What Is An Atom?: The Smallest Unit of Matter

Do you require information on an Atom? Then, read through this article to get all the information you require. This article is equipped with all the information you need on an atom.

For the purpose of clarity, I will begin with an overview of atoms. After which, we will discuss the atomic model.

Immediately after that, we will discuss the properties of an atom. Here, we will discuss the atomic number, atomic mass, isotopes, orbits, energy levels, etc.

This will lead us to discuss the three basic subatomic particles – protons, electrons, and neutrons. After which, we will discuss the models of atoms.

Lastly, I will give answers to some frequently asked questions on atoms.

What Is An Atom; Overview

An atom is the smallest unit of matter. It is the unit into which matter you can divide without releasing electrically charged particles.

Also, it uniquely defines a chemical element. Simply, it is a basic piece of matter.

The matter is anything that you can physically touch. Hence, it is anything that has weight and occupies space. It could be solid, liquid, or gas.

Except for energy, everything in the universe is made of matter. Thus, everything is from atoms.

An atom consists of three subatomic particles – protons, neutrons, and electrons. Other particles like alpha and beta particles also exist.

Mostly, you find the mass of an atom in the nucleus. The nucleus is a small dense area at the center of an atom.

This nucleus contains the nucleons. More so, the nucleons are a composition of protons and neutrons. The nucleus houses the positive charges of an atom.

Furthermore, the positive charges contained in the nucleus originate from the protons. Thus the protons have positive charges.

The neutrons have no charge. That is, they possess neutral charges (neither positive nor negative).

On the other hand, the electrons have negative charges. This is because they are located outside the nucleus and revolve around an orbit or shell.

Atomic Model

The matter consists of a collection of molecules that are easily separable. Molecules, in turn, consist of atoms that are joined together by chemical bonds. Thus, making them more challenging to break.

The atom itself constitutes the nuclei and electrons. More so, these particles possess electrical charges, thus creating an electric field.

This electric field holds the electrical charges together.

To illustrate, positive charges in an atom attract electrons. As a result, some electrons have behavior that resembles that of particles that orbit the nucleus.

While others behave like a frozen wave in position around the nucleus.

Such wave patterns describe the distribution of individual electrons – the orbitals. These orbital properties influence the behavior of the atom.

Also, the shells determine the chemical properties of this atom. Shells are simply orbital groupings.

An atom consists mostly of empty space. Its nucleus at the center has positive charges and consists of virtually the entire mass of the atom.

The protons, neutrons, and orbiting electrons are particles that can live long. They are present in all ordinary and naturally occurring atoms.

Furthermore, all atoms are roughly the same size. The convenient unit of length for the measurement of atomic sizes is the angstrom (Å). The angstrom equates to 10−10 meters.

Another unit of length that applies to atoms is the femtometer (fm). The femtometer equates to 10-15m.

The wide variation of the nuclei’s mass explains what they can have from 1 to 300 protons and neutrons. For example, the nucleus of hydrogen, which is the lightest, is 1,836 times higher in mass than that of electrons.

Although, a heavy nucleus can be about 500,000 times more massive.

Basic Properties Of An Atom

Atomic Number

The atomic number is the most important feature of an atom. Its symbol is usually Z.

By definition, the atomic number is the number of protons (positive charge) in the nucleus of the atom. It also determines the position of the atom in the periodic table.

For instance, if an atom has a Z of 8, it means it has 8 protons, and that is oxygen. Also, an atom with a Z of 17 is chlorine.

However, neutral atoms have an equal number of protons and electrons. Thus, this balances the positive and negative charges.

The electrons have a role in determining how an atom interacts with another. On the other hand, the number of protons in the nucleus determines the chemical properties of the atom.

Atomic Mass And Isotopes

The number of neutrons in a nucleus does not affect the chemical properties. However, it affects the mass of the atom.

Hence, two nuclei with an equal number of protons will have the same chemical properties even if the number of neutrons varies. That is, a nucleus with 8 protons and 8 neutrons, and a nucleus with 8 protons and 10 neutrons will have the same chemical properties.

Nevertheless, due to their different number of neutrons, their masses will vary. These two nuclei are isotopes of each other.

On the other hand, the atomic mass number is the sum of the number of protons and the number of neutrons in the nucleus.

Electron Charge, Mass, And Spin

From the oil-drop experiment of Robert Millikan, electrons are negative charges. In recent times, the charge on an electron is accepted to be 1.602176565 × 10−19 coulombs.

The proton has a charge that is equal in magnitude to that of an electron but with an opposite sign. In other words, the proton is positive. Hence, they attract electrons. This attraction keeps the electrons in orbit around the nucleus.

However, the mass of an electron is about 9.109382911 × 10−28 grams. The proton or neutron has a mass that is about 1836 times higher than that of electrons. This is why the nucleus determines the mass of an atom.

Also, the spin is an intrinsic property of an electron. The electrons can not spin arbitrarily. Electrons have a spin of 0.5 times a basic unit of rotation.

Nevertheless, the name that is given to particles with half-integer spin is fermions. These fermions obey Pauli Exclusive Principle. This principle states that two fermions can not occupy the same state.

Due to the intrinsic spin of electrons, they have a magnetic moment that is equal to −9.28 × 10−24 joule per tesla.

Orbit And Energy Levels

Electrons exist in specific locations called allowed orbits. It is a result of Neil Bohr’s quantum mechanics. Also, he explained that the angular momentum of an electron in orbit comes in discrete bundles called quanta.

According to Neil Bohr, you can only find electrons in allowed orbits that have different energies. Quantum mechanics describes the process by which electrons move from one energy level to another.

Furthermore, a quantum leap is a name for the process of electrons leaving one energy level to another. Each shell or orbit is an energy level. Hence, the energy an electron possesses will be different whenever a quantum leap occurs with the electron.

When an electron leaves a higher energy level for a lower one, it loses photons. Thus, the emission of photons is the process by which atoms emit light as a result of a change in energy levels.

Also, if an electron acquires much energy, it can move to a higher energy level. One of the common ways an electron can achieve this energy is to absorb a photon with the right frequency.

However, the closest orbit to the nucleus is the lowest energy level.

Electron Shells

From Bohr’s atomic model, you assign a quantum number n to every allowed electron orbit. This quantum number n runs from 1 to infinity, starting from the closest orbit to the nucleus.

Every orbital that has equivalent n values forms a shell. However, there may be subshells inside each shell. Thus, this shell must correspond to different rotation rates, the orientation of orbitals, and the spin directions of the electrons.

Generally, the closer the shell to the nucleus, the fewer subshell. This possible orbitals arrangement explains a lot about the chemical properties of various atoms.

Furthermore, the electrons in the outermost shell (Valence shell) determine the chemical behavior of an atom. This depends on the number of leftover electrons after filling the interior shells. Also, this determines the position of an atom on the periodic table.

Atomic Bonds

Atomic bonding is the physical or chemical process that is responsible for the interaction between molecules and atoms. There are varieties of bonds. These include covalent, ionic, hydrogen, metallic bonds, and so on.

However, there are three basic ways the outer electrons form bonds. These are:

  • Transferring electrons from one atom to another.
  • Sharing electrons between neighboring atoms.
  • Sharing electrons with all atoms in a material.

These behaviors of the valence electrons explain the types of atomic bonds.

Subatomic Particles


The protons came as a result of Ernest Rutherford’s experiment in 1919. Ernest Rutherford experimented with a gold foil.

Alpha particles of helium nuclei were projected on a gold foil. As a result, there was a deflection of positive alpha particles. Hence, he gave a conclusion that protons exist in a nucleus and are positively charged.

Furthermore, the proton number is also known as the atomic number. The atomic number is the number of protons an atom has. Thus, this atomic number determines an element.


Electrons are found within the area surrounding the nucleus of the atom. This area is called the electron cloud. Also, electrons are abbreviated as e.

They were discovered by Sir J. J. Thomson in 1897. This came as a result of his cathode ray experiments. He made a confirmation that cathode rays are negatively-charged.

Hence, these cathode rays were called electrons. Through his oil drop experiments, Robert Millikan found the electronic charge value. Recently, the charge in an electron is 1.602176565 × 10−19 coulombs.

However, electrons have a higher probability of being found closer to the nucleus. More so, their magnitude equates to that of the protons. Although, their mass is different.

The mass of a proton or neutron is higher than that of an electron. Hence, by consideration, electrons are insignificant. Thus, protons and electrons of unequal amounts form ions.


In 1932, James Chadwick discovered neutrons through his experiment. In addition, he demonstrated the presence of neutral particles in penetrating radiation.

You can only find the neutrons in the nucleus with the protons. This is because the neutrons and the protons make up virtually the entire mass of the atom.

However, you can derive the number of neutrons by subtracting the proton number from the atomic mass number. Thus, the neutron number is the number of neutrons.

The number of neutrons does not necessarily equate to the number of protons. In essence, the neutrons determine the isotope of an atom and its stability.

Models Of An Atom

John Dalton’s Model

John Dalton, in his model, explained the law of conservation of mass and the law of definite proportions using atoms. Also, he gave a proposal that all matter is made of tiny indivisible particles. The name given to these particles is atoms.

Also, Dalton noted that every single atom is the same as other atoms in the same element. However, atoms of one element differ from atoms of another element.

Furthermore, Dalton stated that compounds are a combination of two or more different atoms. An example of this combination is water. Water is a combination of 2 molecules of hydrogen and 1 molecule of oxygen (H₂O).

Lastly, he made a suggestion that chemical reactions do not create or destroy atoms. Instead, however, they only rearrange the atoms.

J. J. Thomson’s Model

J. J. Thomson, in 1898, proposed that the shape of an atom is spherical. Also, the radius of the atom is about 10-10m. More so, the distribution of positive charges is uniform.

This model successfully explained the neutrality of an atom. Although, later models gave contradictions to Thomson’s model.

Ernest Rutherford’s Model

Along with his students, Rutherford performed an experiment with alpha particles (α-Particles). They directed high-energy alpha particles from a radioactive source to a thin gold foil.

From their experiments, they came up with the following results:

  • Most of the alpha particles that were passed undeflected through the foil.
  • A few alpha particles were deflected by small angles.
  • A very minute number of α-particles bounced back.

Therefore, Ernest Rutherford gave the following conclusions:

  • First, most of the atoms have empty space.
  • Positively charged alpha particles are not spread uniformly throughout the atom. Hence, you have to concentrate it in a small volume. This is to aid deflect positively charged particles.
  • The radius of an atom is 10-10m, and the radius of the nucleus is 10-15m.

Neil Bohr’s Model

In 1915, Neil Bohr proposed that electrons do not radiate energy while orbiting the nucleus. Rather, they exist in different states of constant energy.

By implication, the electrons orbit at fixed distances from the nucleus. Therefore, he focused on the emission spectra of hydrogen.

As part of his experiment, Bohr explained that the electrons could move from one orbit to another. When they have more energy, they can move to a higher state. However, when the energy is removed, they return to their initial states.

In summary. Bohr made these postulates:

  • Electrons orbit the nucleus at energy levels that are fixed.
  • You find higher energy levels in orbits further from the nucleus.
  • Electrons emit energy in the form of light when they return to a lower energy level.

Another model of an atom is Erwin Schrodinger’s Model. Also, read about Heisenberg’s uncertainty principle.

What Is An Atom; Frequently Asked Questions

1. What is an atom in a simple definition?

It is a particle of matter that defines a chemical element uniquely.

2. What are atoms made of?

They are made of protons, neutrons, and electrons. Protons have positive charges, electrons have negative charges, and neutrons have zero charges.

3. What are atoms and molecules?

Atoms are the constituents of matter. While molecules are products of atoms with a covalent bond.

4. Are humans made of atoms?

About 99% of the human body is made up of atoms of hydrogen, nitrogen, oxygen, and carbon.

5. Are atoms living or non-living?

They are non-living.

6. What are 3 facts about atoms?

The facts about atoms include:
1. Atoms are the smallest particles that makeup elements.
2. Also, atoms in most cases are empty space.
3. Atom comes from a Greek word that means uncuttable or indivisible.

7. What is the difference between atom and particle?

Atoms are small units of particles, while particles are very little portions of matter.

8. What is smaller than an atom?

A quark is defined as any subatomic particle that carries a fractional electric charge.

9. Is an atom an element?

It is the part of an element.

10 How many atoms are in a human body?


What Is An Atom; Wrap Up

Atom is the basic unit of matter that is not divisible except by electrical discharge. It shows that almost everything in the universe is a product of matter.

The nuclei and the electrons are what make up an atom. Also, the protons and the neutrons make up the nuclei. Even so, the proton is positive charges, the electrons are negative charges, and the neutrons have zero charges.

Furthermore, the model of atoms shows the works of scientists and their discoveries. The result of their experiments are courses of study in schools.

However, this PowerVersity article gives you every single detail of an atom. It has all the theories and recent modifications on the atom, charges, and acceptable constants.

Hence, I hope you found this article helpful.

Then, fill out the “Leave A Reply” form to share your thoughts.

Finally, you may want to read other articles like this; please visit the following pages below:

What Does Current Mean

Christian Jerome
Akan Christian writes for PowerVersity.com. He holds an HND in Electrical/Electronic Engineering. Christian enjoys reading, listening to solemn music, singing, and playing sports. For PowerVersity.com, Christian writes reviews, buying guides, and best pick articles.


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