P4

• Atom


  The smallest part of an element that can exist
• Plum pudding model


  The scientific idea that an atom is a sphere of positive charge, with negatively charged electrons in it
• Electron


  Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons
• Solids cannot be squashed, therefore the atoms which make them up must be solid throughout
• Rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide
• Atom


  The building block of matter
• The idea of the atom as the building block of matter has developed over time
• Alpha particle


  Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus)
• Rutherford's experiment


  1. Directed a beam of alpha particles at a thin gold leaf

  2. Observed the deflection of the alpha particles
• Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking
• If the plum pudding model had been correct then all of the fast, highly charged alpha particles would have whizzed straight through undeflected
• Rutherford's observations


  • Most of the alpha particles did pass straight through the foil
  • A small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil
  • A very small number of alpha particles came straight back off the foil
• Rutherford's conclusions


  • The atom is mostly empty space
  • There is a concentration of positive charge in the atom
  • The positive charge and mass are concentrated in a tiny volume in the atom (the nucleus)
• Nucleus


  The central part of an atom, containing protons and neutrons, and having most of the mass of the atom
• Atom


  The building block of matter
• Nucleus


  The central part of an atom, containing protons and neutrons, and having most of the mass of the atom
• Electron


  Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons
• Proton


  Nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass
• Neutron


  Neutral particle the same size as a proton, keeping the nucleus stable and making up the mass
• Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil to prove the existence of the neutron
• Atom


  Made up of protons, neutrons and electrons
• Isotope


  Atoms of an element with the same number of protons and electrons but different numbers of neutrons
• Ion


  Atom with a different number of electrons than protons
• An element's atomic number defines it
• An element's mass number can vary due to different numbers of neutrons
• If the number of protons changes, then it is a different element
• Atoms are made up of protons, neutrons and electrons
• Isotope


  An atom with a different number of neutrons
• Ion


  An atom that has gained or lost electrons, becoming a charged particle
• Normally, atoms are neutral and have the same number of protons and electrons
• Atoms can lose or gain electrons due to collisions or other interactions, forming charged particles called ions
• Positively-charged ion


  An atom that has lost one or more electrons
• Negatively-charged ion


  An atom that has gained one or more electrons
• With the wrong number of neutrons, nuclei can fall apart
• Radioactive decay


  1. Nucleus emits alpha or beta particles

  2. Nucleus 'cools down' by emitting gamma radiation
• Stable nucleus


  Nucleus has the right amount of neutrons for the number of protons it has
• Elements with fewer protons, such as the ones near the top of the periodic table, are stable if they have the same number of neutrons and protons
• As the number of protons increases, more neutrons are needed to keep the nucleus stable
• Nuclei with too many, or too few, neutrons do exist naturally but are unstable and will decay by emitting radiation
• Radioactive decay


  The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so