5.1 Electrons, Protons, Neutrons, and Atoms

Protons Are What Make Elements Distinct

All matter, including mineral crystals, is made up of atoms.  All atoms are made up of three main particles: protons, neutrons, and electrons. Protons have a positive charge (+1), neutrons have no charge, and electrons have a negative (-1) charge. Protons and neutrons have the same mass, while electrons have a mass that is 10,000 times smaller.

The element hydrogen (H) has the simplest atoms, each with just one proton and one electron. The proton forms the nucleus (the centre of the atom), while the electron orbits around it. All other elements have neutrons as well as protons in their nucleus. Because protons all have a +1 charge, they repel each other.  It is possible to have more than one proton in a nucleus because neutrons hold them together.

What makes an element distinct is the number of protons it has, so it is fitting that the number of protons is referred to as the atomic number. The total number of protons and neutrons is the atomic mass,[1] which is fitting because the protons and neutrons are what account for nearly all of the mass of the atom. So for H, the atomic number and the mass number are 1 because there is one proton and no neutrons. For helium (He), which has 2 protons and 2 neutrons, the atomic number is 2 and the atomic mass is 4.

For most of the 16 lightest elements (up to oxygen) the number of neutrons is equal to the number of protons. For most of the remaining elements, there are more neutrons than protons. This is because these atoms have a large number of protons concentrated in a very small space, and those protons are repelling each other.  Extra neutrons are needed to keep the nucleus together. An example is uranium.  Its most common isotope has an atomic mass of 238.  It has 92 protons, but requires 146 neutrons to keep them together.  The neutrons are only partly successful.  Uranium is radioactive, meaning that its nucleus tends to come apart and release energy.

Electrons Are What Control How Atoms Interact

Electrons orbiting around the nucleus of an atom are arranged in shells (also known as energy levels). The first shell can hold only two electrons, but the next shell holds up to eight electrons. An atom can have many shells of electrons, but there are never more than 8 outermost electrons interacting with surrounding atoms.

The outermost electrons play an important role in bonding between atoms. Elements that have a full outer shell are inert because they do not react with other elements to form compounds. These are the noble gases which all appear in the far-right column of the periodic table (helium, neon, argon, etc). For elements that do not have a full outer shell, the outermost electrons can interact with the outermost electrons of nearby atoms to create chemical bonds. The electron shell configurations for 29 of the first 36 elements are listed in Table 2.1. Note that some of the shells in the table below have more than 8 electrons.  This is because they contain subshells.  For example, the third shell can hold up to 18 electrons because it contains one subshell that can hold 2 electrons, and two subshells that can hold 8 electrons each.

Table 2.1 Electron shell configurations of some of the elements up to element 36. (The inert elements, with filled outer shells, are bolded.)
      Number of Electrons in Each Shell
Element Symbol Atomic No. First Second Third Fourth
Hydrogen H 1 1
Helium He 2 2
Lithium Li 3 2 1
Beryllium Be 4 2 2
Boron B 5 2 3
Carbon C 6 2 4
Nitrogen N 7 2 5
Oxygen O 8 2 6
Fluorine F 9 2 7
Neon Ne 10 2 8
Sodium Na 11 2 8 1
Magnesium Mg 12 2 8 2
Aluminum Al 13 2 8 3
Silicon Si 14 2 8 4
Phosphorus P 15 2 8 5
Sulphur S 16 2 8 6
Chlorine Cl 17 2 8 7
Argon Ar 18 2 8 8
Potassium K 19 2 8 8 1
Calcium Ca 20 2 8 8 2
Scandium Sc 21 2 8 9 2
Titanium Ti 22 2 8 10 2
Vanadium V 23 2 8 11 2
Chromium Cr 24 2 8 13 1
Manganese Mn 25 2 8 13 2
Iron Fe 26 2 8 14 2
. . . . . . .
Selenium Se 34 2 8 18 6
Bromine Br 35 2 8 18 7
Krypton Kr 36 2 8 18 8


  1. You may wonder why we need an atomic mass if the atomic number is unique for each element. The answer is that some elements have different numbers of neutrons in their nucleus. For example, carbon can have 6, 7, or 8 neutrons (the isotopes 12C, 13C, and 14C). This won't affect the chemical reactions in which an atom can participate, but it can affect other behavior of the atom, sometimes in very useful ways.

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