The Nucleus (Cambridge (CIE) O Level Physics): Revision Note

Composition of the Nucleus

• A nucleus is made up of positively charged protons and neutrally charged neutrons

The Protons, Neutrons and Electrons in an Atom

Protons and neutrons are found in the nucleus of an atom

• Protons have a positive charge, whilst neutrons have no charge

  • This is why the nucleus is positive overall

Atoms & Ions

• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons

  • An atom will lose or gain electrons to become more stable

• A stable atom is normally electrically neutral

  • This means it has the same number of protons (positive charge) and electrons (negative charge)

• Positive ions are therefore formed when atoms lose electrons

  • There will be more protons than electrons

• Negative ions are therefore formed when atoms gain electrons

  • There will be more electrons than protons

Neutral, Positively and Negatively Charged Atoms

The difference between positive and negative ions depends on the number of elections

Describing the Nucleus

Proton Number, Z

• The number of protons in an atom is called its proton number (it can also be called the atomic number)

  • Elements in the periodic table are ordered by their atomic number

  • Therefore, the number of protons determines which element an atom is

• The atomic number of a particular element is always the same

• For example:

  • Hydrogen has an atomic number of 1. It always has just one proton

  • Sodium has an atomic number of 11. It has 11 protons

  • Uranium has an atomic number of 92. It has 92 protons

• The atomic number is also equal to the number of electrons in an atom

  • This is because atoms have the same number of electrons and protons in order to have no overall charge

Nucleon Number, A

• The total number of particles in the nucleus of an atom is called its nucleon number (or mass number)

• The mass number is the number of protons and neutrons in the atom

Calculating the Number of Neutrons

• The number of neutrons can be found by subtracting the atomic number from the mass number

Number of Neutrons = Nucleon Number - Proton Number 

• For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12

Nuclide Notation

• A nuclide is a group of atoms containing the same number of protons and neutrons

  • For example, 5 atoms of oxygen are all the same nuclide but are 5 separate atoms

• Atomic symbols are written in a specific notation called nuclide or ZXA notation

Nuclide Notation

Atomic symbols in AZX Notation describe the constituents of nuclei

• The top number A represents the nucleon number or the mass number

  • Nucleon number (A) = total number of protons and neutrons in the nucleus

• The lower number Z represents the proton or atomic number

  • Proton number (Z) = total number of protons in the nucleus

• Note: In Chemistry, the nucleon number is referred to as the mass number and the proton number as the atomic number. 

The Atomic Symbol of Lithium

• An example of an atomic symbol is:

Atomic symbols, like the one above, describe the constituents of nuclei

Isotopes

• Although the number of protons in a particular element is always the same, the number of neutrons can be different

• Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons

  • This means that each element can have more than one isotope

• Isotopes tend to be more unstable due to their imbalance of protons and neutrons

  • This means they're more likely to decay

• In the diagram below are three isotopes of Hydrogen:

The Three Isotopes of Hydrogen

Hydrogen has three isotopes, each with a different number of neutrons

• Isotopes occur naturally, but some are more rare than others

• For example, about 2 in every 10,000 Hydrogen atoms is Deuterium

  • Tritium is even more rare (about 1 in every billion billion hydrogen atoms)