Atoms and Isotopes

• atoms have a radius of about 1 x 10^ -10 metres
• the protons and neutrons have most of the mass and they're in the nucleus - so most of the mass (99%) is concentrated in the nucleus
• ISOTOPES: same number of protons/electrons, different number of neutrons
• radius of a nucleus is 1/10,000 of the radius of an atom
• • PROTON - in nucleus - relative charge of +1
  • ELECTRON - orbit nucleus - relative charge of -1
  • NEUTRON - in nucleus - relative charge of 0
• RELATIVE MASS

  • protons = 1
  • electrons = 1/2000 (~0 or negligible)
  • neutrons = 1
• • electrons are arranged at different distances from the nucleus
  • these are different energy levels with higher energy levels when electrons are further from the nucleus
• • The electron arrangements may change.
  • If electromagnetic radiation is absorbed by an atom, and electron will move further from the nucleus to a higher energy level

  
• • If electromagnetic radiation is emitted by an atom, an electron will move closer to the nucleus to a lower energy level)

  
• • All atoms of a particular element have the same number of protons.
  • carbon has 6 protons and if an atom has 6 protons it MUST be carbon.
  • It can have different numbers of neutrons (this is called an isotope) and a different number of electrons if ionised
• proton number in atom of an element is atomic number

  the total number of protons and neutrons in an atom is its mass number
• • ION: formed when atom loses or gains 1+ electrons
  • an ion has an unequal number of protons and electrons
  • loses one electron = a positive ion
  • gains one electron = a negative ion
  • this is due to the relative charge being unbalanced from 0
• DEMOCRITUS

  • theorised matter was made from invisible particles that cannot be divided
  • just a theory
• DALTON

  • manchester 1800s, made particle model of matter
  • conclusions reached were:
  • elements are made of very small particles spheres) called atoms, that can't be subdivided, created or destroyed
  • BILLIARD BALL MODEL

  
• JOSEPH JOHN THOMSON

  • electrons can be removed from atoms
  • made the plum pudding model. states that an atom is a ball of positive charge (pudding) with negative electrons embedded in it (plums)

  
• ERNEST RUTHERFORD #1

  • investigate atom structure with alpha particles
  • if pp model is right, he said that alpha particles would be like fast moving 'shells' which would pass straight through atoms as if they were made of tissue and not be deflected
• ERNEST RUTHERFORD #1

  • he supervised HANS GEIGER and ERNEST MARSDEN firing, alpha particles at very thin gold foil
  • the foil scattered some A-particles, detected by light flashes on a glass plate using a microscope
  • shows most of them passed right through the foil, but 1 in 10,000 were deflected by an angle 90+
  • He concluded that:
  • i) most of the atom is empty space because most alpha particles passed straight through it; and
  • ii) there must be a small, high mass, positively charged nucleus at the centre of the atom which deflected some of the alpha particles.
  • results in the nuclear model

  
• NIELS BOHR

  • uni of Copenhagen, adapted nuclear model and suggested electrons orbit the nucleus at specific distances and energy values (levels)
  • electrons move away from nucleus by absorbing a photon of electromagnetic radiation (eg visible light) and move closer by emitting it

  
• ERNEST RUTHERFORD #2

  • did more experiments later, led to idea that the positive charge of any nucleus can be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge
  • given the name PROTON
• JAMES CHADWICK

  • scientists are confused that the total mass of nuclei is higher than the proton mass, so another particle needs to ne present
  • he did work in Cambridge 1932 to give evidence of the existence of neutrons in a nucleus