Basic Model of the Atom and Atomic Theory

Introduction to Atoms

The three parts of an atom are protons and neutrons, which form the nucleus, and electrons, which orbit the nucleus.
The three parts of an atom are protons and neutrons, which form the nucleus, and electrons, which orbit the nucleus. Encyclopaedia Britannica/UIG, Getty Images

All matter consists of particles called atoms. Atoms bond to each other to form elements, which contain only one kind of atom. Atoms of different elements form compounds, molecules, and objects.

Key Takeaways: Model of the Atom

  • An atom is a building block of matter that cannot be broken apart using any chemical means. Nuclear reactions can alter atoms.
  • The three parts of the atom are protons (positively charged), neutrons (neutral charge), and electrons (negatively charged).
  • Protons and neutrons form the atomic nucleus. Electrons are attracted to the protons in the nucleus, but are moving so quickly they fall toward it (orbit) rather than stick to protons.
  • The identity of an atom is determined by its number of protons. This is also called its atomic number.

Parts of an Atom

Atoms consist of three parts:

  1. Protons: Protons are the basis of atoms. While an atom can gain or lose neutrons and electrons, its identity is tied to the number of protons. The symbol for proton number is the capital letter Z.
  2. Neutrons: The number of neutrons in an atom is indicated by the letter N. The atomic mass of an atom is the sum of its protons and neutrons or Z + N. The strong nuclear force binds protons and neutrons together to form the nucleus of an atom.
  3. Electrons: Electrons are much smaller than protons or neutrons and orbit around them.

What You Need to Know About Atoms

This is a list of the basic characteristics of atoms:

  • Atoms cannot be divided using chemicals. They do consist of parts, which include protons, neutrons, and electrons, but an atom is a basic chemical building block of matter. Nuclear reactions, such as radioactive decay and fission, can break apart atoms.
  • Each electron has a negative electrical charge.
  • Each proton has a positive electrical charge. The charge of a proton and an electron are equal in magnitude, yet opposite in sign. Electrons and protons are electrically attracted to each other. Like charges (protons and protons, electrons and electrons) repel each other.
  • Each neutron is electrically neutral. In other words, neutrons do not have a charge and are not electrically attracted to either electrons or protons.
  • Protons and neutrons are about the same size as each other and are much larger than electrons. The mass of a proton is essentially the same as that of a neutron. The mass of a proton is 1840 times greater than the mass of an electron.
  • The nucleus of an atom contains protons and neutrons. The nucleus carries a positive electrical charge.
  • Electrons move around outside the nucleus. Electrons are organized into shells, which is a region where an electron is most likely found. Simple models show electrons orbiting the nuclear in a near-circular orbit, like planets orbiting a star, but real behavior is much more complex. Some electron shells resemble spheres, but others look more like dumb bells or other shapes. Technically, an electron can be found anywhere within the atom, but spends most of its time in the region described by an orbital. Electrons can also move between orbitals.
  • Atoms are very small. The average size of an atom is about 100 picometers or one ten-billionth of a meter.
  • Almost all of the mass of an atom is in its nucleus; almost all of the volume of an atom is occupied by electrons.
  • The number of protons (also known as its atomic number) determines the element. Varying the number of neutrons results in isotopes. Varying the number of electrons results in ions. Isotopes and ions of an atom with a constant number of protons are all variations of a single element.
  • The particles within an atom are bound together by powerful forces. In general, electrons are easier to add or remove from an atom than a proton or neutron. Chemical reactions largely involve atoms or groups of atoms and the interactions between their electrons.

Does the atomic theory make sense to you? If so, here's a quiz you can take to test your understanding of the concepts.


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Helmenstine, Anne Marie, Ph.D. "Basic Model of the Atom and Atomic Theory." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). Basic Model of the Atom and Atomic Theory. Retrieved from Helmenstine, Anne Marie, Ph.D. "Basic Model of the Atom and Atomic Theory." ThoughtCo. (accessed June 3, 2023).