Atomic structure

The basic building blocks making up all substances are known as atoms. Atoms are the smallest amount of any element we can have, and each element is only made up of one type of atom. Gold is only made up of gold atoms, copper is only made of copper atoms and so on. Atoms contain smaller sub-atomic particles known as protons, neutrons and electrons and each type of atom differs in the number of the sub-atomic particles it has. The structure of the atom is shown in the diagram below:

The protons and neutrons are found in the nucleus of the atom and the electrons are found orbiting around the nucleus in energy shells. The mass of the atom is concentrated in the nucleus. This means that the protons and neutrons are the sub-atomic particles which give the atom its mass and that mass of electrons is almost 0.

Properties of atoms

• Protons have a relative charge of +1 and a relative mass of 1. They are positively charged
• Neutrons have no relative charge and a relative mass of 1
• The nucleus overall has a positive charge because of the protons, and is tiny compared to the rest of the atom
• Electrons move around the nucleus in energy shells. They are negatively charged. They are tiny but can cover a lot of spaces. They have a relative charge of -1 and a relative mass of 1/1836
• Atoms are neutral with no overall electrical charge, as the number of electrons and protons always equals the number of electrons in an atom. The size of electrons and protons are the same. However, if electrons are added or removed, the atom becomes charged and then becomes an ion
• When two or more atoms join chemically, a molecule is formed. A compound is a molecule that has at least two different elements. (All compounds are molecules but not all molecules are compounds
• The atomic number is the number of protons contained in an atom. This is also known as the proton number
• The number of protons and the number of neutrons is known as the mass number

The relative charges and relative masses of the sub-atomic particles are summarised in the table below. You must remember these for the exam.

The arrangement of electrons

In the nucleus of an atom, protons and neutrons are joined together but electrons orbit the nucleus in energy shells. Each energy shell can only hold a certain number of electrons. The lowest energy levels are those closest to the nucleus and are always filled first. The outer shell is the highest energy level and is always filled with electrons last.

How electrons are arranged via their electronic structures

1. In the first and lowest energy shell there is only room for 2 electrons. The energy shells after this can hold a maximum of 8 electrons.
2. The first shell should always be filled up first, then electrons move on to the next energy shells until they are filled.

For example, sodium has an atomic number of 11. This means that it has 11 protons and 11 electrons, as the number of electrons is the same as the number of protons. The electron arrangement for sodium is written as 2, 8, 1.

Nitrogen has 7 protons and therefore 7 electrons. Following the rules above, the first shell can take 2 and the second shell can take 5. So, the electronic configuration for nitrogen would be 2,5.

Potassium has 19 electrons. The first shell holds 2 electrons, the second shell holds 8 electrons, the third shell holds 8 electrons and the fourth shell holds only 1 electron. The electron arrangement for potassium is 2,8,8,1.