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11
© 2006 Brooks/Cole - Thomson
ATOMS & ELEMENTS:ATOMS & ELEMENTS:
COMPOSITION AND MASSCOMPOSITION AND MASS
Fig. 2.18, p.86
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© 2006 Brooks/Cole - Thomson
ATOMIC COMPOSITIONATOMIC COMPOSITION• ProtonsProtons
– + electrical charge+ electrical charge
– mass = 1.672623 x 10mass = 1.672623 x 10-24-24 g g
– relative mass = 1.007 atomic relative mass = 1.007 atomic mass units (u)mass units (u)
• ElectronsElectrons– negative electrical chargenegative electrical charge
– relative mass = 0.0005 urelative mass = 0.0005 u
• NeutronsNeutrons– no electrical chargeno electrical charge
– mass = 1.009 umass = 1.009 u
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© 2006 Brooks/Cole - Thomson
ATOM ATOM COMPOSITIONCOMPOSITION
•protons and neutrons in protons and neutrons in the nucleus.the nucleus.
•the number of electrons is equal to the the number of electrons is equal to the number of protons.number of protons.
•electrons in space around the nucleus.electrons in space around the nucleus.
•extremely small. One teaspoon of water has extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean 3 times as many atoms as the Atlantic Ocean has teaspoons of water.has teaspoons of water.
The atom is mostlyThe atom is mostlyempty spaceempty space
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© 2006 Brooks/Cole - Thomson
Atomic Number, ZAtomic Number, Z
All atoms of the same element All atoms of the same element have the same number of have the same number of protons in the nucleus, protons in the nucleus, ZZ
1313
AlAl
26.98126.981
Atomic numberAtomic number
Atom symbolAtom symbol
Atomic weightAtomic weight
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© 2006 Brooks/Cole - Thomson
IsotopesIsotopes
• Atoms of the same element (same Z) Atoms of the same element (same Z) but different mass number (A).but different mass number (A).
• Boron-10 has 5 p and 5 n: Boron-10 has 5 p and 5 n: 101055BB
• Boron-11 has 5 p and 6 n: Boron-11 has 5 p and 6 n: 111155B B
10B
11B
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© 2006 Brooks/Cole - Thomson
Hydrogen IsotopesHydrogen Isotopes
Hydrogen has _____ isotopesHydrogen has _____ isotopes
1111HH
2211HH
3311HH
__ proton and __ neutrons, tritium
radioactive
__ proton and __ neutrons, deuterium
__ proton and __ neutrons, protium
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© 2006 Brooks/Cole - Thomson
Isotopes Isotopes & Their & Their
UsesUsesHeart scans with Heart scans with radioactive radioactive technetium-99. technetium-99.
99994343TcTc
Emits gamma raysEmits gamma rays
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© 2006 Brooks/Cole - Thomson
IsotopeIsotopess
• Because of the existence of isotopes, the mass Because of the existence of isotopes, the mass of a collection of atoms has an average value.of a collection of atoms has an average value.
• Average mass = Average mass = ATOMIC WEIGHTATOMIC WEIGHT• Boron is 19.9% Boron is 19.9% 1010B and 80.1% B and 80.1% 1111B. B.
That is, That is, 1111B is 80.1 percent abundant on earth. B is 80.1 percent abundant on earth.
• For boron atomic weightFor boron atomic weight
= _____ (_____ u) + _____ (_____ u) = _____ u= _____ (_____ u) + _____ (_____ u) = _____ u
10B
11B
99
© 2006 Brooks/Cole - Thomson
Masses of IsotopesMasses of Isotopesdetermined with a mass determined with a mass
spectrometerspectrometer
1111
© 2006 Brooks/Cole - Thomson
Isotopes & Atomic Isotopes & Atomic WeightWeight• Because of the existence of isotopes, the mass of a collection of atoms has an Because of the existence of isotopes, the mass of a collection of atoms has an
average value.average value.
• 66Li = 7.5% abundant and Li = 7.5% abundant and 77Li = 92.5%Li = 92.5%
– Atomic weight of Li = ______________Atomic weight of Li = ______________
• 2828Si = 92.23%, Si = 92.23%, 2929Si = 4.67%, Si = 4.67%, 3030Si = 3.10%Si = 3.10%
– Atomic weight of Si = ______________Atomic weight of Si = ______________
1212
© 2006 Brooks/Cole - Thomson
Atomic WeightAtomic Weight• The mass of one atom of an element relative to The mass of one atom of an element relative to
one atom of another element.one atom of another element.• OR — the mass of 1000 atoms of one relative to OR — the mass of 1000 atoms of one relative to
1000 atoms of another.1000 atoms of another.• For example, an O atom is approximately 16 times For example, an O atom is approximately 16 times
heavier than an H atom.heavier than an H atom.• Define one element as the standard against which Define one element as the standard against which
all others are measuredall others are measured• Standard = carbon-12Standard = carbon-12• C atom with ____ protons and ___ neutrons is the C atom with ____ protons and ___ neutrons is the
mass standard mass standard C-12 = 12 atomic mass units (u)C-12 = 12 atomic mass units (u)
1 u = 1/12th the mass of a carbon-12 atom1 u = 1/12th the mass of a carbon-12 atom
1313
© 2006 Brooks/Cole - Thomson
Counting Counting AtomsAtoms
Mg burns in air (OMg burns in air (O22) to ) to produce white produce white magnesium oxide, MgO. magnesium oxide, MgO.
How can we figure out how How can we figure out how much oxide is produced much oxide is produced from a given mass of Mg?from a given mass of Mg?
1414
© 2006 Brooks/Cole - Thomson
Counting AtomsCounting Atoms
Chemistry is a quantitative Chemistry is a quantitative science—we need a science—we need a “counting unit.”“counting unit.”
1 mole is the amount of 1 mole is the amount of substance that contains as substance that contains as many particles (atoms, many particles (atoms, molecules) as there are in molecules) as there are in 12.0 g of 12.0 g of 1212C.C.
1 mole is the amount of 1 mole is the amount of substance that contains as substance that contains as many particles (atoms, many particles (atoms, molecules) as there are in molecules) as there are in 12.0 g of 12.0 g of 1212C.C.
MOLEMOLE
518 g of Pb, 2.50 mol
1515
© 2006 Brooks/Cole - Thomson
Particles in a Particles in a MoleMole
6.02214199 x 106.02214199 x 102323
Avogadro’s NumberAvogadro’s Number
There is Avogadro’s number of particles in a mole of any substance.There is Avogadro’s number of particles in a mole of any substance.
Amedeo AvogadroAmedeo Avogadro1776-18561776-1856
1616
© 2006 Brooks/Cole - Thomson
Molar MassMolar Mass
1 mol of 1 mol of 1212C C = _______ g of C= _______ g of C = _______ atoms of C = _______ atoms of C
12.00 g/mol of 12.00 g/mol of 1212C is its C is its
MOLAR MASSMOLAR MASS
Taking into account all of Taking into account all of
the isotopes of C, the the isotopes of C, the
molar mass of C is molar mass of C is
______ g/mol______ g/mol
1919
© 2006 Brooks/Cole - Thomson
PROBLEM: How many moles of PROBLEM: How many moles of Mg are represented by 0.200 g? Mg are represented by 0.200 g? PROBLEM: How many moles of PROBLEM: How many moles of Mg are represented by 0.200 g? Mg are represented by 0.200 g?
Mg has a molar mass of ___________.
How many atoms in this piece of Mg?
2020
© 2006 Brooks/Cole - Thomson
PROBLEM: What is the mass of PROBLEM: What is the mass of 4.2 x 104.2 x 102323 atoms of sodium? atoms of sodium? PROBLEM: What is the mass of PROBLEM: What is the mass of 4.2 x 104.2 x 102323 atoms of sodium? atoms of sodium?
Fig. 2.12a, p.82