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Structure of the Atom

by Ron Kurtus (updated 8 February 2022)

The structure of the atom determines its properties and how atoms combine.

Although originally thought of as the smallest form of matter, atoms are really made up of even smaller particles: a nucleus consisting of protons and neutrons and electrons in shells around the nucleus.

The most common way to picture an atom is the solar system model. The number of protons in the nucleus determines the element's atomic number. The number of electrons typically equals the number of protons.

The mass of an atom is approximately the sum of the number of protons and neutrons in it nucleus.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Bohr model

The Bohr model or solar system model of the atom is that the atom is like a tiny solar system, with the nucleus in the center and electrons rotating around the nucleus in orbits, similar to how the planets rotate around the Sun. It is usually called the Bohr model, after Neils Bohr, who discovered electron shells in 1913.

Electrons rotating around the nucleus

Electrons rotating around the Lithium nucleus

It is not certain whether the electrons actually rotate around the nucleus or if they stay in place, but it is more likely that they are in motion.

Newer models

There are some newer models of how the electrons appear in the atom.

Electron cloud model

One model of the atom states that instead of electrons being tiny particles in orbits, they are in the form of an electron cloud in shells around the nucleus. This model is due to the Uncertainty Principle and is useful in Quantum Mechanics.

String theory

The latest theory says that the electrons are neither particles or clouds. Instead, they say they are shaped like pieces of string. String Theory has just been stated in the past few years.

Will stick with old

Neither theory is easy to visualize, so for the most part we will stick with the older, solar system model. It answers most questions about the atom.


The nucleus consists of a number of positively charged protons and uncharged neutrons. They are approximately the same size and weight.

Since like charges repel, there is some sort of "nuclear glue" that holds the nucleus together. There must usually be more neutrons than protons for a nucleus to be stable, so it has been thought that neutrons had something to do with this nuclear glue. Present theories state that there is a sub-nuclear particle called a gluon that holds the nucleus together.

Possible nucleus configuration (protons are red with + in picture)

Possible nucleus configuration (protons are red with + in picture)


To keep an atom electrically stable, it has the same number of negatively charged electrons in orbit around the nucleus as there are positively charged protons in the nucleus. In situations where there are either more or less electrons in orbit than there are protons in the nucleus, the atom is called an ion. This happens in static electricity and in some chemical solutions.

Electron shells

Electrons are arranged in shells or orbits around the nucleus.

Maximum number

There is a definite arrangement of the electrons in these shells and a maximum number of electrons possible in each shell.

Shell or Orbit Number






Maximum Number of Electrons






The most electrons possible in the first shell are 2. After the first shell is filled, the second shell starts filling up, according to the number of positive charges in the nucleus. The most allowed in the second shell is 8 electrons. Then the third shell starts to fill.

Electron shells for a Sodium atom (atomic number 11, with 11 electrons)

Electron shells for a Sodium atom
(atomic number 11, with 11 electrons)

Filling order complicated

After the second orbit or shell is filled, things start to get complicated. The third shell fills until it gets to 8, and then the fourth shell starts adding electrons until it too has 8 electrons. Then the third shell fills until it gets to 18.

Outer shell basis of Chemistry

The number of electrons in each shell can be seen by using the Periodic Table, which is a lesson in Chemistry. In fact, the basis of chemical combinations depends on the number of electrons in the outer orbit or shell.

Energy levels

Electrons have potential energy, depending on their shell or orbit. Energy levels are assigned to each orbit. There are some situations when an electron will jump from one orbit—or energy level—to another. When that happens, the electron gives off electromagnetic energy of one wavelength or color of light.

Since each element has its own electron shell or energy level configuration, it also has its own set of colors for the different possible electron orbit jumps. This fact is used in identifying elements that are giving off light.


The solar system model of the atom is a good way to describe an atom. The nucleus is at the center of the atom and consists of protons and neutrons. There are normally just as many electrons as protons in an atom. The electrons are distributed in shells around the nucleus, according to certain rules. The outer shell is the basis for Chemistry.

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