Proton Number and Nucleon Number

• Proton number: the number of protons in the nucleus of an atom. 
  • \(\text{Proton number} = \text{Number of proton }\)
• In a neutral atom, the number of electrons and protons are the same.
• Nucleon number: the total number of protons and neutrons. 
  • \(\text{Nucleon number} = \text{Number of protons}+ \text{Number of neutrons}\)
• The nucleon number of an atom is about the same as the mass of the atom. 

Standard Representation of an Atom

• The chemical symbol is written to represent an element. 
• The standard representation of an element is as follows; 

Electron Arrangement of an Atom

• Neils Bohr suggested there is an energy level for each electron that occupy the orbits.
• The orbits with definite energy levels are known as the shell.
  • The first shell can hold up to two electrons.
  • The second shell can hold up to eight electrons.
  • The third shell can hold up to eight electrons.

• For a simpler representation of the electron arrangement of the elements, the number of electrons in each shell is written and separated by a dot. 

• Valence electrons are the electron in the outermost shell

• The valence electron has a great significance in determining the chemical properties of the elements. 

• Element with the same number of valence electrons has the same chemical properties

• Below shows examples of the electron arrangement of the first twenty elements in the Periodic Table of Elements.