early ideas about atoms

• Atom


  The smallest part of an element that can exist
• Atoms


  • Consist of a nucleus containing protons and neutrons, surrounded by electrons in shells
  • The number of subatomic particles in an atom can be calculated from the atom's atomic number and mass number
• Atomic number


  The number of protons in the nucleus of an atom
• Mass number


  The total number of protons and neutrons in the nucleus of an atom
• Isotopes


  Atoms of the same element with the same number of protons but different numbers of neutrons
• Relative atomic mass


  The average mass of the atoms of an element compared to the mass of a carbon-12 atom
• Early ideas about atoms

  • John Dalton thought atoms were tiny spheres that could not be divided
  • J.J. Thomson discovered the electron and proposed the plum pudding model
  • Rutherford's experiment led to the nuclear model of the atom
• Plum pudding model


  An early model of the atom in which an atom is a ball of positive charge with negative electrons embedded in it
• Nuclear model


  The scientific idea that an atom has electrons surrounding a nucleus that contains protons and neutrons
• Alpha particle


  Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus)
• Nucleus


  The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom
• The mass of an atom is concentrated at its centre, the nucleus
• The nucleus is positively charged
• Atom


  Consists of a nucleus containing protons and neutrons, surrounded by electrons in shells
• Subatomic particles in an atom

  Can be calculated from the atom's atomic number and mass number
• Niels Bohr's nuclear model


  • Electrons orbit the nucleus in shells at certain distances from the nucleus
  • Calculations agreed with experimental observations
• Proton


  Subatomic particle with a positive charge and a relative mass of 1
• Neutron


  Uncharged subatomic particle, with a mass of 1 relative to a proton
• James Chadwick found evidence for the existence of neutrons in the nucleus
• Atom


  Consists of a nucleus containing protons and neutrons, surrounded by electrons in shells
• Nucleus


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


  Subatomic particles with a negative charge and a negligible mass relative to protons and neutrons, arranged in shells around the nucleus
• The number of subatomic particles in an atom can be calculated from the atom's atomic number and mass number
• Protons


  Subatomic particles with a positive charge and a relative mass of 1
• Neutrons


  Uncharged subatomic particles, with a mass of 1 relative to a proton
• The radius of a nucleus (1 × 10 -14 m) is less than 1/10,000 of the radius of an atom (0.1 nm or 1 × 10 -10 m)
• The mass of an electron is very small compared to a proton or a neutron
• Most of the mass of an atom is concentrated in its nucleus, which contains protons and neutrons
• Protons are positive, and neutrons are neutral