutrons and electrons are the three main parts of an atom, which is the smallest unit of any element in the periodic table. Atoms are the basic units of matter and the defining structure of elements. For some perspective, in just one droplet of water, there are approximately five sextillion atoms (a five followed by 21 zeros).

Atom structure. Source: Live Science

If you were able to enlarge the size of an atom by 1 trillion, the electron cloud would be the size of a football field and the nucleus would be the size of a small marble. In fact, 99.9999999999999% of an atom is actually empty space.

“We must be clear that when it comes to atoms, language can only be used as in poetry.” -Niels Bohr

Finally, when zooming out from atoms we find molecules. Molecules are the smallest unit of any chemical compound and are made of two or more atoms bound together by chemical bonds.

In that same water droplet that contained five sextillion atoms, there are 1.67 sextillion water molecules.

A water molecule. Source: Chemistry World

All ordinary matter that we interact with in our daily lives, including every atom on the periodic table of elements, is made of only three types of matter particles: up and down quarks, and electrons.

Somewhere, the four fundamental forces come in. But how do they work? What role do they play in your daily life? Check out the third part of this series, The Standard Model: Enter the Forces.

A New York City high school sophmore passionate about redefining space exploration.