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atomic structure (IGCSE) 
  • State the relative charges and approximate relative masses of protons, neutrons and electrons
Particle in atom Relative mass Relative charge Location
Proton 1 +1 Nucleus
Neutron 1 0 Nucleus
Electron 1/1840 -1 Electron shell
  • Define proton number (atomic number): the number of protons in the nucleus of an atom
  • Define nucleon number (mass number): the total number of protons and neutrons in the nucleus of an atom
  • Use proton number and the simple structure of atoms to explain the basis of the Periodic Table, with special reference to the elements of proton number 1 to 20
Example: sulfur atom

Proton number is 16. Mass number is 32.
Since mass number = protons + neutrons, number of neutrons = mass number – proton number.
In this case is 32-16 = 16 neutrons.  
For an atom, number of protons is equal to number of electrons, so the number of electrons for sulfur atom is 16.

• Describe the build-up of electrons in ‘shells’ and understand the significance of the noble gas electronic structures and of the outer shell electrons.

How to draw electronic structure of atom:
first electron shell holds maximum 2 electrons.
Second electron shell hold maximum 8 electrons.
Third electron shell hold maximum 8 electrons (Not true for A levels, IB syllabus).

Lastly remember to write the symbol of the atom in the middle of the atom.
Electronic structure of sulfur atom with 16 electrons will look like this:

Electronic configuration shows distribution of electrons of an atom in electron shells without drawing.
First number stands for number of electrons in first shell, and second number, third and fourth number shows for number of electrons in second, third and fourth shell respectively. The numbers are separated by ‘dot’.

Electronic configuration of sulfur atom will be 2.8.6 as it has 16 electrons. 

The last number of the electronic configuration will show the valence/outermost electrons of the atom. Sulfur has 6 valence electrons. Sulfur atom also has 3 electron shells.

Elements in the periodic table are arranged in increasing proton number.
Atoms of Elements in same group (vertical column) have same number of valence electrons and similar chemical properties.
Atoms of elements in the same period (horizontal row) have same number of electron shells.

Noble gases (Group VIII) have fully filled electron shells and stable electronic configuration so they do not need to gain or lose electrons thus noble gases are inert and unreactive.
  • Define isotopes: atoms of the same element which have the same proton number but a different nucleon number
Example: carbon- 12, carbon-13 and carbon-14 are isotopes of carbon as they have same proton number of 6. But carbon- 12, carbon-13 and carbon-14 have 6 neutrons, 7 neutrons and 8 neutrons respectively.

• State the two types of isotopes as being radioactive and non-radioactive.

Example: Carbon -14 is radioactive, its nucleus is unstable. The atom breaks down naturally or decays, giving out radiation in the form of rays and particles and large amount of energy.
• State one medical and one industrial use of radioactive isotopes

Medical uses: Cobalt-60 used in radiotherapy to cure cancer, Cobalt-60 and cesium-137 used to sterilise syringes and other disposable medical equipment.

Industrial use: in tracers such as Geiger counter to check oil and gas pipes for leaks

  • Understand that isotopes have the same properties because they have the same number of electrons in their outer shell.
Example: carbon- 12, carbon-13 and carbon-14 are isotopes of carbon as they have same proton number of 6. All of their atoms have 6 electrons. Their electronic configuration is 2.4. They have the same chemical properties as they all have same number of valence electrons which is 4 but different physical properties since their mass numbers are different.