Mass Number, Atomic Number and Isotopes

• Atoms of each element are represented by a CHEMICAL SYMBOL which can be found in the PERIODIC TABLE. E.g. Carbon has the symbol C.
• They can be represented as below:

  

• The ATOMIC NUMBER indicates how many protons are in an atom's nucleus.
• The MASS NUMBER is the total count of both protons and neutrons.
• To find the number of NEUTRONS, SUBTRACT the atomic number from the mass number.

 

 

Elements

• An ELEMENT is a substance made up of ONE TYPE OF ATOM.
• All the atoms in an element contain the SAME NUMBER of PROTONS in their nucleus.
• When atoms have different numbers of PROTONS, they are classified as DIFFERENT elements.
• This means the ATOMIC NUMBER determines the ELEMENT. For example, Carbon atoms ALWAYS have 12 protons (has an atomic number of 12). 

Positive Ions

• Sometimes, radiation can cause electrons to be REMOVED from an atom in a process known as IONISATION.
• When this happens, an atom LOSES electrons to turn into a POSITIVE ION.

Isotopes

• All atoms of a particular ELEMENT have the same ATOMIC NUMBER but may have different numbers of NEUTRONS.
• ISOTOPES are different forms of the same element, having the SAME NUMBER OF PROTONS (same atomic number) but DIFFERENT NUMBERS OF NEUTRONS (different mass numbers).
• A common example is carbon-12, carbon-13 and carbon-14 which all have six protons but differ in the number of neutrons:

 

• The number of neutrons in an atom does NOT affect the charge of the atom, only its mass.
• This is because neutrons have no charge but do have mass.