Transcript C7.2 Lecture

G8 Science
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Most pure elements are solid a room
temperature
11 of the 92 naturally occurring elements are
gas: H, He, N, O, F, Ne, Cl, Ar, Kr, Xe, Rn
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2 of the 92 are liquids: Br, and Hg
Elements are solid when intermolecular forces
are stronger than thermal motion.
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Noble gases (group 18) are gases because
their valence shell is full, with 8 electrons,
and so they have a weak intermolecular force
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First elements have
a low melting and
boiling point. They
increase until you
reach the middle of
the table, then
decrease again.
Melting and boiling
points reflect strong
intermolecular
forces.
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Intermolecular
forces are strongest
when the valence
shell is half full –
because a half full
valence shell has the
most electrons to
participate in
bonding.
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Good electrical conductors allow electrons to
flow easily through them. This is a property of
metals.
Most metals are good thermal conductors
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Elements to the right of the table are good
insulators, i.e. poor conductors.
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An alloy is a solid mixture of one or more
elements, of which one is a metal.
Steel – iron and carbon (rusts but stronger
than iron)
Stainless steel – iron, carbon, chromium,
vanadium, manganese (does not rust)
Bronze – copper, tin
Brass – copper, zinc
Aluminum alloys
Titanium alloys
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< .01% of Earth’s crust
Most molecules that make up plants and
animals are build around carbon (Organic
Chemistry)
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Pure carbon
◦ Diamond
 Tetrahedral crystal,
 Hardest natural substance (diamond tip drill bits)
 Highest thermal conductivity
◦ Graphite
 Sheet like
 Lubricant for locks and keys
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Silicon
◦ Second most
abundant element in
Earth’s crust (after
Oxygen)
◦ Sand, rocks, minerals
◦ Glass (SiO2)
◦ Semiconductors (also
make with
Germanium)
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Nitrogen
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Colorless, tasteless, odorless
78% of Earth’s atmosphere
N2
Boils at -196°C
Crucial to to life
Oxygen
◦ 21% of Earth’s atmosphere
◦ 46% of Earth’s crust
◦ Crucial to life
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Phosphorus
◦ Key ingredient in
DNA
◦ Glow in the dark