The Nucleus | Physics for GCSE/IGCSE - Year 11 PDF Download

Composition of the Nucleus

• The nucleus consists of a positively charged center made up of protons and neutrons, surrounded by negatively charged electrons orbiting around it.

• Protons carry a positive charge, while neutrons possess no charge.
  • Hence, the nucleus exhibits an overall positive charge.

Describing the Nucleus

Proton Number, Z

• The number of protons in an atom is known as its proton number, which is also called the atomic number.
• Elements in the periodic table are arranged based on their atomic number, determining the identity of an element.
• For instance:
  • Hydrogen has an atomic number of 1, indicating it has one proton.
  • Sodium has an atomic number of 11, corresponding to 11 protons.
  • Uranium has an atomic number of 92, representing 92 protons.
• The atomic number is equal to the number of electrons in an atom, ensuring overall charge neutrality.

Nucleon Number, A

• Number of Neutrons: The number of neutrons in an atom can be calculated by subtracting the atomic number (number of protons) from the mass number (nucleon number).
• For instance, consider a sodium atom with a mass number of 23 and an atomic number of 11. Therefore, the number of neutrons would be 23 - 11 = 12.

Nuclide Notation

• A nuclide refers to a cluster of atoms that possess an identical count of protons and neutrons. For instance, if there are 5 atoms of oxygen, they all constitute the same nuclide, even though they exist as separate entities.
• Atomic symbols are denoted using a specific notation known as nuclide or ZXA notation.

• The top number denoted as A signifies the nucleon number or mass number, representing the total count of protons and neutrons within the nucleus.
• The lower number denoted as Z signifies the proton or atomic number, indicating the total count of protons within the nucleus.
• In Chemistry, the nucleon number corresponds to the mass number, and the proton number corresponds to the atomic number. The periodic table is arranged by atomic number.
• Example of an Atomic Symbol: An atomic symbol, such as the one shown in the image below, provides information about the components of atomic nuclei.

• When you are presented with an atomic symbol, it is possible to determine the total number of protons, neutrons, and electrons in the atom:
  • Protons: Protons are equivalent to the proton number of the atom.
  • Electrons: In a neutral atom, the number of electrons, which are negatively charged, matches the number of protons, which are positively charged.
  • Neutrons: Neutrons can be calculated by subtracting the proton number from the nucleon number.

• The Concept of Nucleons and Nuclides:

  • Nucleons: Nucleons refer to particles found in the nucleus, such as protons or neutrons.
  • Nuclides: A nuclide is a nucleus characterized by a specific combination of protons and neutrons.

Isotopes

• Isotopes are atoms of the same element having the same number of protons but varying numbers of neutrons. This disparity in neutron count leads to different isotopes within an element.
• Each element can exhibit multiple isotopes due to the presence of varying neutron numbers alongside a constant proton count.
• Isotopes, characterized by their neutron-proton imbalances, are generally less stable than elements with balanced neutron-proton ratios. Consequently, they possess a higher likelihood of undergoing decay processes.

• Isotopes are naturally-occurring, but their abundance varies.
• For instance, approximately 2 out of every 10,000 hydrogen atoms are Deuterium.
  • Tritium is even scarcer, with only about 1 in every billion billion hydrogen atoms being Tritium.