Atomic Structure - 4.1.2 Mass Number, Atomic Number and Isotopes (GCSE Physics AQA)

Mass Number, Atomic Number and Isotopes

Electrical Charge of Atoms

• Electrons and protons are charged. In an atom, the electrons are negatively charged and the protons are positively charged. Each electron has a charge of -1, whilst each proton has a charge of +1. Therefore, the protons and electrons have an equal and opposite charge.
• Atoms have no overall electrical charge. Since there are equal numbers of protons and electrons in an atom, the positive charge of the protons will be cancelled out by the negative charge of the electrons. This means that atoms have no overall negative charge.

Changing Electrical Charge

• The electrical charge of an atom can be changed. If an atom loses electrons, then it will turn into a positive ion. This occurs in the following way:

Mass Number and Atomic Number

• Atomic number is related to protons. The number of protons in an atom is called the atomic number. All the atoms of one element have the same number of protons.
• Mass number is related to protons and neutrons. The total number of protons and neutrons in an atom is called the mass number. Sometimes, atoms of one element can have a different number of neutrons.

Representing Atoms

We can represent atoms of an element in a very simple way. Let’s look at this example, involving Sodium.

1. We have represented the element Sodium using its symbol,’Na’.
2. The top number is the mass number. As previously mentioned, the mass number represents the protons and neutrons in an atom. Sodium has a mass number of 23.
3. The bottom number is the atomic number. As previously mentioned, the atomic number represents the protons in an atom. Sodium has an atomic number of 11.

Using the representation of an atom, we can work out exactly how many protons, neutrons and electrons it has.

• To work out protons: Simply look at the atomic number. This is the number of protons.
• To work out electrons: Simply look at the atomic number. We know that the number of protons is equal to the number of electrons.
• To work out neutrons: Use the ‘mass number – atomic number’. The mass number is the protons and neutrons, whilst the atomic number is just the protons. By taking atomic number away from mass number, we can find the number of neutrons.

Isotopes

• Atoms of one element can have varying numbers of neutrons. Previously, we mentioned that atoms of the same element can have different number of neutrons. These kinds of atoms are called isotopes:

Isotopes are atoms of the same chemical element, with the same number of protons, same number of electrons but differ only in the number of neutrons they contain.

• Isotopes are different to atoms. The isotope of an element will have a different mass number to an atom of an element. This means that an isotope affects various physical properties of the element, but not the chemical properties.

Properties of Isotopes

• The physical properties of isotopes can vary. Several physical properties are determined by the mass of the atom. Such as, density, boiling point and the melting point.
• The chemical properties of isotopes are similar. They are determined by the number and arrangement of electrons which do not change.