Structure of the Atom

The Structure
of the Atom

Chapter 13

Joseph F. Alward, PhD
Department of Physics
University of the Pacific

Applets used in this eLecture:
Periodic Table
(Link to applet before lecture and they'll
be ready when you need them.)

Atoms

Hydrogen was the earliest atom, formed when
quarks and an electron came together shortly
after the Big Bang.

Other heavier atoms were formed by smashing
hydrogen atoms together; this took place in the
early stars.

There are 88 naturally occuring basic types of
atoms--called elements.

Carbon (C), hydrogen (H), oxygen (O),
and nitrogen (N) are the four most abundant
elements in living things.

The Plum Pudding Model of the Atom

The atom was once thought to be a solid
ball of positive material of pudding-like
consistency in which were embedded
discrete negatively charged objects
called electrons.
In this (incorrect) model of the atom, the
atom is a ball of uniform density.

Ernest Rutherford

Ernest Rutherford won
1908 Nobel Prize for
studies in radioactivity;
discovered nucleus in
1911. Ernest Rutherford was
the first to show that
the the atom does not
have uniform density,
and that most of the its
mass is located at its
center.

The Nucleus

Rutherford Model of the Atom

Based on his alpha-particle
scattering experiment on
gold, Rutherford concluded
that the atom consisted of
a hard central core where
most of the mass of the
atom rested.

Nuclear Structure

Facts about the nucleus:

Protons and neutrons have roughly the
same mass, and each is about 2000
times as massive as the electron.

The number of protons is the same
as the number of electrons (not shown)
which orbit the nucleus.

For any given element, all nucleii
have the same number of protons,
but the number of neutrons will
vary.

Periodic Table
Shows neutrons and protons, orbiting electrons, and emission
spectra for the first 36 elements. Takes two minutes to load,
but it's worth the wait.

Isotopes

Nucleii of a given element which have
different numbers of neutrons are said
to be isotopes.

Isotopes--meaning "same type"-- have
the same number of protons in their
nucleii, and therefore have the same
electronic and chemical properties,
since it is the number of protons in
the nucleus--and therefore the number
of electrons about the nucleus--which
determines these properties.

The Periodic Table

Atomic number is the number
of protons in the nucleus, which
is the same as the number of
electrons in the neutral atom.

Atomic mass is the number of
neutrons and protons in the
nucleus. Shown in the figure
is the average number.

What is the total number of
neutrons and protons in
methane, CH₄ ?

Relative Size of Nuclei

One fermi (fm) = one billionth of a meter / million

Structure of Sub-Atomic Particles

Quarks

Murray Gell-Mann took
the name quark from
"Three quarks for muster
Mark", in James Joyce's
book Finnegan's Wake.
(1963)
Whimsical names--
called "flavors"--for
the quarks.

Molecules, Atoms, and Nuclei

Nuclei, Nucleon, and Quarks

Line Spectra

Atomic Spectra

Atoms in heated
gases emit and
absorb light of
certain
wavelengths.

Shown at the
left are three
emission spectra
and one
absorption
spectrum.

Neils Bohr Explains the Hydrogen Atom

Neils Bohr, a Danish physicist, treated the
hydrogen atom as if it were an electron of
charge -e orbiting in a circular path about
a proton of charge +e.

Energy Levels
in Hydrogen

Energy Transitions in Atoms

Energy of photon = Energy lost by electron

Louis deBroglie: Hydrogen Electrons Behave Like Waves

Prince Louis deBroglie
(1892-1987)
French aristocrat

l = h / mv
An integral multiple of
wavelengths must fit
in the length 2pr,
otherwise destructive
interference would
occur.

DeBroglie Waves in Bohr's Model

Quantum Mechanics and Probability Clouds