Atomic structure

• All matter is made up of particles called atoms

  These atoms are made up of even simpler sub-atomic particles

  These are called protons, electrons and neutrons
• Protons and neutrons both have mass but no charge

  
• At the centre of an atom is a positive nucleus

  The nucleus is made up of positive protons and neutral neutrons

  The nucleus is surrounded by electrons

  These electrons are arranged in layers ,or shells, sometimes these shells are called energy levels.
• Each element has a fixed number of protons in its nucleus

  Scientists have identified 118 different types of atom- Means there are 118 different elements
• ATOMS-

  -Proton number doesn't change

  -Electron number changes- IONS

  -Neutron number changes- ISOTOPES
• The mass number of an atom is the total number of protons and neutrons in a nucleus.

  The atomic number tells us how many protons are in the nucleus. No other element has the same atomic number as lithium.
• Mass of element= number of protons + neutrons

  Number of protons =number of electrons

  Number of neutrons =Mass of element- number of protons
• ISOTOPES:

  Are atoms with the same number of protons but different number of neutrons

  They have identical chemical properties

  Some elements have decimal point numbers because its the average mass from the different isotopes of the element
• -Electrons are arranged in shells around the nucleus

  -The innermost shell (lowest energy level) of electrons is filled first. This shell can contain maximum of 2 electrons.

  -2nd shell fills, holds maximum of 8 electrons

  -When this is filled, electrons go into the 3rd shell, which holds a maximum of 8 electrons

  -4th shell begins to fill