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SECTION SUMMARY
5.1 Models of Atom Summary:
Rutherford’s planetary model states that in the nuclear atom, the protons and neutrons
are located in the nucleus. The electrons are distributed around the nucleus and
occupy almost all the volume of the atom. It could not explain the chemical
properties of elements. For example, it could not explain why metals have colors
when heated in flame. To be able to explain the chemical properties of elements or
compounds, scientists need to have a model that can describe the behavior of
electrons within atoms. The chemical properties of atoms are related to the
arrangement of the electrons within them.
John Dalton proposed the first modern atomic theory. He portrayed the atom as a
solid, indivisible mass. Next, J. J. Thomson discovered the electron and proposed the
plum-pudding model in which negatively charged electrons were embedded in a
positively charged mass.
Bohr proposed that electron move only in specific circular paths, or orbits, around the
nucleus. Electrons cannot be between energy levels. To move from one energy level
to another, an electron must gain or lose just the right amount of energy. In general,
the higher an electron is on the energy leader, the farther it is from the nucleus.
The quantum mechanical model determines the allowed energies an electron can have
and how likely it is to be found in various locations around the nucleus. It is the
modern description of the electron in atoms. It estimates the probability of finding an
electron within a certain volume of space surrounding the nucleus.
Each sublevel of a principal energy level corresponds to an orbital shape describing
where the electron is likely to be found. There is one orbital in s sublevel, three
orbitals in p sublevel, five orbitals in d sublevel, and 7 orbitals in f sublevel.
5.1 Models of Atom Vocabulary Terms:
Energy levels:
the fixed energies an electron can have
Quantum:
the amount of energy required to move an electron
from one energy level to another energy level
Quantum mechanical model:
the modern description of the electrons in atoms
Atomic orbital:
a region of space in which there is a high
probability of finding an electron
The End of the Summary