Structure of the atom

An atom has a small central nucleus containing protons and neutrons. Electrons surround the nucleus and are arranged in shells.

The three particles differ in their relative charges and masses:

• Proton: charge $$+1$$; mass $$1$$.
• Neutron: charge $$0$$; mass $$1$$.
• Electron: charge $$-1$$; mass approximately $$\frac{1}{1836}$$.

These masses are relative to the mass of a proton. Because electrons have much less mass than protons and neutrons, nearly all of an atom’s mass is concentrated in its nucleus.

A neutral atom has equal numbers of protons and electrons. Their positive and negative charges balance, giving the atom an overall charge of zero.

Atomic number and mass number

An atom has a central nucleus containing protons and neutrons. Two numbers describe how many of these particles the nucleus contains.

The atomic number, also called the proton number, is the number of protons in the nucleus. It identifies the element: every atom of a particular element has the same number of protons.

The mass number, also called the nucleon number, is the total number of protons and neutrons in the nucleus. Protons and neutrons are together called nucleons. The mass number is a count of particles, not the measured mass of the atom.

Because the mass number includes both protons and neutrons, subtracting the number of protons leaves the number of neutrons:

For example, a carbon atom with atomic number $$6$$ and mass number $$12$$ has $$6$$ protons and $$12 - 6 = 6$$ neutrons.

Atoms of the same element can have different mass numbers because they can have different numbers of neutrons. A carbon atom with mass number $$14$$ still has $$6$$ protons, but it has $$14 - 6 = 8$$ neutrons.

Electronic configurations of atoms and ions

Electrons occupy shells around an atom’s nucleus. An electronic configuration shows how many electrons are in each shell, starting with the shell closest to the nucleus. For example, $$2,8,3$$ means that there are 2 electrons in the first shell, 8 in the second and 3 in the third.

The shell counts must add up to the total number of electrons in the atom or ion.

Filling electron shells

For elements with proton numbers from 1 to 20, electrons fill the shells in this order:

1. The first shell holds up to 2 electrons.
2. The second shell holds up to 8 electrons.
3. The third shell fills up to 8 electrons before the fourth shell begins to fill.

For example, a neutral calcium atom has 20 electrons. Its configuration is $$2,8,8,2$$: after the first three shells contain 2, 8 and 8 electrons, the remaining 2 enter the fourth shell. The rule of filling the third shell to 8 before starting the fourth applies to the elements in this range; it is not a general statement that the third shell can never hold more than 8 electrons.

Configurations of atoms and ions

A neutral atom has equal numbers of protons and electrons. Its proton number therefore tells us how many electrons to distribute among its shells. For example, a sodium atom, $$\ce{Na}$$, has proton number 11 and 11 electrons, so its configuration is $$2,8,1$$.

An ion forms when an atom loses or gains electrons. Its proton number does not change:

• A positive ion has lost electrons, so it has fewer electrons than protons.
• A negative ion has gained electrons, so it has more electrons than protons.

A sodium atom can lose its outer electron to form $$\ce{Na+}$$. The ion still has 11 protons, but it has 10 electrons and the configuration $$2,8$$.

A chlorine atom, $$\ce{Cl}$$, has 17 protons and 17 electrons, arranged as $$2,8,7$$. It can gain one electron to form $$\ce{Cl-}$$. The ion still has 17 protons, but its 18 electrons are arranged as $$2,8,8$$.

Electron shells and periodic-table position

Electrons in an atom occupy shells around the nucleus. The arrangement of these electrons helps locate the element in the periodic table: its group is a vertical column, and its period is a horizontal row.

Group and outer-shell electrons

For elements in Groups I to VII, the group number equals the number of electrons in the outer shell. For example, an element in Group III has three outer-shell electrons.

Group VIII contains the noble gases, which have full outer electron shells. Helium’s only shell is full with 2 electrons. The other noble gases among the first 20 elements—neon and argon—each have 8 electrons in their outer shell.

Period and occupied shells

The period number equals the number of occupied electron shells: shells that contain electrons. An element with electrons in three shells is in Period 3.

For example, chlorine has the electron arrangement 2, 8, 7. It has 7 outer-shell electrons, so it is in Group VII. It has 3 occupied shells, so it is in Period 3.