Atomic Weights

Atomic Weights

1. Number of Protons = Number of Electrons

2. Atomic Number = Number of Protons

   • All atoms of a particular element have the same number of protons
   • The atoms of different elements have different numbers of protons
   • For example, all oxygen atoms have 8 protons and all sodium atoms have 11 protons

3. Mass Number = Number of Atoms + Number of Neutrons

   • The mass number of an atom is never smaller than the atomic number
     • It can be the same but is usually bigger

4. The full chemical symbol for an element shows its mass number at the top and its atomic number at the bottom

   • This symbol tells us that the chlorine atom has 17 protons
     • It will also have 17 electrons as this is as the number of electrons
       • The total number of protons and neutrons is 35, so we can work out that there are 18 neutrons

5. The electrons in an atom occupy energy levels, also known as shells.

   • Each electron in an atom is found in a particular energy level. The lowest energy level (innermost shell) fills with electrons first.
     • Each energy level can only hold a certain number of electrons before it becomes full.
       • The first energy level can hold a maximum of two electrons, the second energy level a maximum of eight, and so on.
         • The electronic structure of an atom is written using numbers to represent the electrons in each energy level.
           • For example, for sodium this is 2,8,1 – showing that there are: 2 electrons in the first energy level 8 electrons in the second energy level 1 electron in the third energy level.

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