Atoms are tiny particles that make up every item in the world. Atoms are extremely small; they have a radius of about 0.1 nanometres (1 x 10-10 m). Atoms are made up of three subatomic particles:
- Protons
- Neutrons
- Electrons
The table below shows the charge, relative mass and location in an atom (found) for protons, neutrons and electrons.
Protons and neutrons are found in the centre of an atom, which is known as the nucleus. The nucleus is tiny in comparison to the size of the rest of the atom; the radius of the nucleus is about 10,000 times smaller than the radius of an atom (the radius of a nuclei of an atom is about 1 x 10-14 m). The nucleus of an atom is positively charged, and this is because of the protons in the nucleus. Almost all of the mass in an atom is found in the nucleus and this is because all of the protons and neutrons are found in the nucleus.
Electrons move around the nucleus in shells and these shells are referred to as electron shells or energy levels. There are many electron shells and each of the electron shells can only hold a specific number of electrons. The first shell holds a maximum of 2 electrons, and the second and third shells can hold a maximum of 8 electrons. An electron will always fill the lowest energy level (the closest shell to the nucleus). When the closest shell is full, the electrons will start filling the next shell outside the shell that is full. Shells do not have to be full. Electrons have virtually no mass; their mass is often taken as zero or 1/2000.
The rest and the majority of the atom is made up of space.
It is the number of protons that determines what element an atom is. Atoms should have no overall charge, which means that the number of protons will be equal to the number of electrons. Protons and electrons have the same magnitude of charge but in opposite directions; protons have a positive charge of +1 and electrons have a negative charge of -1. Neutrons have no charge and therefore they have no effect on the charge of the atom/ element.
A sketch of boron is shown below.
Atoms can lose or gain electrons, which results in atoms becoming positively or negatively charged. We call atoms that are charged ions; both positive and negative atoms are called ions.
If a neutral atom gains some electrons, they become negatively charged because they are gaining electrons which have a negative charge. The number of electrons that the atom gains is the amount of negative charge that they become. If they gained 1 electron, they will have a charge of 1-. If they gained 2 electrons, they will have a charge of 2-. All atoms will be a negative ion if they have more electrons than protons.
If a neutral atom loses some electrons, they become positively charged because they are losing electrons which have a negative charge. The number of electrons that the atoms lose is the amount of positive charge that they become. If they lost 1 electron, they would have a 1+ charge. If they lost 2 electrons, they would have a 2+ charge. All atoms will be a positive ion if they have more protons than electrons.
The rules for ions are shown below.
It is the number of protons in an atom that determines what element an atom is. For example, all atoms with 11 protons will be sodium, all atoms with 9 protons will be fluorine etc.
Isotopes are different forms of the same element with a different number of neutrons. As they are the same element, all isotopes of an element will have the same number of protons (the atomic number of the isotopes will be the same). As isotopes of the same element have a different number of neutrons, the mass number of the isotopes will be different (the mass number is the number of protons and neutrons added together).
A common example of isotopes is carbon-12 and carbon-13. The tiles for carbon-12 and carbon-13 with the number of protons, neutrons and electrons are shown below.
Most elements have many isotopes. Usually only one or two of the isotopes are stable. The other isotopes are unstable and radioactive. This means that they give out radiation and decay into other elements. We are going to look at the different types of radioactive decay in the next few sections.
Electromagnetic radiation can cause electrons to move within or leave an atom. If the electrons absorb EM radiation, they move to a higher energy level that is further away from the nucleus (they move to a shell that is further out). If the electrons release EM radiation, they move to a lower energy level that is closer to the nucleus (they move to a shell that is closer to the nucleus). If an atom loses one or more electrons, they will become positively charged as the atom will have more protons than electrons.
This nuclear model is agreed upon by most scientists. However, if the outcomes of new experiments contradicted what the nuclear model predicts, it may be the case that scientists will modify or completely get rid of the current nuclear model and come up with a new model to explain the structure of atoms.