chapter 2: atoms and atomic theory - university of...

9/13/2002 Chemistry 140 Fall 2002 Dutton

Chapter 2: Atoms and Atomic Theory

P Early chemical discoveries P Electrons and the Nuclear AtomP Chemical ElementsP Atomic MassesP The Mole


Early Discoveries

Law of conservation of mass – Lavoisier 1774

Law of constant composition – Proust 1799

Atomic Theory – Dalton 1803-1888


Dalton’s Atomic Theory

� Each element is composed of small particles called atoms.

� Atoms are neither created nor destroyed in chemical reactions.

� All atoms of a given element are identical

� Compounds are formed when atoms of more than one element

combine


Consequences of Dalton’s theory

¶ In forming carbon monoxide, 1.33 g of oxygen combines with 1.0 g of carbon.

¶ In the formation of hydrogen peroxide 2.66 g of oxygen combines with 1.0 g of hydrogen.

u Law of Definite Proportions: combinations of elements are in ratios of small whole numbers.

¶ Using atomic theory, we conclude that carbon dioxide contains twice as many atoms of oxygen per carbon.


Behaviour of charges


Cathode ray tube


Properties of cathode rays

Electron m/e = -5.6857 x 10-9 g coulomb-1


Charge on the electron

¶ From 1906-1914 Robert Millikan showed ionized oil drops can be balanced against the pull of gravity by an electric field.

¶The charge is an integral multiple of the electronic charge, e.


Radioactivity

Ü Radioactivity is the spontaneous emission of radiation from a substance. eg. X-rays, γ-rays, α-particles, and β-particles

¶ X-rays and γ-rays are high-energy light.

¶ α-radiation is a stream of helium nuclei, He2+ (α- particles).

¶ β-radiation is a stream of high speed electrons that originate

in the nucleus (β-particles).


The nuclear atom

Geiger and Rutherford1909


The α-particle experiment explained¶ Most of the mass and all of the

positive charge is concentrated in a small region called the nucleus .

¶ There are as many electrons outside the nucleus as there are units of positive charge on the nucleus


The nuclear atom

Rutherfordprotons 1919

James Chadwickneutrons 1932


Atomic Diameter 10-8 cm Nuclear diameter 10-13 cm

Nuclear Structure

Particle Mass Chargekg amu Coulombs (e)

Electron 9.109 x 10-31 0.000548 –1.602 x 10-19 –1Proton 1.673 x 10-27 1.00073 +1.602 x 10-19 +1Neutron 1.675 x 10-27 1.00087 0 0

1 Å


Scale of Atoms

Useful units:

¶ 1 amu (atomic mass unit) = 1.66054 x 10-24 kg

¶ 1 pm (picometer) = 1 x 10-12 m

¶ 1 Å (Angstrom) = 1 x 10-10 m = 100 pm = 1 x 10-8 cm

ÜThe heaviest atom has a mass of only 4.8 x 10-22 gand a diameter of only 5 x 10-10 m.

Typical C-C bond length 154 pm (1.54 Å)

Molecular models are 1 Å /inch or about 0.4 Å /cm

240 amu50 Å, 5 nm


Isotopes, atomic numbers and mass numbers

ÜTo represent a particular atom we use the symbolism:

A= mass number Z = atomic number


Measuring atomic masses


Quadrupole Mass Spectrometer

http://www.chem.vt.edu/chem-ed/CHP/copyright.html, Copyright © 2000 by Brian M. Tissue, all rights reserved.


The Periodic tableAlkali Metals

Alkaline Earths

Transition Metals

Halogens

Noble Gases

Lanthanides and Actinides

Main Group

Main Group


The Periodic Table

• Read atomic masses.• Ions formed by main group elements.• Electron configuration.• Trends in physical and chemical properties.

We will discuss these in detail in Chapter 10.


The Mole

• Physically counting atoms is impossible.• We must be able to relate measured mass to

numbers of atoms.– buying nails by the pound.– using atoms by the gram


Avogadro’s number

vThe mole is an amount of substance that contains the same number of elementary entities as there are carbon-12 atoms in exactly 12 g of carbon-12.

NA = 6.02214199 x 1023 mol-1


Molar Mass• The molar mass, M, is the mass of one mole

of a substance.

= 6.022 x 1023 atoms

= 12.00 g 12C

1.000 mol 12C x NA (atoms/mol 12C)

x M (g/mol 12C) x Z (g/atom 12C)

= 12.00 g 12C

M (g/mol 12C) = Z (g/atom 12C) x NA (atoms/mol 12C)


Do it backwards

= 6.022 x 1023 atoms

= 12.00 g 12C

) Z (g/atom 12C) or x 1/Z (atom 12C/g)

) NA (mol 12C/atom) or x 1/NA (mol 12C/atom)

= 1.000 mol 12C


Do it backwards with M

= 1.000 mol 12C12.00 g 12C x 1/M (mol 12C/g)

Practice ALL equations and transformations not only backwards, forwards but also using various paths.

There are often multiple ways of carrying out calculations and all of them may be correct.

Hint:


Example 2-9Combines several factors in a calculation.

Potassium-40 is one of the few naturally occurring radioactive isotopes of elements of low atomic number. Its percent natural abundance among K isotopes is 0.012%. How many 40K atoms do you ingest by drinking one cup of whole milk containing 371 mg of K?

Want atoms of 40K, need atoms of K,

Want atoms of K, need moles of K,

Want moles of K, need mass and M(K).


Convert strategy to plan

mK(g) x (1g/1000mg) è mK (g) x 1/MK (mol/g) è nK(mol)

Convert mass of K(mg K) into moles of K (mol K)

Convert moles of K into atoms of 40K

nK(mol) x NA è atoms K x 0.012% è atoms 40K

nK = (371 mg K) x (10-3 g/mg) x (1 mol K) / (39.10 g K)

= 9.49 x 10-3 mol K

and plan into action

atoms 40K = (9.49 x 10-3 mol K) x (6.022 x 1023 atoms K/mol K)

x (1.2 x 10-4 40K/K)= 6.9 x 1017 40K atoms


Chapter 2 Questions

3, 4, 11, 22, 33, 51, 55, 63, 83

chapter 2: atoms and atomic theory - university of...

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