# Introduction to the Atom

Cloud model of an atom (koto_feja, iStockphoto)

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Learn about the parts of an atom and its history.

## The Atom

According to the Particle Theory of Matter, all matter is made of tiny particles. These particles are either individual atoms or groups of atoms called molecules. There are two main parts to an atom. These are the nucleus and the electrons.

### The Nucleus

In the center of each atom is the nucleus. Within the nucleus there are two kinds of particles. Positively-charged particles called protons and particles with no charges called neutrons. The protons give the nucleus a positive charge. For example, a helium atom has 2 protons and 2 neutrons. It would have a net charge of +2. A carbon atom has 6 protons and 6 neutrons and a net charge of +6.

The number of protons in the nucleus defines the type of atom. For example, all gold atoms have 79 protons and all silver atoms have 47 protons. The protons and neutrons in the nucleus are held together by a force called the strong nuclear force

Did you know?

Neutrons were not discovered until 1932. They were discovered by scientist James Chadwick.

### The Electrons

Moving around the nucleus are tiny, negatively-charged particles called electrons. These particles are 2 000 times smaller than protons and neutrons. This means that most of the weight of an atom is found in the nucleus. Electrons move extremely fast. In fact, electrons whip around the nucleus at a speed of about 2 200 kilometers per second! That is fast enough to get around the Earth in about 18 seconds.

Electrostatic forces keep the electrons moving around the nucleus. These are the forces that pull negatively-charged and positively-charged particles towards each other.

In nature, most atoms are stable. A stable atom has the same number of electrons as protons. For example, a gold atom has 79 protons and 79 electrons. Silver has 47 positively-charged protons and 47 negatively-charged electrons. These positive and negative charges cancel each other out. This means that stable atoms have a neutral charge.

Did you know?

Any particle smaller than an atom is called a subatomic particle. Protons, neutrons and electrons are all subatomic particles.

## Atomic Models

It is pretty tough for people to understand things that they cannot see. This was the case for the atom. Over time, scientists created different scientific models. They did this as they performed experiments and made observations.

### Thomson Plum Pudding Model

At the start of the 20th century, J.J. Thomson did experiments to learn about the atom. He showed that positively-charged and negatively-charged particles made up atoms. What he was not sure about was how they fit together. His idea at the time was that the negative electrons were stuck into a positive sphere. He imagined that the atom looked like a popular Christmas cake that had raisins in it. That is why this model is known as the plum pudding model.

### Rutherford Planetary Model

Scientist’s understanding of the atom changed in 1911. This due to work done by Ernest Rutherford and his team known as the gold foil experiment. They discovered that the positive charge of atoms seemed to be concentrated at their centres. Rutherford called this the nucleus. He also predicted that the electrons would orbit the nucleus, like planets around the Sun. This is why Rutherford's model is also called the planetary model. You have probably seen this atom model. Often a lithium atom, like the one below, is used as a symbol to represent science!

### Bohr Model

Rutherford’s planetary model explained a lot. But it didn’t answer some questions that scientists still had. They wondered where the electrons actually were and could their location be predicted? They also wondered why the orbiting electrons didn’t lose energy and crash into the nucleus. Luckily, Danish scientist Niels Bohr was trying to find those answers.

Bohr was part of a group of scientists interested in a new field of science called quantum mechanics. Quantum mechanics is the study of how atomic particles exist and interact with each other. Bohr was particularly interested in the energy possessed by electrons. Scientists were beginning to understand more about energy and subatomic particles. Based on this, Bohr suggested that electrons orbit the nucleus along specified paths. He called these electron shells. Remember the atomic models of gold and silver above? Those were Bohr models. The electron shells are labelled using either letters (K, L, M, N, O, P, Q) or quantum numbers (n=1 to n=7).

### Electron Cloud Model

The Bohr Model quickly became very popular. It is still used today because it helps us to understand how and why atoms interact with each other. But scientists were not yet finished trying to understand how atoms look.

Austrian physicist Erwin Schrödinger took Bohr’s model a step further. In 1926, he proposed a model in which he described the likelihood of finding an electron in a given place. This model is known as the electron cloud model or the quantum mechanical model. Drawings of electron clouds look like fuzzy shapes. Where the shape is most dense, the chance of finding an electron there is the highest. The place where an electron is most likely to be found is called its orbital.

Remember that the Bohr model showed the location of electrons in shells? That is still important in this model. Within each shell are subshells. Within each subshell are a specific number of orbitals. Each of the subshells can hold a certain number of electrons. It also has a characteristic shape. These shapes can get pretty complex as the number of electrons increases. Below are the shapes of the s, p and d subshells.

Atomic models are a great example of how scientific thinking changes over time. As well as how new tools, such as computer modeling, can lead to new ways of things about how things work.

Eureka Episode 22: Atoms (2011)

This episode of Eureka! (4:51 min.) explores what atoms are.

This episode of Eureka! (4:50 min.) explores electrons and shows a number of atomic models.

The Atom

This resource from Edumedia has a brief history of the atom and includes animations of different atomic models.

This video from SciShow (5:09 min.) explores the models that have been used to describe atoms.

## References

Brittanica. (n.d.). Orbital

Compound Chemistry. (2016, Nov. 16). The History of the Atom

Crash Course. (2013). Orbitals: Crash Course Chemistry #25.

Deziel, C. (2018). What Is an Unstable Atom? Sciencing.com.

Helmenstine, A.M. (2019, May 6). Basic Model of the Atom and Atomic Theory. Thought Co.

Khan Academy. (n.d.). Bohr’s model of hydrogen.