Atoms and Isotopes

• What is the approximate radius of an atom?


  1 x 10$^{-10}$ metres
• What is the composition of an atom's nucleus?


  The nucleus is made up of protons and neutrons
• What is the charge of protons and electrons?


  Protons have a charge of +1 and electrons have a charge of -1
• Why do atoms have no overall electrical charge?


  Atoms have equal numbers of electrons and protons
• Where is most of the mass of an atom located?

  Most of the mass is located in the nucleus
• How does the radius of the nucleus compare to the radius of the atom?


  The radius of the nucleus is less than 1/10000 of the radius of the atom
• How are electrons arranged in an atom?


  Electrons are arranged at different distances from the nucleus in different energy levels
• What happens to an electron's energy level when it absorbs electromagnetic radiation?


  The electron moves to a higher energy level, further from the nucleus
• What occurs when an electron drops to a lower energy level?


  Electromagnetic radiation is emitted
• What is formed when an atom loses one of its outer electrons?


  An atom becomes a positive ion
• What happens to an atom that gains an extra electron?


  The atom becomes a negative ion
• Isotopes

  . All atoms of a particular element have the same number of protons.

  . The number of protons in an atom of an element is called its atomic number.

  . The total number of protons and neutrons in an atom is called its mass number.

  . Atoms of the same element can have different numbers of neutrons.

  . These atoms are called isotopes.

  . For example, carbon has two common isotopes:

  - carbon-12, which contains 6 protons and 6 neutrons

  - carbon-14, which contains 6 protons and 8 neutrons.
• Key Point

  All isotopes of an element contain the same number of protons, otherwise they would be different elements. It is the number of neutrons that is different.
• The Plum Pudding Model


  . The model of the atom has changed over the years.

  . Atoms were once thought to be tiny spheres that could not be divided.

  . The discovery of the electron by J.J. Thompson, in 1897, led to the plum pudding model of the atom, which depicts the atoms as a ball of positive charge with electrons embedded in it, like plums in a pudding.
• Key Point

  New experimental evidence can lead to a scientific model being changed or replaced over time.
• Who adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances?

  Niels Bohr
• What did later experiments suggest about the positive charge of the nucleus?


  It can be divided into a whole number of smaller particles, each with the same amount of positive charge.
• What name was given to the smaller particles that make up the positive charge of the nucleus?

  Proton
• In what year did James Chadwick carry out experiments that led to the discovery of the neutron?


  1932
• What did James Chadwick's experiments provide evidence for?


  That there was another particle within the nucleus called the neutron.
• How did the discovery of the neutron contribute to the nuclear model of the atom?


  It led to further refinement of the nuclear model for the structure of the atom.
• How has the theory of the structure of the atom changed over time?


  • Developed through experiments
  • Refined through peer review
  • Adapted based on new evidence