The Nucleus (CIE IGCSE Physics)

• The structure of the atom is made up of a:
  • Positively charged nucleus at the centre (made up protons and neutrons)
  • Negatively charged electrons in orbit around the nucleus

 

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 overall positive

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
• 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

• 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

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 periodic table is ordered by atomic number

• An example of an atomic symbol is:

 

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

• When given an atomic symbol, you can figure out the total number of protons, neutrons and electrons in the atom:
  • Protons: The number of protons is equal to the proton number
  • Electrons: Atoms are neutral, and so in a neutral atom the number of negative electrons must be equal to the number of positive protons
  • Neutrons: The number of neutrons can be found by subtracting the proton number from the nucleon number

• The term nucleon is used to mean a particle in the nucleus – ie. either a proton or a neutron
• The term nuclide is used to refer to a nucleus with a specific combination of protons and neutrons

ANSWER:  D

Step 1: Determine the atomic and mass number

• • The gold atom has an atomic number of 79 (lower number) and a mass number of 197 (top number)

Step 2: Determine the number of protons

• • The atomic number is equal to the number of protons
  • The atom has 79 protons

Step 3: Calculate the number of neutrons

• • The mass number is equal to the number of protons and neutrons
  • The number of neutrons is equal to the mass number minus the atomic number

197 - 79 = 118

• • The atom has 118 neutrons

Step 4: Determine the number of electrons

• • An atom has the same number of protons and electrons
  • The atom has 79 electrons

• 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:

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)

Answer: B

• In nuclide notion, the top number is the nucleon number (number of protons and neutrons) and the bottom number is the proton number (number of protons)
• Isotopes are two of the same elements
  • This eliminates option D since one is oxygen (O) and the other nitrogen (N)
• Which have the same number of protons
  • This eliminates option C and A
  • Their proton numbers are different for the same element
• But a different number of neutrons
  • Therefore, the correct answer is B