Atomic Structure

• All substances and elements are made of atoms.

  An atom is the smallest part of an element that

  can exist.

  This makes atoms VERY TINY, we cannot see them

  even with microscopes.
• The nucleus is in the middle of the atom.  It contains protons and neutrons. Protons are positively charged. Neutrons have no charge, they are neutral. So the nucleus has a positive charge overall because of the protons. The size of the nucleus is tiny compared to the rest of the atom. Its radius is 1 x 10-14m, which is about 1/10 000 the size of an atom.

  The electrons move around the nucleus. They are negatively charged and tiny but cover a lot of space. They occupy shells around the nucleus. Electrons do not have much mass at all compared to protons and neutrons, so nearly all the mass in located in the nucleus.
• Democritus believed that all matter is made of invisible particles called atoms
• What did Democritus believe about atoms in 400 BC?

  Atoms are indivisible
• Thomson's atomic model is called the plum pudding model.
• What discovery is J.J Thomson known for around 1900?

  The electron
• Thomson's plum pudding model suggested that atoms are positively charged with negative electrons embedded in them.
• What experiment did Rutherford conduct to prove his atomic model?

  Alpha particle scattering
• Rutherford's experiment showed that atoms have a tiny, positively charged nucleus at their center.
• Rutherford's model suggested that most of an atom is empty space.
• Bohr suggested that electrons orbit the nucleus at fixed distances from the nucleus.
• What did Niels Bohr propose about electron orbits in 1913?

  Electrons orbit in shells
• Which neutral particle in the nucleus was identified by Chadwick?

  The neutron
• Rutherford discovered the proton in 1918.
• Compare Thompson's "plum pudding model" and Rutherford's "nuclear model"

  They both have electrons, which are not in shells and neither of them have neutrons or protons. They also have an overall neutral charge.

  In the plum pudding model, the atoms are a ball of positive charge but for the nuclear model, the mass/positive charge is all in the centre. The plum pudding model also is solid but the nuclear model has empty space.

  
• Atoms are very small, having a radius of about 0.1 nm ($1\times10^{-10}$m).
• The radius of a nucleus is less than 1/10,000 of that of the atom (about $1\times10^{-14}$m).
• The mass number is the number of protons and neutrons added together.

  The atomic number is the number of protons/neutrons.

  To find the amount of electrons you do; mass number - atomic number

  Eg. For sodium there are:

  • 11 protons
  • 11 neutrons
  • 12 electrons (23-11)