The structure of the atom, atomic number and mass number, isotopes, electronic configuration, and the arrangement of the periodic table into groups and periods.

What this dot point is asking

Edexcel wants you to describe the structure of the atom, define atomic number and mass number, explain isotopes, work out electronic configurations, and describe how the periodic table is arranged into groups and periods.

The structure of the atom

The relative masses are: proton $1$, neutron $1$, electron very small (about $\frac{1}{1835}$, treated as negligible).

Atomic number and mass number

• Atomic number = the number of protons (this defines the element).
• Mass number = the number of protons + neutrons.
• Number of neutrons = mass number $-$ atomic number.

In a neutral atom, the number of electrons equals the number of protons.

Isotopes

Electronic configuration

Electrons occupy shells, filling the inner shells first. The first shell holds up to $2$ electrons and the next shells up to $8$. For example, sodium (11 electrons) has the configuration $2, 8, 1$. The number of electrons in the outer shell decides how the element reacts.

The periodic table

The modern periodic table is arranged in order of increasing atomic number. It has:

• Groups (vertical columns): elements in the same group have the same number of outer electrons, so they have similar chemical properties. The group number equals the number of outer electrons (for the main groups).
• Periods (horizontal rows): elements in the same period have the same number of electron shells.

Metals are on the left and centre; non-metals are on the right. The table was developed by Mendeleev, who arranged the known elements in order of atomic mass but left gaps for elements that had not yet been discovered, and even swapped the order of a few pairs so that elements with similar properties lined up. When the missing elements were later found with the properties he had predicted, it gave strong support to his table. The modern table is now ordered by atomic number rather than mass, which removes the few anomalies in Mendeleev's version.

The position of an element tells you a lot about it: the group gives the number of outer electrons (and so the charge of the ion it tends to form), and whether an element is a metal or a non-metal tells you the kind of bonding it will take part in. This is why the periodic table is such a powerful tool for predicting chemical behaviour.

Try this

Q1. State what the atomic number of an element tells you. [1 mark]

• Cue. The number of protons in the nucleus.

Q2. Define isotopes. [2 marks]

• Cue. Atoms of the same element with the same number of protons but different numbers of neutrons.