Download Ancient and Modern Atomic Theory PPT

4.1 Studying Atom
Early evidence used to develop models
of atoms.
Democritus
• said that all matter consisted of
extremely small particles that could
NOT be divided
• called these particles atoms from the
Greek word atomos, which means
“uncut” or “indivisible.”
Aristotle
• thought that all substances were made of
only four elements—earth, air, fire, and
water.
• Aristotle’s views on the structure of
matter were accepted for many centuries.
• By the 1800s, scientists had enough
experimental data to support an atomic
model.
Dalton’s Atomic Theory
Dalton proposed the theory
that all matter is made up
of individual particles
called atoms, which cannot
be divided.
Dalton’s Theory
• All elements are composed of
atoms.
• All atoms of the same element
have the same mass, and
atoms of different elements
have different masses.
Dalton’s Theory
• Compounds contain atoms of
more than one element.
• In a particular compound,
atoms of different elements
always combine in the same
way.
Thomson’s Model of the Atom
Thomson’s experiments
provided the first evidence
that atoms are made of
even smaller particles
(subatomic particles).
Thomson’s Model
• Thomson revised Dalton’s model to
account for these subatomic particles.
• The atom has neither a positive nor a negative
charge, but there must always be some
positive charge in the atom.
• The atom is filled with a positively charged
mass of matter that has negative charges
evenly scattered throughout it.
Rutherford’s Atomic
Theory
According to Rutherford’s
model, all of an atom’s
positive charge is
concentrated in its nucleus.
Rutherford’s Model
• Rutherford proposed a new model.
• The positive charge of an atom is not evenly
spread throughout the atom.
• Positive charge is concentrated in a very
small, central area.
• The nucleus of the atom is a dense,
positively charged mass located in the
center of the atom.
4.3: Modern
Atomic
Theory
Bohr’s Model of the Atom
• Focuses on electrons and their
arrangement.
• Bohr stated that electrons move with
constant speed in fixed orbits around the
nucleus, like planets around a sun.
• Bohr correctly assigned energy levels to
electrons, but electrons do not move in
fixed orbits around the nucleus.
What are Energy Levels?
• The possible energies that electrons in an
atom can have are called energy levels.
– When an atom gains or loses energy, the
energy of an electron can change.
• An electron in an atom can move from one
energy level to another when the atom gains or
loses energy.
– An electron cannot exist between energy
levels.
Bohr’s Model of the Atom
Electron
Nucleus
Electron Cloud Model
• The electron cloud model replaced
Bohr's vision of electrons moving in
predictable paths.
• An electron cloud is a visual model
of the most likely locations for
electrons in an atom.
– probability of finding an electron is higher in the
denser regions of the cloud.
Electron Cloud Model
• An electron cloud is a good
approximation of how electrons
behave in their orbitals.
– An orbital is a region of space around the
nucleus where an electron is likely to be
found.
– The electron cloud represents all the
orbitals in an atom.
Electron Cloud Model
Electron Cloud Model
• The level in which an electron has the least
energy—the lowest energy level—has only
one orbital. Higher energy levels have more
than one orbital.
Electron Configuration
• An electron configuration is
the arrangement of electrons
in the orbitals of an atom.
• The most stable electron configuration is the
one in which the electrons are in orbitals with
the lowest possible energies.
We will draw Bohr
Models to illustrate the
number of electrons for
an element.
Before drawing a Bohr Model…
• Horizontal rows on the Periodic Table are called
periods.
– The row number tells you how many energy levels, or
rings, the element has.
– Elements in the same row have the same number of
rings.
– The number of rings increases as you move DOWN
the Periodic Table.
• Example: Sodium (Na) and Magnesium (Mg) are
in the third row so they both will have three rings
in their Bohr Model.
Horizontal Rows=Period=Number of Rings
1
2
3
4
5
6
Will not draw Bohr Models for Transition Metals
Before drawing a Bohr Model…
• Vertical columns on the Periodic Table are called
groups.
– Group numbers tell you how many valence electrons
each element in that column has.
• Valence electrons are electrons that are located on the LAST
energy level.
– The number of valence electrons increases as you
move ACROSS the Periodic Table.
• Example: Sodium and Potassium are both in the
1st column, so they both will have one valence
electron in their last energy level.
– If the group number is higher than nine, so 10-18,
drop the one to determine the number of valence
electrons.
Vertical Columns=Groups=Number of Valence
Electrons
1
2,8
3 4 5 6 7
2
Will not draw Bohr Models for Transition
Metals
Drawing Bohr Models
1. Determine the number of rings, or energy levels.
(Look at the period, or row, number.)
2. Determine the number of protons, neutrons,
and electrons.
3. Determine the number of valence electrons.
(Look at the group, or column, number.)
4. Draw the correct number of rings.
5. Draw the correct number of electrons on each
ring, or energy level. (Use Energy Levels,
Orbitals, and Electrons table.)