Download Chapter 2 ATOMS AND ELEMENTS

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Fig. 2.18, p.86
ATOMS & ELEMENTS:
COMPOSITION AND MASS
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ATOMIC COMPOSITION
• Protons
– + electrical charge
– mass = 1.672623 x 10-24 g
– relative mass = 1.007 atomic
mass units (u)
• Electrons
–
negative electrical charge
– relative mass = 0.0005 u
• Neutrons
no electrical charge
– mass = 1.009 u
–
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ATOM
COMPOSITION
The atom is mostly
empty space
•protons and neutrons in
the nucleus.
•the number of electrons is equal to the
number of protons.
•electrons in space around the nucleus.
•extremely small. One teaspoon of water has
3 times as many atoms as the Atlantic Ocean
has teaspoons of water.
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Atomic Number, Z
All atoms of the same element
have the same number of
protons in the nucleus, Z
13
Al
26.981
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Atomic number
Atom symbol
Atomic weight
Isotopes
• Atoms of the same element (same Z)
but different mass number (A).
• Boron-10 has 5 p and 5 n: 105B
• Boron-11 has 5 p and 6 n: 115B
11B
10B
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Hydrogen Isotopes
Hydrogen has _____ isotopes
1 H
1
__ proton and __ neutrons,
protium
2 H __ proton and __ neutrons,
1
deuterium
3 H
1
__ proton and __ neutrons,
tritium
radioactive
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Isotopes &
Their Uses
Heart scans with
radioactive
technetium-99.
99
43Tc
Emits gamma rays
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Isotopes
11B
10B
• Because of the existence of isotopes, the mass
of a collection of atoms has an average value.
• Average mass = ATOMIC WEIGHT
• Boron is 19.9% 10B and 80.1% 11B.
That is, 11B is 80.1 percent abundant on earth.
• For boron atomic weight
= _____ (_____ u) + _____ (_____ u) = _____ u
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Masses of Isotopes
determined with a mass spectrometer
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Mass spectrum of C6H5Br
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Isotopes & Atomic Weight
• Because of the existence of isotopes, the mass of a
collection of atoms has an average value.
• 6Li = 7.5% abundant and 7Li = 92.5%
– Atomic weight of Li = ______________
•
28Si
= 92.23%, 29Si = 4.67%, 30Si = 3.10%
– Atomic weight of Si = ______________
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Atomic Weight
• The mass of one atom of an element relative to
one atom of another element.
• OR — the mass of 1000 atoms of one relative to
1000 atoms of another.
• For example, an O atom is approximately 16 times
heavier than an H atom.
• Define one element as the standard against which
all others are measured
• Standard = carbon-12
• C atom with ____ protons and ___ neutrons is the
mass standard
C-12 = 12 atomic mass units (u)
1 u = 1/12th the mass of a carbon-12 atom
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Counting Atoms
Mg burns in air (O2) to
produce white
magnesium oxide, MgO.
How can we figure out how
much oxide is produced
from a given mass of Mg?
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Counting Atoms
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Chemistry is a quantitative
science—we need a
“counting unit.”
MOLE
1 mole is the amount of
substance that contains as
many particles (atoms,
molecules) as there are in
12.0 g of 12C.
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518 g of Pb, 2.50 mol
Particles in a Mole
Avogadro’s Number
Amedeo Avogadro
1776-1856
6.02214199 x
23
10
There is Avogadro’s number of
particles in a mole of any substance.
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Molar Mass
1 mol of 12C
= _______ g of C
= _______ atoms of C
12.00 g/mol of 12C is its
MOLAR MASS
Taking into account all of
the isotopes of C, the
molar mass of C is
______ g/mol
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One-mole Amounts
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PROBLEM: How many moles of
Mg are represented by 0.200 g?
Mg has a molar mass of ___________.
How many atoms in this piece of Mg?
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PROBLEM: What is the mass of
4.2 x 1023 atoms of sodium?
Fig. 2.12a, p.82
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MASS

g/mol
g
MOLES



22.4

VOLUME
of gas @STP
L
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6.02 x 10 23
particles/mol
mol
L/mol

PARTICLES
atoms
molecules
formula units