Edexcel IGCSE Physics: Double Science 复习笔记:7.1.1 Atomic Structure

Edexcel IGCSE Physics: Double Science 复习笔记:7.1.1 Atomic Structure

Atomic Structure



  • Atoms are the building blocks of all matter
  • They are incredibly small, with a radius of only 1 × 10-10 m
    • This means that about one hundred million atoms could fit side by side across your thumbnail


  • Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
  • The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom





Diagram showing the structure of a Lithium atom. If drawn to scale then the electrons would be around 100 metres away from the nucleus!




Parts of the Atom

  • The nucleus contains:
    • Protons - positively charged particles with a relative atomic mass of one unit
    • Neutrons – no charge, and also with a relative atomic mass of one unit


  • Almost all of the atom is empty space, but moving around the nucleus there are:
    • Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)






  • The properties of each of the particles are shown in the table below:




Charge in the Atom

  • Although atoms contain particles of different charge, the total charge within an atom is zero
    • This is because the number of electrons is equal to the number of protons


  • The following table sets out the calculation of the total charge in the Lithium atom in the diagram above:



Total Charge Calculation Table


  • If an atom loses electrons, then it is said to be ionised
  • Symbols are used to describe particular nuclear by their element symbol, atomic number and mass number
    • This notation is called nuclear notation



Carbon 12 in nuclear notation



Worked Example

A nucleus of carbon-12 is shown below.

How many electrons are there in an atom of carbon-12?


Step 1: Count the number of protons in the carbon nucleus

    • There are 6 protons in the carbon atom



Step 2: Determine the number of electrons


    • Remember, the number of electrons in an atom is equal to the number of protons
    • Therefore there must be 6 electrons in the carbon atom



Exam 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.

Atomic & Mass Number


Atomic Number

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





Mass Number

  • The total number of particles in the nucleus of an atom is called its mass number (it can also be called the nucleon 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 Neutron = Mass Number – Atomic 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



Nuclear Notation

  • The mass number and atomic number of an atom are shown by writing them with the atomic symbol
    • This is called nuclear notation


  • Here are three examples:

Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium



  • The top number is the mass number
    • This is equal to the total number of particles (protons and neutrons) in the nucleus


  • The lower number is the atomic number
    • This is equal to the total number of protons in the nucleus


  • The atomic and mass number of each type of atom in the examples above is shown in this table:


Number of Protons, Neutrons & Electrons Table


Worked Example

The element symbol for gold is Au. How many protons, neutrons and electrons are in the gold atom?



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