Nuclear physics

• Atoms are the building blocks of all matter
• they are incredibly small, with a radius of 1x10^-10 m (100 million atoms could fit side by side across a thumbnail)
• atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
• the radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all the mass of the atom
• atoms consist of small, dense positively charged nuclei surrounded by negatively charged electrons
• structure of an atom

  
• in 1909, a group of scientists were investigating the plum pudding model:


  Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment
• rutherford's experiment involved the scattering of alpha (α) particles by a sheet of thin metal, and this supports the nuclear model of the atom:


  a beam of alpha particles (He2+ ions) were directed at a thin sheet of gold foil

  they expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
• findings of rutherford's experiment:

  • most of the alpha particles passed straight through the foil
  • some of the alpha particles changed direction but continued through the foil
  • a few of the alpha particles bounced back off the gold foil

  the bouncing back could not be explained by the plum pudding model, so a new model had to be created- this was the first evidence of the structure of the atom
• rutherford's experiment:when alpha particles are fired at thin pieces of gold foil:

  • THE MAJORITY OF THEM GO STRAIGHT THROUGH
  • this happens because the atom is mainly empty space
  • SOME ARE DEFLECTED THROUGH SMALL ANGLES
  • this happens as positive alpha particles are repelled by the positive nucleus which contains most of the mass of the gold foil atoms
  • A VERY SMALL NUMBER ARE DEFLECTED STRAIGHT BACK
  • this is because the nucleus is extremely small

  
• an ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons- an atom will lose or gain electrons to become more stable

  a stable atom is normally electrically neutral- same number of protons (+ charge) and electrons (- charge)
• positive ions are formed when atoms lose electrons (there will be more protons than electrons)
• negative ions are formed when atoms gain electrons (there will be more electrons than protons)
• positive vs negative ions

  
• the structure of an atom is made up of:

  • a positively charged nucleus at the centre (made of protons and neutrons)
  • negatively charged electrons in orbit around the nucleus

  
• we can describe the nucleus of an atom by defining its atomic number Z and nucleon number (mass number) A and be able to calculate the number of neutrons in a nucleus

  
• the proton number (Z) is the number of protons in an atom. elements in the periodic table are ordered by their proton number, so it determines which element an atom is
• the proton (atomic) number of a particular element is always the same:

  • hydrogen always has 1 proton, so atomic nr. 1
  • sodium has 11 protons and an atomic number of 11
  • uranium has an atomic nr. of 92, so 92 protons
• the atomic nr. is also equal to the number of electrons in an atom, as atoms have no overall charge (are neutral) so have the same amount of protons and electrons
• the nucleon number, or mass number, is the total number of particles in the nucleus of an atom, so is the number of protons and neutrons in the atom
• the number of neutrons can be found by subtracting the atomic number from the mass number

  • NUMBER OF NEUTRONS= NUCLEON (MASS) NR. - PROTON (ATOMIC) NR.
  • e.g. sodium atom has mass nr 23, atomic nr 11 --> 23-11=12 neutrons in the atom
• electrons are not counted as part of the mass number due to the fact that their mass is negligible compared to protons and neutrons in the nucleus
• a nuclide is a group of atoms containing the same number of protons and neutrons (5 atoms of oxygen are all the same nuclide but 5 separate atoms)

  atomic symbols are written in a specific notation called nuclide or ZXA notation
• the top number A = nucleon number/ mass nr- total number of protons and neutrons in the nucleus

  lower number Z = proton/ atomic number- total number of protons in the nucleus
• when given an atomic symbol, you can figure out the total number of electrons, protons and neutrons in the atom:

  • PROTONS- nr. of protons=proton nr
  • ELECTRONS- atoms are neutral->in a neutral atom, negative electrons=positive protons
  • NEUTRONS- subtracting proton nr from mass nr/nucleon number
• nucleon- measures a particle in the nucleus (proton/ neutron)
• nuclide- nucleus with a specific combination of protons and neutrons
• 197

  Au

  79?


  -79 protons

  -79 electrons

  -118 neutrons
• isotopes- atoms of the same element that have an equal number of protons but a different number of neutrons--> each element can have more than one isotope
• although the number of protons is always the same in a particular element, number of neutrons can differ->isotopes
• isotopes tend to be more unstable due to their imbalance of protons and neutrons, so they are more likely to decay
• isotopes occur naturally, but some are rarer than others (e.g. 2 in 10,000 Hydrogen atoms is Deuterium, and 1 in every billion billion hydrogen atoms is Tritium)
• relative mass is a way of comparing particles, and is measured in amu (atomic mass units)

  • a relative mass of 1 = 1.67x10^-27 kg
• charge can be positive or negative, and relative charge can be used to compare particles:

  • the fundamental charge is equal to the size of the charge on a proton and an electron, however the electron's charge is negative
• table of relative charge and mass of the subatomic particles:

  
• if a particle has 0 relative charge, it is NEUTRAL
• both the proton and neutron have relative mass 1--> have the same mass
• nuclear charge is normally stated as the relative charge of the nucleus ('relative' means the charge of the particle divided by the charge of the proton)
• proton nr= nr of protons in the nucleus

  as nuclei are made up of only protons and neutrons, the proton nr decides the relative charge on a nucleus
• What is the relative charge of the Chromium nucleus:

  52

  Cr

  24?


  1. determine proton nr (24)
  2. state the relative mass of 1 proton (1 proton- relative charge of +1)
  3. multiply relative charge by proton nr (24x1=+24)
  4. REMEMBER TO INCLUDE THE + OR - SIGN