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How do we know about atoms?
Atomic Theory
Democritus (460 BC- 370 BC)His mentor Leucippus
Everything is composed of “atoms” Atoms are physically indivisible
Between atoms is empty space Atoms are always in motionInfinite number of atoms and kinds of atoms
which differ in shape and size
John Dalton (1766-1844)Postulated the following:
that all matter was made of some type of particle- called atoms which could not be broken up or destroyed
Atoms could combine with different atoms to form compounds
All atoms of the same element are the same size (mass) and have the same properties
Atoms can be rearranged during a chemical reaction to form new substances
Discovery of ElectronJJ Thomson in 1897 Cathode Ray Tubes- A cathode ray forms when
high voltage is applied to a partially evacuated tube. The ray passes through a hole in the negative end of the tube and hits the coated end of the tube to produce a glow
Plum Pudding ModelKnow that there are electrons (negative)Must be a positive part as well in order to be
neutral
Discovery of the NucleusErnest Rutherford designed an experiment to
test the Plum Pudding Model. Expected all particles to go straight through
the foil
They didn’t all go straight through?
Why?
New Model = Nucleus
NucleusContains the mass of the atom.
20 years later, James Chadwick discovered the neutron because the entire mass of the nucleus could not be accounted for by just protons.
Re-examining Dalton’s Atomic TheoryWhich postulates had to be modified after JJ
Thomson and Ernest Rutherford’s experiments?
All atoms are the same size- isotopes
Atoms can’t be changed- nuclear reactions
All mater is made of atoms – a proton is still matter
Atomic Symbol -
IsotopesAll atoms of an element are identical in
atomic number, but not in atomic mass. Isotopes = different number of neutrons. Electrons determine chemical properties of
an element. Can determine neutrons from atomic mass
and atomic number
Using the info on the PTHow many protons, neutrons and electrons
does an atom of iron have?
Atomic Masses on the PTHow come they have decimals?
Does a single neutron or proton weight exactly 1 amu?
Atomic Mass Unit (amu)The mass of an atom is measured relative to
the mass of an atomic standard. The standard = carbon-12 atom. 1 carbon-12 atom has exactly 12 amu
Atomic MassThe mass represented on the periodic table is
the average of all of the naturally occurring isotopes weighted accordingly to their abundances.
PROBLEMS: find the abundances of isotopes based on the atomic mass. OR predict the atomic mass based on the abundances of certain isotopes
Finding Atomic MassMagnesium has 3 naturally occurring
isotopes, 24Mg (23.9850 amu, abundance 78.99%), 25Mg (24.9858 amu abundance 10.00%), and 26Mg (25.9826 amu, abundance 11.01%). Calculate the atomic mass of magnesium.
Sample ProblemBoron has 2 naturally occurring isotopes.
Find the percent abundance of Boron-10 and Boron-11 given the atomic mass = 10.81 amu. The isotopic mass of Boron-10 = 10.0129 amu, and the isotopic mass of Boron-11 = 11.093 amu.
Homework: Pg 69: # 1 (a,b,c), 2, 9, 20 (a,b,c) 21, 22, 23 (a,b), 29, 32,
91 Read about mass spectroscopy on page 48, and
summarize what it is