1 © 2006 brooks/cole - thomson atoms & elements: composition and mass

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© 2006 Brooks/Cole - Thomson

ATOMS & ELEMENTS:ATOMS & ELEMENTS:

COMPOSITION AND MASSCOMPOSITION AND MASS

Fig. 2.18, p.86

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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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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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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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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

66


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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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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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


Masses of IsotopesMasses of Isotopesdetermined with a mass determined with a mass

spectrometerspectrometer

1010


Mass spectrum of CMass spectrum of C66HH55BrBr

1111


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


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

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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


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


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


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

1717


One-mole AmountsOne-mole Amounts

1818


1919


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


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

2121


MOLES

mol

PARTICLESatoms

moleculesformula units

MASS

g

VOLUMEof gas @STP

L

6.02 x 10 23

particles/mol

22.4

L/mol

g/mol

1 © 2006 brooks/cole - thomson atoms & elements: composition and mass

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