Atomic Theory
• Chemistry I
• Chapter 3
Dalton’s Atomic Theory
Dalton developed a theory to explain why the
elements in a compound always join in the
same way.
Dalton proposed the theory that all
matter is made up of individual
particles called atoms, which can
not be divided.
Dalton’s Atomic 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.
• 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
He used an electric current to learn more
about atoms.
Thomson’s experiments provided the first
evidence that atoms are made of even
smaller particles.
Thomson’s “plum pudding” model suggested
that the negative charges in an atom were
evenly scattered throughout an atom filled
with a positively charged mass of matter.
Rutherford’s Atomic Theory
• Rutherford’s model concluded that all of
an atom’s positive charge is concentrated
in its nucleus.
• He used the gold foil experiment to
determine the behavior of the alpha
particles when they struck different
locations in the gold foil. He originally
thought that the path of the alpha particles
would only be slightly bent. He was
wrong. Some of the particles bounced
straight back.
Rutherford’s Gold Foil
Experiment
Subatomic Particles
• Protons are positively charged subatomic
particle that are found in the nucleus. +1 charge
• Electrons are negatively charged subatomic
particles that are found in the space outside the
nucleus. -1 charge
• Neutrons are neutral subatomic particles that
are found in the nucleus. 0 charge or neutral
• Protons, electrons, and neutrons can be
distinguished by mass, charge, and location in
an atom.
Atomic Number and Mass Number
• The atomic number of an element is the same as the
number of protons in an atom of that element.
Atoms of different elements have different numbers of
protons.
Hydrogen has 1 proton so its atomic number is 1.
Silver has 47 protons so its atomic number is 47.
• The mass number of an atom is the sum of the
protons and neutrons in the nucleus of that atom.
To find number of neutrons:
Mass of neutrons = Mass number – Atomic number
Atoms are neutral
• When they have an equal number of
protons and electrons.
• If Hydrogen has 1 proton, it has 1 electron.
+1 + -1 = 0
It is neutral.
• If Silver has 47 protons, it has 47
electrons.
+47 + -47 = 0
It is neutral.
Isotopes
Isotopes are atoms of the same
element that have different
numbers of neutrons and
different mass numbers.
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Bohr’s Model of the Atom
• Bohr’s model focused on the electrons in an
atom.
• Bohr’s model stated that electrons move with
constant speed in fixed orbits around the
nucleus, like planets around a sun.
• When an atom gains or loses energy, the
energy of an electron can change.
• Possible energies that electrons in an atom
have are called energy levels.
• An electron in an atom can move from one
energy level to another when the atom gains
or loses energy.
Bohr Model and Subatomic
Particles
Atomic number is
the number of
protons.
Mass number is
the number of
protons and
neutrons.
Electron Cloud Model
An electron cloud is a visual model
of the most likely locations for
electrons in an atom.
The Modern Periodic Table
is organized so that the elements are
arranged in ascending order of atomic
number.
The symbols are a combination of ancient
and modern symbols, from several
languages (such as Latin, English, and
German). All symbols consist of one or
two letters, with the first capitalized and
the second lower case.
Nitrogen is N, Calcium is Ca, Silver is Ag
The location of electrons
is not known at any
given time, except that
they move within
specific energy levels
and have a certain
amount of energy.
Electrons in the first
energy level have
lower total energy than
electrons farther away
from the nucleus.
Energy Levels
Energy Level
1
# of orbitals
Max # of electrons
1
2
(Energy Level Squared)
(2 x # of orbitals)
2
4
8
3
9
18
4
16
32
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.