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Chapter 4
The structure of the atom
Atom
• Smallest part of an
element that retains
the properties of the
element
Democritus:Greek (460-370 BC)
• Proposed the first
atomic theory
• called the tiny
individual particles
“atomos”
• Aristotle said that he
was wrong
John Dalton: Eng (1766-1844)
• School teacher
• 1808- proposed the
first accepted atomic
theory
Joseph John (J.J) Thomson
• English physicist
(1856-1940)
• 1897- used the
cathode ray tube
experiment to
discover the electron
• Called plum pudding
model
Robert A. Millikan
• American Physicist(1868-1953)
• 1909- used the oil droplet
experiment to determine
the charge of an electron
and calculate the mass of
the electron
• 1923- Nobel Prize
• Thomson’s and Millikan’s
works was combined to
conclude there must be a
positive particle- equal in
charge, but more massive
than the electron
Ernest Rutherford
• New Zealander (18711937)
• 1911- Gold Foil
experiment- proved the
atom was mostly empty
space
• Concluded there was
small positive center and
called it the nucleus
• “discovered” and named
the nucleus
• 1908- Nobel Prize
James Chadwick
• English (1891-1974)
• 1932- discovered the
neutron
• Subatomic particles=
electron, proton,
neutron
• Nuclear forces- short
range forces that hold
nuclear particles
together
• Atomic number =
number of protons in
an atom
– In a neutral atom = #
of electrons
Mass number
• Sum of the protons
and neutrons in a
nucleus
• Mass number –
atomic number =
neutrons
Average atomic mass
• Weighted average of the atomic masses of the naturally
occurring isotopes of an element
• # on chart
• (%)(mass)+ %(mass)= average mass
Example: Carbon
mass number
exact
weight
percent abundance
12
12.00
98.90
13
13.00
1.10
(12.00) (0.98) + (13.00) (0.01) = 12.011
isotope
• Same element, same
number protons,
same number
electrons, different
number of neutrons
Methods of writing isotopes
Nuclear form and hyphen form
Nuclear form=
Mass
atomic #
Hyphen= name-mass
Ex. Carbon-12, carbon-14
12
6
C
Atomic mass unit (AMU)
• 1/12 of the mass of a C-12 atom
• Not exactly 1 proton or 1 neutron
Nuclear reactions
• Reactions which
involve as change in
an atoms nucleus
Radioactivity
• Substances
spontaneously
emitting radiation
• Radiation- rays and
particles
• Radioactive
substances decay
until they become
stable
4 types of radiation
1.Alpha particle- a helium nucleus
2 protons and 2 neutrons
2.Beta particle- fast moving electron
4
He
2
0
-1
3. Gamma rays- high energy radiation
4.Positron- same mass as electron with (+) charge
Proton decays to a positron and a neutron
e
The Mole
• Equal to the number
of particles in exactly
12g of carbon-12
3 equivalents of 1 mole
1. Molar mass: gram atomic mass, formula
mass, molecular mass
•
•
Mass number from chart
Add for compounds
Calculate the molar mass of Al(NO3)3
(1 x 27) + (3 x 14) + (9 x 16) = 213.00 g/mol
213.00 grams is the mass of one mole of aluminum nitrate.
213.00 grams of aluminum nitrate contains 6.02 x 10^23 entities of Al(NO3)3
2. Avogadro’s number of
representative particles
•
•
•
•
6.022 x 1023
Elements = atoms
Ionic = formula units
Covalent = molecules
Ex: One mole of donuts contains 6.022 x 1023 donuts
3. 22.4 L of a gas at STP
• Standard temp= 0o C, 273 K
• Standard pressure= 1atm, 760 mmHg, 760
torr, 101.325 kPa