Describe the structure of the atom in terms of protons, neutrons and electrons, define proton number and nucleon number, explain isotopes, and write the electronic configuration of the first twenty elements

What this dot point is asking

SEAB wants you to describe the atom in terms of protons, neutrons and electrons, define proton (atomic) number and nucleon (mass) number, explain what isotopes are and why they behave the same chemically, and write the electronic configuration of the first twenty elements. This is the structure that explains the Periodic Table and all bonding, so it is worth getting completely secure.

The answer

The structure of the atom

An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons arranged in shells. The three sub-atomic particles have these properties:

• Proton: relative mass $1$, charge $+1$, in the nucleus.
• Neutron: relative mass $1$, charge $0$, in the nucleus.
• Electron: relative mass almost $0$, charge $-1$, in shells around the nucleus.

In a neutral atom the number of protons equals the number of electrons, so the positive and negative charges balance. Almost all the mass is in the nucleus, because electrons are so light.

Proton number and nucleon number

• The proton number (atomic number) is the number of protons in the nucleus. It defines the element; every atom of an element has the same proton number.
• The nucleon number (mass number) is the total number of protons and neutrons.

So the number of neutrons is the nucleon number minus the proton number. An atom is written as $^{A}_{Z}\text{X}$, where $Z$ is the proton number and $A$ is the nucleon number.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same proton number but different nucleon numbers. For example, chlorine has $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$.

Isotopes of an element react in exactly the same way chemically, because chemical reactions involve only the electrons, and isotopes have the same number of electrons in the same arrangement. Their physical properties (such as density) differ slightly because they have different masses.

Electronic configuration

Electrons fill shells starting from the one nearest the nucleus. The shells hold up to $2$, then $8$, then $8$ electrons for the first twenty elements. The configuration is written as the number in each shell, separated by commas. For example:

• Sodium (11 electrons): $2, 8, 1$.
• Chlorine (17 electrons): $2, 8, 7$.
• Calcium (20 electrons): $2, 8, 8, 2$.

The number of shells gives the period, and the number of electrons in the outer shell gives the group, in the Periodic Table.

Examples in context

Example 1. Carbon dating uses isotopes. Carbon exists as ordinary carbon-12 and a small amount of radioactive carbon-14, isotopes with the same chemistry but different masses. Because they behave identically in living things, the decay of the heavier isotope after death can be used to estimate age, an application that depends on isotopes sharing chemical behaviour.

Example 2. Why the Periodic Table works. Sodium ($2, 8, 1$) and potassium ($2, 8, 8, 1$) both have one outer electron, so both are reactive Group I metals. Their electronic configurations explain why elements in the same group behave alike, linking atomic structure directly to the Periodic Table.

Try this

Q1. State the relative mass and charge of a neutron. [1 mark]

• Cue. Relative mass $1$, charge $0$.

Q2. An atom has proton number $12$ and nucleon number $24$. State its number of neutrons and its electronic configuration. [2 marks]

• Cue. Neutrons $= 24 - 12 = 12$; configuration $2, 8, 2$.

Q3. Explain why two isotopes of the same element have identical chemical properties. [2 marks]

• Cue. They have the same number of electrons in the same arrangement, and chemical reactions depend on the electrons, so their chemistry is the same.