Describe the structure of the atom, define proton number and nucleon number, work out the numbers of protons, neutrons and electrons, and draw electron shell arrangements

What this dot point is asking

The syllabus wants you to describe what an atom is made of, to define proton number and nucleon number, to work out the numbers of protons, neutrons, and electrons in an atom or ion, and to draw the electron shell arrangement for the first twenty elements. You also need to understand isotopes. This topic is the foundation for bonding and the Periodic Table, so the counting and the shell diagrams must be secure.

The answer

The structure of the atom

An atom has a tiny central nucleus containing protons and neutrons, with electrons moving around it in shells (energy levels). The three particles are:

• Proton: charge $+1$, mass $1$, found in the nucleus.
• Neutron: charge $0$, mass $1$, found in the nucleus.
• Electron: charge $-1$, almost no mass, found in shells around the nucleus.

An atom is neutral overall because it has equal numbers of protons and electrons, so the charges cancel.

Proton number and nucleon number

Two numbers describe an atom, written as $^{A}_{Z}\text{X}$:

• The proton number (atomic number) $Z$ is the number of protons. It is also the number of electrons in a neutral atom, and it tells you which element it is.
• The nucleon number (mass number) $A$ is the total number of protons and neutrons.

So the number of neutrons is the nucleon number minus the proton 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. Because they have the same number and arrangement of electrons, isotopes have the same chemical properties; they differ only slightly in mass.

Drawing electron shell arrangements

Electrons fill shells from the inside out, and each shell holds a maximum number:

• the first shell holds up to $2$ electrons,
• the second shell holds up to $8$ electrons,
• the third shell holds up to $8$ electrons (for the first twenty elements).

For example, the $17$ electrons of chlorine fill as $2, 8, 7$. The number of electrons in the outer shell controls how the element reacts and which group it is in.

Examples in context

Example 1. Why the Periodic Table is ordered by proton number. The elements are arranged in order of increasing proton number, so each element has one more proton (and one more electron) than the one before. This steady change in electron arrangement is what produces the repeating pattern of properties down and across the table.

Example 2. Carbon dating uses an isotope. Carbon has isotopes including the common $^{12}\text{C}$ and the rare radioactive $^{14}\text{C}$. They behave identically in chemistry because they have the same electrons, but differ in mass and stability, which is why one of them can be used to date old materials.

Try this

Q1. An atom has a proton number of $8$ and a nucleon number of $16$. State the number of protons, neutrons, and electrons. [3 marks]

• Cue. Protons $= 8$, electrons $= 8$, neutrons $= 16 - 8 = 8$.

Q2. Draw or write the electron arrangement of a magnesium atom, which has $12$ electrons. [2 marks]

• Cue. Fill from the inside: $2, 8, 2$, so magnesium has two electrons in its outer shell.

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

• Cue. They have the same number and arrangement of electrons, and chemical reactions depend on the electrons, so they behave the same; only the number of neutrons (and so the mass) differs.