Describe the structure of the atom, define proton number and nucleon number and isotopes, and relate electronic configuration to the position of an element in the periodic table

What this dot point is asking

SEAB wants you to describe the atom as a nucleus of protons and neutrons surrounded by electrons in shells, to define proton (atomic) number and nucleon (mass) number, to explain isotopes, and to relate the electronic configuration to an element's group and period in the periodic table. The marks come from accurate counting and from linking the outer electrons to the group number.

The answer

The structure of the atom

An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels). The relative charges and masses are:

• proton: charge $+1$, mass $1$,
• neutron: charge $0$, mass $1$,
• electron: charge $-1$, mass almost zero.

A neutral atom has equal numbers of protons and electrons, so the charges cancel.

Proton number and nucleon number

The proton number (atomic number) is the number of protons in the nucleus; it identifies the element. The nucleon number (mass number) is the total number of protons and neutrons. So:

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 electron arrangement, isotopes have identical chemical properties.

Electronic configuration

Electrons fill shells from the innermost outward: the first shell holds up to $2$, the next up to $8$, and the next up to $8$ at this level. The configuration is written as the number in each shell, for example oxygen is $2, 6$.

Link to the periodic table

The number of occupied shells gives the period (row), and the number of electrons in the outermost shell gives the group (column) for the main groups. Sodium ($2, 8, 1$) is in Group I, Period 3. Elements in the same group react similarly because they have the same number of outer electrons.

Examples in context

Example 1. Why Group 0 gases are unreactive. The noble gases such as neon ($2, 8$) have a full outer shell of electrons. A full outer shell is very stable, so these atoms have no tendency to gain, lose or share electrons, which is why they are chemically inert.

Example 2. Carbon dating uses isotopes. Carbon has the common isotope carbon-12 and the radioactive carbon-14, which has two extra neutrons. They are chemically identical, so living things take in both, but carbon-14 slowly decays, letting scientists estimate the age of old remains.

Try this

Q1. State the relative charge and relative mass of a neutron. [2 marks]

• Cue. A neutron has a relative charge of $0$ and a relative mass of $1$.

Q2. An atom has proton number $9$ and nucleon number $19$. Find the number of neutrons. [1 mark]

• Cue. Neutrons $= 19 - 9 = 10$.

Q3. Explain why magnesium ($2, 8, 2$) is placed in Group II of the periodic table. [2 marks]

• Cue. Magnesium has two electrons in its outermost shell, and for main-group elements the number of outer electrons equals the group number, so it is in Group II.