The Nucleus (Cambridge O Level Physics)

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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

atom-proton-neutron-electron, IGCSE & GCSE Physics revision notes

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

Examiner Tip

Be careful with your terminology:

  • Atom = nucleus (proton and neutron) and electrons
  • Nucleus = protons and neutrons at the centre of the atom

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

5-1-1-positive-and-negative-ions-cie-igcse-23-rn

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

Examiner Tip

You may hear the term 'net charge'. This just means the 'overall' charge of the atom. If an atom has 5 protons, 5 neutrons and 6 electrons, it has a net negative charge because it's a negative ion (more electrons than protons). 

Remember which way around the charges are by proton being positive.

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

Examiner Tip

You may have noticed that the number of electrons is not part of the mass number. This is because electrons have a tiny mass compared to neutrons and protons. We say their mass is negligible when compared to the particles in the nucleus.

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

AZX Notation, downloadable AS & A Level Physics revision notes

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 symbol, IGCSE & GCSE Physics revision notesAtomic symbol, IGCSE & GCSE Physics revision notes

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

Worked example

The element symbol for gold is Au. How many protons, neutrons and electrons are in the gold atom?Gold atom, downloadable IGCSE & GCSE Physics revision notes

  Protons Neutrons Electrons
A 79 79 79
B 197 79 118
C 118 118 79
D 79 118 79

 

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

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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

Isotopes of Hydrogen, downloadable AS & A Level Physics revision notes

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)

Worked example

Which of the following elements are isotopes of each other?

A Cl presubscript 17 presuperscript 35 and Cl presubscript 18 presuperscript 35
B straight U presubscript 92 presuperscript 238 and U presubscript 92 presuperscript 235
C C presubscript 6 presuperscript 12 and C presubscript 8 presuperscript 14
D straight O presubscript 8 presuperscript 16 and straight N presubscript 7 presuperscript 14

   

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 options C and A
    • Their proton numbers are different for the same element
  • But a different number of neutrons
    • Therefore, the correct answer is B

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Ann H

Author: Ann H

Expertise: Physics

Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students no matter their schooling or background.