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Chapter 4
Atomic Structure
Section 4.1, Studying Atoms
 Timeline
of Atomic Models
 Ancient Greek Models of Atoms
 About
540 B.C., the Greek scientist
Democritus proposed the first model of the
atom.
 All matter is made of small invisible,
indivisible particles called atomos, which later
became our word atom.
 His theory was rejected by the Greek
scientific community.
Section 4.1, Studying Atoms
 Timeline
of Atomic Models
 John Dalton’s Atomic Theory (1803)
 Dalton was an English scientist born in
1766.
Section 4.1, Studying Atoms
 Timeline
of the Atomic Model
 Dalton’s atomic theory had four main parts
 1.
All Elements are composed of atoms.
 2. All atoms of the same atom have the
same mass and different elements
have
different masses.
 3. Compounds contain elements of
more than one type of atom.
Section 4.1, Studying Atoms
 Timeline
of the Atomic Model
 Dalton’s atomic theory (continued)
4. In a particular compound, atoms of
different elements always combine in
the same proportion (ratio). Also
known as the law of definite
proportions.
Daltons model of the
atom was a sphere.
He was not sure
how the spheres
(atoms) were
connected, but knew
that they did
combine in definite
ratios or
proportions.
Section 4.1, Studying Atoms
 Timeline
of the Atomic Model
 J. J. Thomason Model (1909)
 J.J.
Thomason (1856 – 1940), an English born
scientist developed a model of the atom
composed of positively and negatively
charged particles.
 He had a partner named Nagayoka which
your book doesn’t mention.
Section 4.1, Studying Atoms
 J.J.
Thomason Model (1909)
 Thomason’s model showed us the
following:
 1.
Atoms were made of smaller (subatomic particles.
 2. The sub-atomic particles carried
positive and negative charges.
 3. Overall atomic charge is neutral.
 4. Negative charges balance the
positive charges.
Thomason-Nagaoka Model 1909
Section 4.1, Studying Atoms
 J.J.
Thomason Model (1909)
 Thomason had the nucleus (positively
charged area) and the electrons (negatively
charged particles) in one spherical mass.
 The model had several nicknames: The
Plum Pudding Model, The Watermelon
Model, and The Raisin Cookie Model.
Section 4.1, Studying Atoms
 Rutherford’s
Atomic Theory (1911)
 Ernest Rutherford was an Australian born
physicist.
 His
first major discovery was the alpha
particles given off in the radioactive decay of
Uranium.
 Alpha particles are very small and consist of 2
protons and 2 neutrons (a helium nucleus)
and carry a positive charge.
Section 4.1, Studying Atoms
 Rutherford’s
Atomic Theory
 It
shows that there is a lot of empty space in
atoms between the nucleus and electrons.
 It also (unlike Thomason-Nagaoka) places the
electrons outside the nucleus.
 The
Gold Foil Experiment
 Rutherford
used Polonium (Po), another
radioactive substance to direct a beam of
alpha (α) particles at a thin sheet of gold foil.
Section 4.1, Studying Atoms
 The
Gold Foil Experiment (continued)
 Rutherford
predicted that the mass and
charge of the gold atoms in the foil would not
be great enough to deflect the α particles and
they would travel straight through.
 As the diagram shows, many of the alpha
particles were deflected to varying degrees.
 Rutherford felt that the deflection was caused
by the α particles coming close to a positively
charged particle.
Section 4.1, Studying Atoms
 The
Gold Foil Experiment (Continued)
 This evidence led Rutherford to the
following conclusions:
 Deflected
α particles had come close to a
positively charged particle.
 + and – charges are not equally distributed in
an atom.
 The nucleus is dense and positively charged.
 Electrons are located outside the nucleus.
Notice, that in
Rutherford’s Atomic
Model, the
electrons
(negatively charged
particles) are
located outside the
nucleus, located in
the center of the
atom.
Section 4.1, Studying Atoms
 The
Gold Foil Experiment (Continued)
 From
the time of Democritus (540 BC) to
The Gold Foil Experiment (1911), we have
the classical development and study of our
atomic model. From this point forward, we
will study modern atomic theory, which will
begin with Neils Bohr (a student of
Rutherford’s), but first we will learn of a few
of the sub-atomic particles that we have
discovered.
Section 4.2, The Structure of an Atom
 Properties
of Sub-Atomic Particles (cont)
 The three sub-atomic particles we will
study are: Protons, Neutrons, and
Electrons.
 Electrons
charged (-1 per electron e-)
Found outside the nucleus
Low mass low density particles
Outer e- are involved in chemical bonding
Negatively
Section 4.2, The Structure of an Atom
 Properties
of Sub-Atomic Particles
 Protons
charged (+1 per proton p+)
Found in the nucleus of the atom
Very dense, high mass particles (equal to
the density/mass of a neutron)
Positively
Section 4.2, The Structure of an Atom
 Properties
of Sub Atomic Particles (cont.)
 Neutrons
Carries
a “0” or neutral charge (symbol n)
Found inside the nucleus
Very dense, high mass particle
Discovered by the English physicist James
Chadwick in 1932.
Differences in numbers of neutrons in the
same type of atoms account for isotopes.
Section 4.2, The Structure of an Atom
Section 4.2, The Structure of an Atom
Section 4.2, The Structure of an Atom
 Atomic
Number and Mass Number
 Atomic Number
 The
atomic number of an atom equals the
number of protons in its nucleus.
 Atoms of the same element have the same
number of protons.
 Each proton has a charge of +1.
 The number of protons is balanced by an
equal number of electrons, giving the atom an
overall charge of “0”.
Section 4.2, The Structure of an Atom
 Atomic
Number and Mass Number (cont.)
 Mass Number
 The
mass number of an atom is the sum of
the number of protons and neutrons in the
nucleus (#Protons + #Neutrons = Mass #)
 All atom of an element must have the same
number of protons, but they can have
different numbers of neutrons.
 Most mass numbers are fractional due to the
presence of isotopes.
Section 4.2, The Structure of an Atom
 Atomic
Number and Mass Number (cont.)
 Isotopes
 Isotopes
are atoms of a specific element with
the same number of protons and electrons,
but different numbers of neutrons.
 Different isotopes have different mass
numbers.
 Different isotopes can have different chemical
and physical properties.
Section 4.3, Modern Atomic Theory
 Bohr’s
 Neils
Model of the Atom
Bohr (1885 – 1962) was a former
student of Ernest Rutherford.
 Unlike Rutherford’s model, Bohr’s model of
the atom focused more on the electrons.
Where individual electrons were located
around the nucleus.
How the electrons moved in their orbits.
Section 4.3, Modern Atomic Theory
 Bohr’s
Model of the Atom (1913)
 Bohr’s
model had electrons in fixed orbitals
(energy levels) around the nucleus.
 The Bohr model was sometimes referred to as
the “Planetary Model”.
Section 4.3, Modern Atomic Theory
 The
Bohr Model of the Atom (1913)
 Energy
levels closest to the nucleus are
considered to be the lower energy levels and
those farther away are higher energy levels.
 By absorbing or releasing energy, an electron
can change its energy level
 Electrons can’t stay between energy levels.
Section 4.3, Modern Atomic Theory
 Electron
Cloud Model (also referred to as
the Quantum-Mechanical Model).
 Electrons
do not have fixed orbits as in the
Bohr model.
 Electron movement is based on probable
areas around the nucleus where electrons
might be found (the electron cloud).
Current Quantum-Mechanical
Section 4.3, Modern Atomic Theory
 Atomic
 An
Orbitals
orbital is an area in space around an
atomic nucleus in which an electron is most
likely to be found.
 There are different types and shapes of
orbitals.
 An electron cloud is a good approximation of
how electrons behave in their orbits.
Shape of a Typical S – Orbital
Shapes of typical P - Orbitals
Shape of Typical d - Orbitals
Comparison
s, p, andAtomic
d Orbital
Shapes
Section 4.3,ofModern
Theory
Notice how each of the different orbitals
have different shapes.
Element
Atomic
Number
Number of
Electrons
Number of
Protons
Atomic
Mass
Number of
Neutrons
Elemental
Symbol
Section 4.3, Modern Atomic Theory
Section 4.3, Modern Atomic Theory
Section 4.3, Modern Atomic Theory
 Electron
Energy Levels and Configurations
 Electrons
in the lowest energy levels have the
least energy.
 When possible, electrons travel in pairs.
 The most stable electron configuration of an
atom is when the outer orbital is filled.
 This
is called the octet rule.
 The noble gases are an example of this rule.
 All atoms that don’t have complete outer orbits
try to complete them by bonding with other
atoms
Flame Photometry
Comparison of Protons, Neutrons and
Electrons