Atom
•An atom is the smallest particle of an element that
retains the chemical properties of that element.
•Atoms consist of two regions. The nucleus is at the
center of the atom and contains at least one
positively charged particle called a proton and one
or more neutral particles called neutrons.
electron
neutron
proton
•Surrounding the nucleus is a region occupied by
negatively charged particles called electrons (this
region is very large compared to the nucleus.
•Protons, neutrons, and electrons are usually
referred to as subatomic particles.
electron
neutron
proton
Discovery of the Electron
•The discovery of the electron occurred in the late
1800s when scientists were investigating the
relationship between electricity and matter. The
experiments were carried out in cathode-ray tubes.
Cathode-Ray Tube
A simple cathode-ray
tube. Particles pass
through the tube from
the cathode, the metal
disk connected to the
negative terminal of
the voltage source,
to the anode, the
metal disk connected
to the positive
terminal.
•The experiments carried out by Joseph John
Thomson in 1897, showed the particles that
composed the cathode ray were negatively charged.
•Thomson also thought that atoms must contain
some positive charge.
•Watch video clip.
•Thomson suggested that an atom
consisted of a cloud of positive
charge with the negative electrons
embedded in it. This model is
often called the “plum-pudding”
model.
•In 1909 experiments conducted by Robert
A. Millikan showed that atoms were
electrically neutral and must contain a
positive charge to balance the negative
electrons. They also showed that atoms
must contain other particles that account
for most of their mass.
•Watch video clip.
Discovery of the Atomic Nucleus
•In 1911 Ernest Rutherford, Hans Geiger, and Ernest
Marsden bombarded a thin piece of gold foil with
fast-moving alpha particles. Some particles were
redirected back toward the source.
•Rutherford concluded that this was caused by a
densely packed bundle of matter with a positive
charge (nucleus).
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance
path of invisible
-particles
Most particles passed through.
So, atoms are mostly empty.
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
Composition of the Atomic Nucleus
•The nuclei by atoms of different elements differ in
the number of protons they contain (the number
of protons in an atom’s nucleus determine the
atom’s identity.
When two protons are extremely close to each other, there
is a strong attraction between them.
A similar attraction exists when neutrons are very close to
each other or when protons and neutrons are very close
together.
These short-range proton-neutron, proton-proton, and
neutron-neutron forces hold the nuclear particles together
and are referred to as nuclear forces.
Nuclear Forces
The Sizes of Atoms
•The sizes of atoms are expressed in picometers
( 1 pm = 1 x 10-12 m).
•To get an idea of how small a picometer is,
consider that 1 cm is the same fractional part of
103 km (about 600 miles) as 100 pm is of 1 cm.
•Atomic radii range from about 40 to 270 pm.