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ATOMIC STRUCTURE
A History of Atomic Theory
Democritus
• Around 400 BC, the
Greek philosopher
Democritus proposed that
matter was made of tiny
particles that he called
atoms
• His idea was ignored for
2000 years
Dalton
• In 1808, John Dalton
once again proposed
the idea of atoms
• This time, the idea
was taken seriously
by scientists
Dalton’s Model of the Atom
• Dalton pictured an atom
as a solid sphere
• He proposed that atoms
of an element were alike,
and different elements
had atoms with different
masses
Thomson
• In 1898, J.J. Thomson
discovered the
existence of negatively
charged particles, which
he called electrons
Thomson’s Model of the Atom
• In this model, negatively
charged electrons are
embedded in a
positively charged
sphere
• Electrons are able to
leave the atom
• Thomson’s atomic model
became known as the
“Plum Pudding” model
Rutherford
• In his famous1911
experiment, Ernest
Rutherford shot a
stream of positively
charged alpha particles
at a very thin sheet of
gold foil
Here’s what he observed
• Most of the particles
went right through
the foil
• A few were
deflected or
bounced back
• What does this tell
you about the
structure of the gold
foil?
Rutherford’s Atomic Model
• Rutherford’s new model
of the atom consisted of a
positively charged
nucleus surrounded by
orbiting electrons and a
lot of empty space
Bohr
• In 1913 Neils Bohr
studied the spectra
of elements and
decided that the
electrons must be
located in different
levels or orbitals
Bohr model
• Bohr’s new “solar
system” model of
the atom showed
electrons in orbitals
or energy levels
(also called electron
shells)
• His model is still
used today
Atomic Theory Today
• An atom is the smallest particle of an
element
• Atoms are made of smaller subatomic
particles
• Atoms of different elements contain
different numbers of subatomic particles
Subatomic particles
Atoms consist of three
main types of
subatomic particles:
• Protons (+ charge)
• Neutrons (no charge)
• Electrons (- charge)
• Protons and neutrons
are found in the nucleus
• Electrons are found
outside the nucleus
The Nucleus
• The nucleus of an
atom contains
protons and neutrons
• These are held
together by powerful
forces and cannot
easily escape
Atomic Number
• The atomic number of
an element is the
number of protons in an
atom of the element
• Every atom of the same
element has the same
number of protons
Mass number
• Most of the mass of
an atom is found in
the nucleus
• The mass number of
an element = the
number of protons +
neutrons in an atom
Isotopes
• Atoms of the same element can have
different numbers of neutrons
• These different forms are called isotopes
Atomic mass
• The atomic mass (shown on your
periodic table) is the average mass
number of atoms of the element
• Round this number to the nearest whole
number, to find the most common mass
number for that element
Calculating the number of
subatomic particles in an
atom:
• Number of protons = atomic number
• Number of neutrons = mass number atomic number
• Number of electrons = number of
protons
Electrons
• Electrons are found in
the area surrounding
the nucleus
• Electrons have very
little mass
• Electrons are found in
“shells” or energy levels
• Electrons can move out
of their positions
Electron shells
• Each electron shell has
room for a specific
number of electrons
• 1st electron shell:
2
• 2nd electron shell: 8
• 3rd electron shell: 8
Bohr models
• Show number of protons
and neutrons in the
nucleus
• Draw in electron orbitals
• Fill from the inside out (28-8-…)
Magnesium
New Findings in Atomic
Theory
• The study of quantum
physics has led to many
new discoveries and
theories about the
structure of atoms
• Protons and neutrons
are made of even
smaller particles called
quarks
• Electrons shells are not really circular orbits,
but in a series of orbitals with different shapes
• The exact location
of an electron can’t
actually be
determined - just the
probability of finding
in a certain place
and time
• Particle accelerators are used to do research
on atomic structure