Atoms

The Building Blocks of Matter

Chapter 3

OBJECTIVES The Atom: Philosophy to Science 3.1 Explain the law of conservation of mass, the

law of definite proportions, and the law of multiple proportions.

Summarize the five essential points of Dalton’s atomic theory.

Explain the relationship between Dalton’s atomic theory and the 3 Laws

Foundations of Atomic TheoryThe Philosophers

400 B.C. – Particle Theory of Matter Greek Philosophers

Democritus“atmos” - indivisible

Aristotle matter is continuous, did

not believe in atomsNeither view was supported by experiments until the 18th century

Alchemy 700 B.C. – 1700’s

Transmutation – metals are made of varying proportions of sulfur and mercury - achieving the right combination would produce gold

Some used alchemy to make medicines

Balance Scale – Quantitative Analysis

The Laws of Chemistry

Measuring the masses of elements and compounds it was observed that when elements react to form compounds they combine in fixed proportions by mass.

Three basic laws of chemistry were proposed.

1. Law of Conservation of Mass

1782 - Antoine Lavoisier observed that the mass of the reactants before the reaction and the mass of the products after the reaction are the same.

Mass is neither created nor destroyed during ordinary chemical reactions or physical changes.

2. The Law of Definite Proportions

Regardless of the size of the sample or the source of a chemical compound, it is composed of a fixed ratio of elements by mass.

3. Law of Multiple Proportions

If two or more different compounds are made of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers.

Examples:

P. 87 #2

Dalton’s Atomic Theory

1808, an English school teacher and chemist, John Dalton, proposed a scientific explanation of these three laws based upon the idea of atoms of elements.

Dalton’s Atomic Theory1. All matter is made of tiny particles called

atoms.2. Atoms of the same element are identical,

those of different elements are different.3. Atoms cannot be subdivided, created , or

destroyed.4. Atoms of different elements combine in

small whole number ratios to form compounds

5. In chemical reactions, atoms are combined, separated or rearranged.

Dalton’s Atomic TheoryHow it Supported the laws: Conservation of mass - atoms are not created or destroyedDefinite proportions – a given compound is always the same proportion of atomsMultiple proportions – in 2 different compounds made of the same elements the ratio of the second atoms to the first atoms is a small whole number

Modern Atomic Theory

Dalton turned Democritus’ idea of the atom into a scientific theory that could be tested by experiment.

Not all aspects of Dalton’s theory have been proven correct. The theory has been modified by new discoveries.

OBJECTIVES The Structure of the Atom 3.2 Define atom. Summarize the observed properties of

cathode rays that led to the discovery of the electron.

Summarize the experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus.

List the properties of protons, neutrons, and electrons.

Atom

The smallest particle of an element that retains the chemical properties of the element.

Discovery of the electron

1897 -J. J. Thomson - English physicist.

Used a cathode ray tube.

It is a vacuum tube - all the air has been pumped out and replaced with a low pressure gas

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

An electric current was passed through the An electric current was passed through the tube from the tube from the cathodecathode (the negative (the negative electrode) to the electrode) to the anodeanode (the positive (the positive electrode). electrode).

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electromagnetic field By adding an electromagnetic field he found that the moving particles he found that the moving particles were negative were negative

+

-

Thomson’s Experiment

Movement of a paddlewheel in the path of the electrodes led scientist to conclude that the rays have mass.By using the cathode ray tube, Thomson determined that electrons have a very high charge and a very low mass.

Thomson’s Model

“plum pudding model”

An atom made of negative particles surrounded by positive material with the mass and charges uniformly distributed

Millikan Oil Drop Experiment.In 1909, Robert Millikan, an

American physicist, confirmed that the electron has the smallest possible negative charge and that all other negative charges are whole number multiples of the charge of the electron.

Rutherford’s experiment

(1910) Ernest Rutherford - English physicist.

Believed in the plum pudding model of the atom.

Used radioactivity to test it Alpha particles - positively charged pieces

given off by uranium Shot them at gold foil which can be made

a few atoms thick

Lead block

Uranium

Gold Foil

Florescent Screen

He expected the alpha particles to pass through He expected the alpha particles to pass through without changing direction very muchwithout changing direction very much

Because the positive charges were spread out Because the positive charges were spread out evenly. Alone they were not enough to stop the evenly. Alone they were not enough to stop the alpha particlesalpha particles

He thought the mass was evenly distributed in the atom

What he got

Most of the particles passed through the foil indicating that the atom is mostly empty space.

+

Very few of the particles bounced back, but Very few of the particles bounced back, but with great force,with great force,

What Rutherford concluded:

+

The atom is mostly empty space

A small, very dense, positively charged core within the atom

The nucleus

Protons and Neutrons

In 1919 Rutherford discovered the proton.

The neutron was discovered in 1932 by an English scientist, James Chadwick.

Composition of the Atom – Subatomic Particles

Protons are subatomic particles located in the nucleus of the atom with high mass and a positive charge equal in magnitude to the negative charge of the electron.

The nucleus also contains neutrons which are electrically neutral and have a mass ~ equal to a proton

Electrons surround the nucleus in an electron cloud. They have very little mass and a negative charge

Atoms are neutral due to the presence of equal numbers of protons and electrons.

Subatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

n0

-1

+1

0

.0005

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Subatomic Particles

POSIT IVECHARG E

PROT ONS

NEUT RALCHARG E

NEUT RONS

NUCLEUS

NEG AT IVE CHARG E

ELECT RONS

AT OM

NUCLEUS ELECTRONS

PROTONS NEUTRONS NEGATIVE CHARGE

POSITIVE CHARGE

NEUTRAL CHARGE

ATOM

QUARKS

Homework

P. 74 # 1-5

OBJECTIVES Counting Atoms 3.3 Explain what isotopes are. Define atomic number and mass number, and

describe how they apply to isotopes. Given the identity of a nuclide,determine its

number of protons,neutrons, and electrons. Define mole in terms of Avogadro’s number,

and define molar mass. Solve problems involving mass in grams,

amount in moles, and number of atoms of an element.

Atomic NumberAtomic Number (Z) = number of

protons in the nucleus of atomAtomic Number Identifies the

element# of protons determines kind of

atom# protons = # electrons in the

neutral atom

Atomic Number – The identity

3LiLithium6.941[He]2s1

The atomic numberin this periodic-table entry revealsthat an atom of lithium has threeprotons in its nucleus

Mass Numbermass # = protons

+ neutrons

always a whole number

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Isotopes

Atoms of the same element with different numbers of neutrons –so different mass numbers.

C126Mass #

Atomic #

Nuclear symbol:

Hyphen notation: carbon-12

Isotopes

© Addison-Wesley Publishing Company, Inc.

Designating Isotopes

Isotopes of Hydrogen and Helium – p. 77

Isotopes

Chlorine-37

atomic #:

mass #:

# of protons:

# of electrons:

# of neutrons:

17

37

17

17

20Cl37

17

Symbols of Nuclides

Find the # p+

# n0

# e-

Atomic numberMass Number

F19 9

Symbols of Nuclides Find the Find the

# p# p++

# n# n00

# e# e--

–Atomic numberAtomic number

–Mass NumberMass Number

Br80 35

Nuclides if an element has an atomic if an element has an atomic

number of 34 and a mass number of 34 and a mass number of 78 what is the number of 78 what is the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Complete symbolComplete symbol

Nuclides if an element has 91 protons if an element has 91 protons

and 140 neutrons what is the and 140 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of electronsnumber of electrons

–Complete symbolComplete symbol

Nuclides if an element has 78 if an element has 78

electrons and 117 neutrons electrons and 117 neutrons what is the what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of protonsnumber of protons

–Complete symbolComplete symbol

Using the periodic table for nuclides

How many protons, neutrons, and electrons are in an atom of

Uranium-235

Uranium-238

Relative Atomic MassThe mass of an atom expressed in atomic mass

units is called the atomic mass of the atom.

1 p = 1.007276 amu

1 n = 1.008665 amu

1 e- = 0.0005486 amu

© Addison-Wesley Publishing Company, Inc.

atomic mass unit (amu or u)

1 amu = 1/12 the mass of a 12C atom

Average Atomic Mass

weighted average of all naturally occurring isotopes of an element

Periodic Table shows the average

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Avg.AtomicMass

Average Atomic Mass

EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.

100

(18)(0.20)(17)(0.04))(16)(99.76 16.00amu

Avg.AtomicMass

Average Atomic Mass

Ex. copper consists of 69.17% copper-63, which has an atomic mass of 62.94 u, and copper-65, which has an atomic mass of 64.93 u.

10

.83)(64.93)(30.17)(62.94)(69

63.55 amu

Magnesium has three isotopes. 78.99% magnesium - 24 with a mass of 23.99 amu,

10.00% magnesium - 25 with a mass of 24.99 amu,

magnesium - 26 with a mass of 25.98 amu.

What is the average atomic mass of magnesium?

The Mole

Avogadro

What is the Mole?

A counting number (like a dozen)

Avogadro’s number

1 mol = 6.022 1023 items

A large amount!!!!

1 mole of pennies would cover the Earth 1/4 mile deep!

1 mole of basketballs would fill a bag the size of the earth!

Molar Mass

Mass of 1 mole of an element or

compound.

Molar mass tells the … grams per mole (g/mol) Use average atomic mass on the

periodic table – same # different unit

Molar Mass Examples

carbon

aluminum

zinc

12.01 g/mol

26.98 g/mol

65.39 g/mol

Molar Conversions

molar mass

(g/mol)

MASS

IN

GRAMS

MOLESNUMBER

OF

PARTICLES

6.022 1023

(particles/mol)

Mole

Molar Conversions – grams to mol

How many moles of carbon are in 26.0 g of carbon?

26.0 g C 1 mol C

12.01 g C= 2.16 mol C

Molar Conversions – mol to grams

What is the mass in grams of 3.50 mol of Cu?

3.50 mol Cu 63.55 g Cu

1 mol Cu= 222 g Cu

Molar Conversions Using Avogadro’s numberHow many atoms are in 2.50

moles of lead?

2.50 mol

6.022 1023

atoms

1 mol= 1.51 1024

atoms Pb

Molar Conversions Using Avogadro’s numberHow many moles of Ag are in

3.01 1023 atoms of silver?

3.01 1023 atoms 1 mol Ag

= 0.500 mol Ag6.022 1023 atoms

Molar Conversions

Find the mass of 2.1 1024 atoms of Copper.

2.10 1024

atoms 1 mol

6.022 1023

atoms

= 222 g Cu

63.55 g

1 mol