Science - 2018-19

CH.2 a-c, i - Properties & Models of Atoms

The student will investigate and understand that the placement of elements on the periodic table is a function of their atomic structure. The periodic table is a tool used for the investigations of:
a) average atomic mass, mass number, and atomic number;
b) isotopes, half lives, and radioactive decay;
c) mass and charge characteristics of subatomic particles;
i) historical and quantum models.

Bloom's Levels:  Analyze; Understand

Adopted: 2010


  • The structure of an atom determines its properties.
  • Atoms are composed of smaller particles.
  • All elements have isotopes.
  • Some nuclei can change due to radioactive decay.

  • I can explain why the masses of elements on the Periodic Table appear as decimals.
  • I can calculate how long medicine will stay in my body.
  • I can explain how chemical bonding occurs.
  • I can communicate how the model of the atom has changed over time.


  • The periodic table is arranged in order of increasing atomic numbers.
  • The atomic number of an element is the same as the number of protons. In a neutral atom, the number of electrons is the same as the number of protons. All atoms of an element have the same number of protons.
  • The average atomic mass for each element is the weighted average of that element's naturally occurring isotopes.
  • The mass number of an element is the sum of the number of protons and neutrons. It is different from each element's isotopes.
  • An isotope is an atom that has the same number of protons as another atom of the same element but has a different number of neutrons. Some isotopes are radioactive; many are not.
  • Half-life is the length of time required for half of a given sample of radioactive isotope to decay.
  • Electrons have little mass and a negative (-) charge. they are located in electron clouds or probability clouds outside the nucleus. 
  • Protons have a positive (+) charge. Neutrons have no charge. Protons and neutrons are located in the nucleus of the atom and comprise most of its mass. Quarks are also located in the nucleus of the atom.
  • Discoveries and insights related to the atom's structure have changed the model of the atom over time. Historical models have included solid sphere, plum pudding, nuclear, and planetary models. The modern atomic theory is called the quantum mechanical model.


In order to meet this standard, it is expected that students will

a)  determine the atomic number, atomic mass, the number of protons, and the number of electrons of any atom of a particular element using a periodic table.

determine the number of neutrons in an isotope given its mass number.

b)  perform calculations to determine the “weighted” average atomic mass.

     perform calculations involving the half-life of a radioactive substance.

     differentiate between alpha, beta, and gamma radiation with respect to penetrating power, shielding, and composition.

c)  differentiate between the major atom components (proton, neutron and electron) in terms of location, size, and charge.

i)  identify key contributions of principal scientists including:

  •      atomos, initial idea of atom – Democritus
  •      first atomic theory of matter, solid sphere model – John Dalton
  •      discovery of the electron using the cathode ray tube experiment, plum pudding model – J. J. Thomson
  •      discovery of the nucleus using the gold foil experiment, nuclear model – Ernest Rutherford
  •      discovery of charge of electron using the oil drop experiment – Robert Millikan
  •      energy levels, planetary model – Niels Bohr
  •      periodic table arranged by atomic mass – Dmitri Mendeleev
  •      periodic table arranged by atomic number – Henry Moseley
  •      quantum nature of energy – Max Planck
  •      uncertainty principle, quantum mechanical model – Werner Heisenberg
  •      wave theory, quantum mechanical model – Louis de Broglie.
  •      differentiate between the historical and quantum models of the atom.


atom, atomic mass, atomic mass unit, atomic number, cathode ray, Dalton's atomic theory, electron, half-life, isotope, mass number, neutron, nucleus, proton

Updated: Nov 19, 2017