World of Chemistry Chapter 11—Modern Atomic Theory
I.
II.
III.
IV.
Price
Rutherford’s atom
A. Rutherford discovered the atomic nucleus but could not
describe the electrons in his model
B. Rutherford imagined that the electrons moved around the
nucleus in circular orbits like planets around the sun
C. Rutherford could NOT explain why the negative electrons did
not collapse into the positively charged nucleus.
Emission of Energy by Atoms
A. When atoms are exposed to energy they become “excited,”
that is, they take in some of the energy (typically heat or light)
B. Atoms do not like being excited and try to get back to a lower
energy state. To do this they must emit, or give away some of
their energy.
C. An atom will emit a specific quantity of energy when it returns
to its lowest (ground) state. This specific quantity of energy
can be emitted as a wave of light at a color corresponding to
the amount of energy emitted.
The Quantized Nature of Atoms
A. The lowest energy state is called the “ground” state and
excited atoms are always trying to get back to this lowest
energy state.
B. The fact that excited atoms always emit the same quantity of
energy (i.e. wavelength of light) is profoundly significant. This
implies that there are only a few energy levels that an excited
atom can reach before returning to its ground state.
C. By looking at the energy released by an excited atom we can
obtain a “signature” of the excited atom. By matching the
pattern of light emitted by an excited atom (the spectrum) to
those of known atoms, we can identify unknown atoms. This is
known as “spectroscopy”.
Niels Bohr
A. This “quantized” model of the atom was first proposed by a
Danish physicist named Niels Bohr (1885-1962)
B. Bohr still envisioned circular orbits around a positive nucleus
but thought that electrons could “jump” to higher orbits when
they absorb energy and “fall” back to lower orbits by emitting
energy.
C. Although Bohr’s model worked well for hydrogen it failed to
describe and predict the behavior of other atoms.
Nonetheless, Bohr paved the way for the wave mechanical
model of the atom widely accepted today.
V.
The Wave Mechanical Model of the Atom
A. In Bohr’s model the electron moved around the nucleus in
circular orbits. In the wave mechanical model electrons are
found to occupy certain areas around the nucleus called
orbitals. Orbitals are nothing like orbits!!!
B. An orbital is an area where we will probably find an electron
90% of the time. We can never know where an electron is or
predict where it will be next.
C. The wave mechanical model provides us with 4 types of
orbitals.
s orbitals are sphereical
p orbitals have 2 lobes
d orbitals have 4 lobes
f orbitals look like this
VI.
VII.
Spin and Exclusion
A. The wave mechanical model provides us with one more
characteristic of electrons: spin
B. Electrons seem to spin around an axis just like a child’s toy top.
2 electrons can share an orbital only if they have opposite
spins.
C. Each orbital can only hold 2 electrons!
D. Pauli Exclusion Principle: “An atomic orbital can hold a maximum
of two electrons, and those two electrons must have opposite
spins.”
Modern Electron Configurations
A. The wave mechanical model gives us a more sophisticated way
of describing electrons in their orbitals—No More Shells!
(They didn’t work that great anyway!)
B. Refer to daigrams 11.31 and 11.32 on p.345 of your book for the
order and pattern of orbital filling (be very careful with the
lanthanides and actinides!)
C. Shortcut: To save time, space and energy put the preceding
noble gas in square brackets and start your electron
configuration from the beginning of the period!