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2.28: Atoms

Difficulty Level: At Grade Created by: CK-12
Atoms Practice
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What could this hilly blue surface possibly be? Do you have any idea? The answer is a single atom of the element Cobalt. The picture was created using a scanning tunneling microscope. No other microscope can make images of things as small as atoms. How small are atoms? You will find out in this lesson.

What Are Atoms?

Atoms are the building blocks of matter. They are the smallest particles of an element that still have the element’s properties. Elements, in turn, are pure substances—such as nickel, hydrogen, and helium—that make up all kinds of matter. All the atoms of a given element are identical in that they have the same number of protons, one of the building blocks of atoms (see below). They are also different from the atoms of all other elements, as atoms of different elements have different number of protons.


Size of Atoms

Unlike bricks, atoms are extremely small. The radius of an atom is well under 1 nanometer, which is one-billionth of a meter. If a size that small is hard to imagine, consider this: trillions of atoms would fit inside the period at the end of this sentence. Although all atoms are very small, elements vary in the size of their atoms. The Figure below compares the sizes of atoms of more than 40 different elements. The elements in the figure are represented by chemical symbols, such as H for hydrogen and He for helium. Of course, real atoms are much smaller than the circles representing them in the Figure below.

Atomic size chart

Q: Which element in the Figure above has the biggest atoms?

A: The element in the figure with the biggest atoms is cesium (Cs).

Subatomic Particles

Although atoms are very tiny, they consist of even smaller particles. Three main types of particles that make up all atoms are:

  • protons, which have a positive electric charge.
  • electrons, which have a negative electric charge.
  • neutrons, which are neutral in electric charge.

The model in the Figure below shows how these particles are arranged in an atom. The particular atom represented by the model is helium, but the particles of all atoms are arranged in the same way. At the center of the atom is a dense area called the nucleus, where all the protons and neutrons are clustered closely together. The electrons constantly move around the nucleus. Helium has two protons and two neutrons in its nucleus and two electrons moving around the nucleus. Atoms of other elements have different numbers of subatomic particles, but the number of protons always equals the number of electrons. This makes atoms neutral in charge because the positive and negative charges “cancel out.”

Model of a helium atom

Model of a helium atom.

Q: Lithium has three protons, four neutrons, and three electrons. Sketch a model of a lithium atom, similar to the model above for helium.

A: Does your sketch resemble the model in the Figure below? The model has three protons (blue) and four neutrons (gray) in the nucleus, with three electrons (red) moving around the nucleus.

Model of a lithium atom

Q: All atoms of carbon have six protons. How many electrons do carbon atoms have?

A: Carbon atoms must have six electrons to “cancel out” the positive charges of the six protons. That’s because atoms are always neutral in electric charge.


  • Atoms are the building blocks of matter. They are the smallest particles of an element that still have the element’s properties.
  • All atoms are very small, but atoms of different elements vary in size.
  • Three main types of particles that make up all atoms are protons, neutrons, and electrons.


  1. What is an atom?
  2. Which of the following statements is true about the atoms of any element?
    1. They have the same number of protons as the atoms of all other elements.
    2. They have protons that are identical to the protons of all other elements.
    3. They have the same size as the atoms of all other elements.
    4. They have the same number of protons as neutrons.
  3. Explain why atoms are always neutral in charge.

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Smallest particle of an element that still has the element’s properties.

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Difficulty Level:
At Grade

Concept Nodes:

7 , 8
Date Created:
Oct 31, 2012
Last Modified:
Sep 13, 2016
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