Atomic Structure

Atomic Structure

1. History of the Atom

   • J.J Thomson (1898), England, discovered the electron and developed the plum-pudding model of the atom.
   • Ernest Rutherford (1909), England - used the results of his gold foil experiment to prove that atoms consist of electrons revolving around a positively charged nucleus.
   • Chadwick (1931), England - discovered the neutrally charged neutron.

2. Present day Atom

   • The atom is composed of a very small, dense, positively charged nucleus composed of protons and neutron, with negatively charged electrons in shells orbiting the nucleus.
   • These three particles, protons, neutrons, and electrons are known as the subatonic particles.
   • Protons are found in the nucleus of an atom with a charge of +1 and a relative mass of 1.
     • Neutrons are found in the nucleus and have a charge of 0, they also have a relative mass of 1.
       • Electrons are found of the shells of atoms, have a charge of -1 and a relative mass of 1/1840.

3. Atomic number and mass number

   • Atomic number is the number of protons in an atom.
   • Mass number is the number of protons and number of neutrons in an atom.
   • Atoms are equally neutral so they must contain equal number of protons and electrons.

4. Gaining Electrons

   • If an atom has a partially filled outer shell; this makes it unstable and reactive.
   • When atoms lose or gain electrons they do it to have a full outer shell. This full outer shell is stable
   • cation - positive ion anion - negative ion

5. Charged Particles - Ions

   • An ion is a particle formed when an atom loses or gains electrons.

6. Electron Structure

   • Shell 1 can hold up to 2 electrons. All other shells can hold up to 8 electrons.
   • The group number is the number of electrons in the outer shell (except group 0 which all have 8 except Helium which has 2)
   • Period number is equal to the number of shells

7. Isotopes

   • Isotopes are atoms of an element with the same atomic number but a different mass number indicating a different number of neutrons
   • Proton number never changes; the only thing that changes is the number of neutrons and mass number

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