Bohr Models

One way to show the arrangement of electrons in an atom is with a Bohr model diagram. A Bohr model diagram shows how many electrons are in each shell surrounding the nucleus. This type of diagram is named after Niels Bohr, a Danish physicist.

The regions surrounding the nucleus of an atom are sometimes called electron shells. The shell nearest the nucleus can hold 0, 1, or 2 electrons. The next two shells outward can each hold up to 8 electrons. Any remaining electrons will fill the fourth shell, to a maximum of 18.

This pattern of 2, 8, 8, and 18 applies to all atoms, although not all atoms have that many electrons.

Borh Model Diagrams

A Bohr model diagram usually contains the element symbol, the number of protons in the atom, and a way to show where the electrons are. Some diagrams show the electrons as dots. Others just have figures indicating the number of electrons in each shell. The diagram below shows three ways you could draw a Bohr model diagram for the element potassium (atomic number 19).

Valence Electrons and Chemical Families

The diagram below shows how the electrons are arranged in each atom of the first 18 elements. Notice that the first electron shell is filled (2 electrons) before the second electron shell is filled. Likewise, the second electron shell is filled (8 electrons) before the third shell. The third shell is filled (8 electrons) before the fourth shell, which can hold up to 18 electrons.

The electrons in the outermost shell (those farthest from the nucleus) have the strongest influence on the properties of an atom. These electrons in the outermost shell are called valence electrons. The shell that contains the outermost electrons is called the valence shell.

You will notice several striking patterns from the diagram.

• Most elements in the same family have the same number of valence electrons. For example, halogens have seven valence electrons. Helium, a noble gas, is an exception to the pattern with only two valence electrons.
• Elements in the same period have valence electrons in the same shell.
• The period number indicates the number of shells that have electrons.

Noble Gas Stability

The noble gases (He, Ne, Ar, Kr, Xe, Rn) are normally unreactive, which means the atoms do not form new substances with other atoms.

Why?

Because their atoms have filled valence shells—the maximum number of electrons in their outermost shells. That makes them stable. For two atoms to join together to make a new substance, atoms must gain, lose, or share electrons. But atoms with filled valence shells will not easily trade or share electrons. They have what we call noble gas stability.

Atoms from other families often try to achieve a kind of stability similar to the noble gases. To do this, they will gain or lose electrons. For example, metals, which usually have one, two, or three valence electrons, will often lose them all. When metals lose their valance electrons, their remaining electrons will have the same arrangement as the noble gas in their row in the periodic table.

What about non-metals? They gain one, two, or three extra electrons in order to achieve noble gas stability. They will gain exactly enough electrons to achieve the same electron arrangement as the noble gas in their row in the periodic table.

How Atoms Become Ions

Recall that an atom that has gained or lost electrons is called an ion. Ions carry an electric charge, because the number of protons (positive) in them is not equal to the number of electrons (negative).
Note that:
• an atom of any metal can lose electrons and form positive ions
• an atom of any non-metal except a noble gas can gain electrons and form negative ions
• ions have the same electron arrangement as the nearest noble gas

The charge on an ion is shown on the symbol of an element by adding a superscript number followed by a plus or minus sign. The diagram below shows some examples of Bohr model diagrams for atoms and their ions. The charge on an ion is equal to the sum of the charges on its protons and electrons. For example, the magnesium ion Mg²⁺ has 12 protons (12+) and 10 electrons (10–) for a total charge of 2+.