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Matter, Atoms, Compounds

● Democritus - c
...

○ Thought: Matter is ultimately composed of indivisible particles (atoms)
● Democritus’s theory was suppressed for 2000 years because Aristotle said it was
impossible for “nothing” to exist
● Robert Boyle
○ 17th Century
■ Argued that an element is composed of simple bodies (atoms)
● An early 20th century burst of creativity led to the current model of an atom with a
complex internal structure
Elements, Compounds, and Mixtures: An Atomic View
● Based on the composition, matter can be classified into three types
○ Elements
■ Called a substance (matter with a fixed composition)
■ The simplest type of matter with unique physical or chemical methods
■ All have names
● Ex
...
oxygen is diatomic molecules
○ diatomic - two atoms
○ Compounds
■ Called a substance (matter with fixed composition)
■ Consists of two or more different elements that are bonded chemically
■ Elements in a compound are not just mixed together
● Their atoms have joined in a chemical reaction
■ Many consist of molecules
● Ex
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sodium sulfate, silicon dioxide
■ All compounds have two defining features

Matter, Atoms, Compounds
● The elements are present in fixed parts by mass (fixed mass ratio)
○ Because each unit of the compound consists of a fixed
number of atoms of each element
■ Ex
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Table salt
■ Soft, silvery sodium metal + yellow-green,
poisonous chlorine gas = white, crystalline sodium
chloride (common table salt)
■ Can be broken down into simpler substances, its component elements
■ Ex
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mixture of sodium chloride and water can have many
different parts of mass of salt to water
■ Retains many of the properties of its components
● Because it consists of its component elements and or compounds
■ Can be separated into their components by physical changes
The Observations that Led to an Atomic View of Matter
● Any model of the composition of matter had to explain three mass laws:
○ The Law of Mass Conservation
■ Most fundamental chemical observation of the 18th century
■ The total mass of substances does not change during a chemical reaction
● The # of substances may change and their properties must, but the
total amount of matter remains constant
○ Ex
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two compounds of carbon and oxygen (carbon oxides I
and II)
■ Carbon oxide I is 57
...
9
mass % carbon
■ Carbon oxide II is 72
...
3
mass % carbon
○ Tells us that in two compounds of the same elements, the
mass fraction of one element relative to the other element
change in increments based on ratio of small whole
numbers
...
A chemical reaction cannot possibly result in a mass change
because atoms combine differently
...

■ Each element in a compound must constitute a fixed fraction of the total
mass
○ Multiple proportions
■ Atoms of an element have the same mass and are indivisible
...
When electric power source was turned on, a
“ray” (cathode ray) could be seen striking the phosphor coated end of the
tube, which emitted a glowing spot of light
...

● 1
...
J
...

○ By comparing this value with the mass/charge ratio for the
lightest charged particle in solution, Thomson estimated
that the cathode ray particle weighed less than 1/1000 as
much as hydrogen
...
Charge: In 1909, Robert Millikan measured the charge of the
electron
○ He observed the movement of tiny oil droplets in an
apparatus that contained electrically charged plates and an
x-ray source
...

○ Millikan found that the total charge of the various droplets
was always some whole number multiple of a minimum
charge
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Circular zinc

Matter, Atoms, Compounds

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●

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sulfide screen registers the deflection (scattering angle) of the
alpha particles emerging from the foil by emitting light flashes
when the particles strike it
...
He reasoned an electron
should not be able to deflect any alpha particle more than a Pingpong ball deflecting a basketball
...

The data showed very few particles deflected at all and only 1 in
20,000 deflected by more than 90 degrees
...
Calculations based on the mass, charge, velocity of the
alpha particles showed that
○ At atom is mostly space occupied by electrons
○ In the center is a tiny region called the nucleus, that
contains all the positive charge and essentially all the mass
of the atom
Proposed that positive particles (protons) lay within the nucleus

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The Atomic Theory Today
● Structure of the Atom
○ Atom - electrically neutral, spherical entity composed of a positively charged
central nucleus surrounded by one or more negatively charged electrons
■ Diameter is about 20,000 times the diameter of its nucleus
■ Nucleus contributes 99
...

○ Mass number - total number of protons and neutrons in the nucleus of each of its
atoms
○ Each proton and neutron contributes one unit to the mass number
■ Ex
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C for Carbon
○ Number of neutrons = mass number - atomic number N = A-Z
○ All atoms of an element have the same atomic number but not the same mass
number
○ Isotopes
■ Atoms that have different numbers of neutrons and therefore different
mass numbers
○ Most elements occur in nature in a particular isotopic composition
■ Specifies the proportional abundance of each of its isotopes
● Ex
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89% of naturally occurring carbon atoms have 6 neutrons
○ The chemical properties of an element are primarily determined by the number of
electrons
■ All isotopes of an element have nearly identical chemical behavior
● Even though they have different masses
● Atomic Masses of the Elements
○ Mass of an atom is measured relative to the mass of atomic standard
○ The modern standard is the carbon 12 atom whose mass is defined as exactly 12
atomic mass units
○ Atomic mass unit (amu) is the 1/12 mass of carbon-12 atom
○ the H atom has a mass of 1
...

○ A cation or anion derived from a single atom is called a monatomic ion
○ The Case of Sodium Chloride
■ All binary ionic compounds are solid arrays of oppositely charged ions
■ The oppositely charged ions attract each other, and the similarly charged
ions repel each other
○ Coulomb’s Law
■ The strength of the ionic bonding depends to a great extent on the net
strength of these attractions and repulsions
■ The energy of attraction (or repulsion) between two particles is directly
proportional to the product of the charges and inversely proportional to the
distance between them



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