ATOMS
The Building Blocks of Matter
History of the Atom
• Around 400 B.C. Democritus was the first person
to use the term atom.
– Atom is from the Greek word “indivisible”
• 1704 Isaac Newton proposed a mechanical
universe with small solid masses in motion
• 1808 John Dalton proposed his atomic theory
based upon measurable properties of mass
Properties of Mass
• There are several basic laws which drive Atomic Theory
• Law of Conservation of mass
– Matter is never created nor destroyed during ordinary physical
and chemical changes
• Law of Definite Proportions
– States that a chemical compound contains the same elements in
exactly the same proportions by mass regardless of the size of
the sample or source of the compound
• Law of Multiple Proportions
– If two or more different compounds are composed of the same
two elements, then the ratio of the masses of the second
element combined with a certain mass of the first element is
always a ratio of small whole numbers
Dalton’s Atomic Theory
• In 1808, an English schoolteacher named
John Dalton proposed an explanation for
the different properties of mass.
• Reasoned that elements were composed
of atoms and combine to form compounds
Daltons Atomic Theory
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All matter is composed of extremely small particles
called atoms.
Atoms of a given element are identical in size, mass,
and other properties; atoms of different elements differ
in size, mass, and other properties
Atoms cannot be subdivided, created, or destroyed
Atoms of different elements combine in simple wholenumber ratios to form chemical compounds
In chemical reactions, atoms are combined, separated,
or rearranged
Dalton’s WRONG!!!!
• We now know that atoms can be divided
into smaller particles.
• An element can have atoms of different
masses, called isotopes.
Structure of the Atom
• Nucleus
– Dense, positively charged center of the atom
– Composed of protons and neutrons
– Makes up the mass of an atom
• Protons
– Positively charged
• Neutrons
– No charge
• Electrons
– Negatively charged, located outside the nucleus
– Makes up the size of the atom
75047
Electrons
• In 1897, J.J. Thomson, using cathode ray
tubes, found negatively charged particles
called electrons
– https://www.youtube.com/watch?v=O9Goyscbazk
• In 1909, Robert Millikan measured the
charge of the electron
– Oil drop experiment
• https://www.youtube.com/watch?v=XMfYHag7Liw
Plum Pudding Model
• Proposed by Thomson
• Negative electrons
were spread
throughout the positive
charge of the atom
• Like plums in plum
pudding, thus the
name…get it
• He was wrong!!!
Glowing Matter
• Around 1900, Max Planck explained the
phenomenon of hot glowing matter.
• Explained this using discrete units of
energy he called “quanta” or “quantum”
• This idea would later be used to “correctly”
place electrons in an atom.
Behold…The Nucleus
• In 1911, Ernest Rutherford found a dense,
positively charged bundle of matter he called the
nucleus
• Used the Gold Foil Experiment to find the
nucleus
• https://www.youtube.com/watch?v=Zd6_zVdMgJk&feature=related
• https://www.youtube.com/watch?NR=1&v=kHaR2rsFNhg
• Nuclear forces are short range forces that hold
protons and neutrons very close together
Rutherford’s Model
• Ernest Rutherford used
his research to develop
his own model of the atom
• Dense positively charged
center, nucleus
• Electrons traveled around
the nucleus in empty
space
• He was Wrong!!!!
STAY TUNED
To see if someone got the correct
model of the atom!!!
The Power of the Proton
• In 1914, H.G.J. Moseley using x-ray tubes,
determined the charges on the nuclei of
most atoms.
• He wrote "The atomic number of an
element is equal to the number of protons
in the nucleus".
• This work was used to reorganize the
periodic table based upon atomic number
instead of atomic mass.
The rest of the Nucleus
• In 1932, James Chadwick, using alpha
particles discovered a neutral atomic
particle with a mass close to a proton.
• Thus was discovered the neutron
Counting Atoms
• Atomic number (Z) is the number of protons an
atom contains
– Change the number of protons, change the element
• Isotopes are atoms with the same number of
protons but with varying numbers of neutrons
– Two was to symbolize isotopes, hyphen notation and
nuclear symbol notation
• Mass Number is the total number of protons and
neutrons in an atom
• Nuclide is a general term for a specific isotope of
an element
Masses of Atoms
• Relative atomic mass uses units called “atomic
mass units (amu)”
– An amu is defined as the 1/12th the mass of a carbon12 atom
– Since a Carbon-12 atom contains 6 protons and 6
neutrons, 1 amu equals the approximate mass of
either a proton or neutron
• Atomic Weight is the weighted average of atomic
masses of all known isotopes
– The most abundant isotopes contribute more to the
overall mass of the element