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Introduction to Atoms
Introduction to Atoms
Chapter 10 – Section 1
 Atom: the smallest unit of an element that
maintains the chemical properties of that element.
The idea of an atom was developed by Democritus.
All matter is made of atoms. 440 B.C.
 Aristotle disagreed!
Section 1
 Scientists had discovered that all things combine in
set ratios! John Dalton (school teacher/chemist)
wanted to know why????
 Dalton’s Atomic Theory: 1803
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All substances are made of atoms. Atoms are small particles
that cannot be created, divided, or destroyed.
Atoms of the same element are exactly alike, and atoms of
different elements are different.
Atoms join with other elements to make new substances.
This was NOT ALL CORRECT!
Section 1
 Pg. 262: Cathode-ray tube
 There are small particles inside an atom. Atoms can
be divided into smaller parts. Electrons = negative
charge.
 Plum-pudding model: proposed by J.J.
Thompson….thought that electrons were mixed
throughout an atom like plums in pudding.
 http://www.youtube.com/watch?v=XU8nMKkzbT8
 Ernest Rutherford further tested this…pg. 263.
Section 1
 1911 Rutherford (former student of Thompson) revised
the atomic theory. He developed the idea of a nucleus.
He had electrons as free floating.
 http://www.rsc.org/chemsoc/timeline/pages/1911.html
 Niels Bohr: Bohr’s electron levels (1913): electrons move
around in certain paths or energy levels. There are no
paths between levels but electrons can jump between
levels if energy is added (heat). According to Bohr,
electrons will remain at lowest energy level until enough
energy is added. Bohr did a lot of experiments with light.
Section 1
 Current atomic theory: Schrodinger and
Heisenberg: electrons do not travel in definite
paths as Bohr suggested. Their path cannot be
predicted. There are regions where they are likely
to be found. This is called an electron cloud.
Section 2
 Atoms are composed of electrons, protons and
neutrons---protons and neutrons are found in the
nucleus.
 Electron cloud: a region around the nucleus of an
atom where electrons are likely to be found
Section 2
 Electron: a subatomic particle that has a negative
charge. Much smaller than neutrons and protons.
Takes 1800 to equal the mass of 1 proton.
 Neutron: a subatomic particle that has no charge
and that is found in the nucleus of an atom
 Proton: a subatomic particle that has a positive
charge and that is found in the nucleus of an atom.
 A single penny contains 20 thousand billion billion
atoms.
Section 2
 Nucleus: in physical science, an atom’s central region,
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which is made up of protons and neutrons.
Protons and Neutrons are the most massive particles in an
atom. They are about the same size but neutrons have the
slightest bit more mass.
Protons and electrons may differ in size but their charges
cancel each other out.
If charges are unequal then you will have an ion. -1 electron
then positive ion. +1 electron then negative ion.
The diameter of the nucleus is 1/100,000 the diameter of
the atom.
Introduction to Atoms
Chapter 10 – Section 2
 Atomic mass unit: a unit of mass that describes
the mass of an atom or molecule. Scientist made this
specifically for atoms because they are so small.
Each proton has a mass of 1 amu. Neutrons are
larger (mass) but not by much. So their mass can
also be estimated to 1 amu. Nucleus is small and
very dense. If a nucleus were the size of a grape it
would weigh 9 million metric tons.
 Atomic number: the number of protons in the
nucleus of an atom; the atomic number is the same
for all atoms of an element
Introduction to Atoms
Chapter 10 – Section 2
 Isotope: an atom that has the same number of
protons (or the same atomic number) as other atoms
of the same element but has a different number of
neutrons (and thus a different atomic mass).
 Unstable isotopes tend to be radioactive. They will
change over time. This is half-life.
 Mass number: the sum of the number of protons
and neutrons in the nucleus of an atom. This is how
you can tell isotopes apart. Electrons have very little
effect on the total mass of an element.
Section 2
 Naming isotopes: name of element and hyphen, then
mass number. Ex. Carbon-12.
 Atomic mass: the mass of an atom expressed in
atomic mass units. Why are mass numbers not all
whole numbers???
Section 2
 Forces: pg. 272
 Gravitational Force: Force acts between all objects all the time.
Pulls objects toward one another. Us toward the earth.
 Electromagnetic Force: Objects with opposite forces attract
while like forces repel.
 Strong Force: what keeps protons in the nucleus. The strong
force is stronger than the electromagnetic force. Protons want
to push away from each other (positive charges repel).
 Weak Force: assists in radioactive change----neutrons
changing to protons and electrons. It plays a role in
radioactive decay.