What Is an Atom? Atom Definition

What Is an Atom
An atom is the basic building block of matter, consisting of protons, neutrons, and electrons.

An atom is the smallest unit of matter that forms a chemical element and cannot be divided using any chemical means. Every solid, liquid, gas, and plasma consists of neutral or ionized atoms. An atom consists of a nucleus of protons and neutrons that is surrounded by electrons.

What Is an Atom?

  • An atom is fundamental building block of matter.
  • It consists of protons, neutrons, and electrons.
  • While it is not the smallest particle of matter, it is the smallest unit that cannot be broken by any chemical means.

The Structure of an Atom

Atoms consist of protons, neutrons, and electrons. A proton carries a positive electrical charge, a neutron is neutral, and an electron has a negative charge. In a neutral atom, there are equal numbers of protons and electrons.

These particle organize in a characteristic way:

  • Nucleus: The nucleus, located at the center of the atom, contains protons and neutrons. Protons are positively charged, while neutrons have no charge. So, the nucleus carries a net positive charge.
  • Electrons: Electrons are negatively charged particles that orbit the nucleus in electron shells. Electrons orbit very quickly, so while they feel attraction to protons, the two don’t stick to each other. The electrons organize into shells that maximize the stability of the atom.

Elements, Isotopes, and Ions

  • Element: The number of protons in the nucleus is the atomic number and it determines the element identity of the atom. For example, all atoms with 6 protons are carbon atoms. All atoms with 18 protons are argon atoms.
  • Isotope: Isotopes are forms of an element that have the same number of protons but different numbers of neutrons. This variation affects the atomic mass but not the element identity of the atom or its chemical properties. Examples of isotopes of the same element (hydrogen) are protium, deuterium, and tritium.
  • Ion: An ion is an atom that has lost or gained one or more electrons, giving it a net positive or negative charge. For example, a neutral lithium atom has 3 protons and 3 electrons, while a lithium ion with 3 protons and 2 electrons has a charge of +1.

Atom Properties

Atoms are incredibly small, typically around 100 picometers in diameter. About 99.94% of an atom’s mass is in its nucleus, with protons and neutrons being almost 2,000 times heavier than electrons. If all of an atom’s components froze in time, its volume would be around 99.9999% empty space. This is because the electrons are tiny compared to the nucleus and orbit far away from it. But, the subatomic particles are always in motion, so an atom feels solid. It’s sort of like how there is empty space between the blades of a fan, but when it seems solid when it’s on.

Chemical and Nuclear Reactions

On of the hallmarks of an atom is that is can’t be broken into smaller parts by any chemical reaction. However, nuclear reactions can break apart atoms or combine them into new elements.

  • Chemical Reactions: These involve the outer electrons of atoms (electron shells) and form or break chemical bonds. The nucleus remains unaffected.
  • Nuclear Reactions: In contrast, nuclear reactions involve changes to the nucleus, such as nuclear fusion or fission.

(Very) Brief History of Atomic Theory

The concept of atoms was first proposed by ancient Greek and Indian philosophers around 400 BC and further developed by scientists like John Dalton, J.J. Thomson, Ernest Rutherford, and Niels Bohr. The modern understanding of the atom has evolved through discoveries like the electron, the nucleus, and quantum mechanics.

Misconceptions About Atoms

There are some misconceptions about atoms:

  • Not every atom contains protons, neutrons, and electrons. Technically, all you need is a proton (which identifies the element). A typical hydrogen atom, for example, only contains a proton and an electron.
  • A common error is thinking that atoms are the smallest particles in existence. In reality, they are composed of subatomic particles (protons, neutrons, and electrons). Additionally, protons and neutrons consist of even smaller particles.
  • Another misunderstanding is thinking that atoms are like mini solar systems, with electrons following circular orbits around the nucleus. Electron orbitals are not necessarily round. In fact, there is a non-zero possibility of an electron passing through the atomic nucleus.
  • Atoms are not really the tiny spheres often depicted in illustrations. While some atoms appear round, others have less regular shapes because of their electron clouds.
  • The subatomic particles making up atoms don’t have a fixed identity. For example, a proton can change into a neutron, and vice versa. While an electron can’t change into a positron, it transforms into a muon from certain rare collisions.

Where Do Atoms Come From?

Atoms form in various ways, including during the Big Bang, through nuclear fusion in stars, and via radioactive decay. The process is involves the transformation and combination of subatomic particles.

Normal Matter vs. Antimatter Atoms

Atoms of matter and antimatter have the same structure and mass, but their subatomic particles differ. In antimatter, these particles have opposite charges compared to their matter counterparts. An antimatter atom has a nucleus consisting of antiprotons and antineutrons, with positrons orbiting this nucleus. For example, an antihydrogen atom consists of a positron (positively charged electron) that orbits around an antiproton (negatively charged proton).

While positrons have the opposite charge of electrons and antiprotons have the opposite charge of protons, both antineutrons and neutrons have no net electrical charge. That being said, these particles differ in that the the antineutron consists of one anti-up quark (charge –2/3) and two anti-down quarks (charge +1/3), while the neutron consists of one up quark (charge +2/3) and two down quarks (charge –1/3).


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