Download Atomic Theory

Atomic Theory
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The Atom
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Early Theory
• Democritus (400 BC) thought the world was
made of tiny indivisible particles
• Aristotle believed that all matter was made up
of 4 elements:
•
•
•
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Fire
Earth
Water
Air
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John Dalton
Believed that…
1. All matter is composed of extremely small,
indivisible particles called atoms
2. All atoms of a given element are identical
3. Atoms cannot be created or destroyed
4. Atoms combine to form compounds
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J.J Thompson
• The discovery of electrons
led to Thompsons plum
pudding model of the atom
(blueberry muffin).
• Positive background
speckled with negative
particles
• Atom is seen as divisible.
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Ernest Rutherford
• Suggested the Nuclear Model
• The atom consists of a
nucleus containing most of
the mass and a strong
positive charge in the middle
of the atom
• Nucleus= protons + neutrons
• Electrons form a cloud
around the nucleus
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The Atomic Structure
• Atoms have no charge and are made up of several
subatomic particles. They are:
• 1. Neutrons: are found in the center of the atom
(nucleus). Have no charge or are neutral.
• 2. Protons: are also found in the center of the atom
(nucleus). Have a positive charge.
• 3. Electrons: are found revolving around the nucleus of
the atom in shells or electron clouds. Have a negative
charge.
• The nucleus: is the center of the atom and is made up of
protons and neutrons. (also has a positive charge)
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Terms:
• Electrons: negatively charged particles
around the nucleus, “cloud” formation.
• Protons: positively charged particles
found in the nucleus
• Neutrons: no charge, or neutral particles
also found in the nucleus
• Nucleus: dense middle of the atom, has a
positive charge due to protons
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Neils Bohr
• Believed electrons to be
found in orbital paths
around the nucleus (like
planets around the sun).
• Electron close to the
nucleus were low in
energy and as the orbit
got larger the energy of
the e- increased.
• Mapped out where the
electrons are most likely
to be found
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What makes atoms different from
one another?
• The number of protons an atom has determines
what kind of atom it is. Example: The oxygen
atom has 8 protons therefore we know it is an
oxygen atom.
• You can determine the number of protons an
atom has by looking at its atomic number found
on the periodic table.
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Atomic Number
• The atomic number of an element is found on
the periodic table. They are arranged in
increasing order going from left to right. So
Hydrogen has an atomic number of 1, Helium is
2 and so on.
• The atomic number tells us how many protons
an atom has. This also tells us how many
electrons an atom has, because the number of
protons and electrons are balanced. So an
atom has the same number of protons as
electrons.
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Finding the number of electrons
and protons an atom has:
• To find the number of protons and
electrons an atom has we must look at the
atomic number.
• Ex: Find the number of protons and
electrons a Titanium atom has.
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Atomic Mass and Mass Number
• The atomic mass of an atom also appears
on the periodic table as the weighted
average (because of the isotopes).
• The mass number of an atom is equal to
the number of protons and neutrons it has
in the nucleus.
• Formula:
• Mass # = # protons + # neutrons
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• Atomic number = number of protons (Z)
• Mass number = p+ + n° (A)
• In an atom #p+ = #e14
Calculating the number of neutrons
an atom has:
• To calculate the number of neutrons an atom
has we must use the previous formula:
• Mass # = # protons + # neutrons
• Or
• # neutrons = Mass # - # protons (atomic #)
• Ex: Calculate the number of neutrons that the
sodium atom has.
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Practice
Nuclear Symbol
A (mass number)
E
(element symbol)
Z (atomic number)
•Find the
• number of protons
• number of neutrons
• number of electrons
• atomic number
• mass number
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F
9
80
35
Br
184
74
W
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Isotopes
• All atoms of the same element have the
same number of protons and electrons,
but they don’t have to have the same
number of neutrons. This can result in an
isotope: atoms of the same element that
have different numbers of neutrons,
(changes the mass number and atomic
mass). There are three different isotopes
of Hydrogen.
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Isotopes: Different forms of the same element
based on the number of neutrons in the nucleus
Protium
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