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P. Sci.
Chapter 4
Atoms
Essential Questions
1) What is an atom?
2) What is the difference between
Dalton’s theory, Bohr’s model
and the Electron Cloud theory?
3) How do you find the number of
protons, neutrons and electrons
of an atom?
4) What is the difference
between atomic mass number
and the average atomic mass?
5) What is an isotope?
Atomic Structure – timeline
• Ancient Greece - Democritus
proposed the atom – a tiny solid
particle that could not be
subdivided.
• 1904 – J.J. Thomson –
discovered that atoms contained
small, negatively charged
particles called electrons.
• 1911 – Ernest Rutherford –
proposed that the atom had two
parts – the nucleus in the center
(most of the mass) surrounded
by the electrons.
• 1913 – Niels Bohr –
hypothesized that
electrons traveled in fixed
orbits around the atom’s nucleus.
• 1913 – James Chadwick –
concluded that the nucleus
contained positive protons and
neutral neutrons.
• 1926 – Erwin Schrodinger –
developed the quantum
mechanical model – which is
based on the wavelike properties
of the electron. (not a particle –
leads to quantum physics)
• 1927 – Werner Heisenberg
– (the Heisenberg
uncertainty Principle)
described that it is
impossible to know
precisely both an
electron’s position and
path at a given time. Led to
the electron cloud theory.
electron
neutron
proton
Atoms
• The smallest particle that has the
properties of an element.
Dalton’s Atomic Theory
• Every element is made of tiny,
unique, particles called atoms that
cannot be subdivided.
• Atoms of the same element are
exactly alike.
• Atoms of different elements can
join to form molecules
Parts of an Atom
• Nucleus – small, dense center of
an atom made up of 2 subatomic
particles that are almost identical
in size and mass.
–Protons – have a
positive charge
–Neutrons – have no charge
Parts of an Atom cont.
• Electrons – are tiny particles that
have very little mass that moves
around the outside of the nucleus.
These particles are negatively
charged and form a
“cloud” around
the nucleus.
• The number of protons and
electrons an atom has is unique
for each element.
Atomic Charge
• Atoms have no overall charge
because the protons (+) cancel
out the electrons (-).
Helium
2 protons - +2
2 neutrons 0
2 electrons - -2
total charge 0
Protons
• positive (+) charge
• Found in the nucleus
• # of protons = atomic #
• The number of protons identify
the element (atomic #)
Neutrons
• no charge
• Found in the nucleus
• Along with protons makes up
atomic mass
• protons and neutrons are made up
of quarks – which three quarks –
determines whether it becomes a
proton or a neutron.
Electrons
• negative (–) charge
• travel in orbitals (or energy
levels) around the nucleus.
(electron cloud)
• valence electrons - the # of
electrons in the outer shell =
oxidation #
• Unit of measure for atomic
particles is Atomic mass unit
(amu) protons and neutrons =
about 1 amu (electrons are
about the size of 1 quark)
Chemical symbols
• The one or two letter abbreviation
of the element name.
Mass Number or Atomic Mass
• the sum of the number of protons
and the number of neutrons in the
nucleus of an atom.
• # of neutrons =
Neutrons
mass # - atomic #
Protons
+
Average atomic mass
• the weighted - average mass of
the mixture of all an atoms
isotopes. The average atomic
mass is close to the mass of its
most abundant isotope.
Isotopes
• Atoms of the same element that
have a different # of neutrons.
(identified by the element name
followed by the mass # )
• ex. C-12, C-14, B-10, B-11)
Carbon 14 = 8 neutrons
Carbon 12 = 6 neutrons