history of the atomic model - vigo county school corporationvigoschools.org/~mmc3/inorganic/inorgo...

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Development of

the Atomic

Model Chapter 2.1-4

. .

.

. .

.

.

. .

. heavy

central

(+) nucleus

e- “about”

nucleus

“sea of e-”

History of the Atomic Model

• Democritus (400 B.C.)

• Believed that matter was composed of invisible particles of matter he called atoms.

• Antoine Lavoisier (1700’s)

• Law of Conservation of Mass – Matter is not created or destroyed.

• Joseph Proust (1700’s)

• Law of constant composition – compounds are composed of atoms in definite ratios.

History of the Atomic Model • John Dalton (Late 1700’s)

• First atomic theory explaining chemical reactions


• Dmitri Mendeleev (1871)

• developed the modern periodic table. He

argued that element properties are

periodic functions of their atomic masses.

History of the Atomic Model • Henri Bacquerel (1896)

• While studying the effect of x-rays on

photographic film, he discovered some

chemicals spontaneously decompose and

give off very penetrating rays.

• J.J. Thomson (1897)

• Discovered the electron using cathode

ray tubes.

• Proposed the Electron Cloud model, or

more commonly called the “Plum

Pudding” model

Thompsons Cathode Ray Experiment (1897)

Using a cathode ray tube, he

determined the charge-to-mass ratio for

the electron as: 1.76 x 108 C/g

2

The Atom, circa 1900:

• “Plum pudding”

model, put forward by

Thompson.

• Positive sphere of

matter with negative

electrons imbedded in

it.


• Marie Curie (1898) • Studied uranium and thorium and called

their spontaneous decay process

"radioactivity,” leading Rutherford to

discover the alpha particle.

• Hantaro Nagaok (1903) • Postulated a "Saturnian" model of the

atom with flat rings of electrons revolving

around a positively charged particle.

• Robert Millikan (1909) • Found the charge and mass of the electron

in his famous “oil-can” experiment.

Milikan Oil Drop

Experiment (1909)

• Using voltage and change

in the rate of fall of

charged oil drops, he was

able to determine the

charge on each drop.

• From Thompson’s charge

to mass ratio, Milikan

determined the charge and

mass of an electron.

Particles of Radioactivity

• Three types of radiation were discovered by

Ernest Rutherford:

particles

particles

rays

Rutherford’s Gold Foil

Experiment (1911)

Ernest Rutherford

shot particles at

a thin sheet of

gold foil and

observed the

pattern of scatter

of the particles.

Since some particles

were deflected at large

angles, Thompson’s

model could not be

correct.

Rutherford’s Gold Foil

Experiment (1911)

3

The Nuclear Atom (1911)

• Rutherford postulated a very small, dense

nucleus of positive charge with the electrons

around the outside of the atom.

• Most of the volume of the atom is empty space.

• Protons were later discovered by Rutherford in 1919.


• Ernest Rutherford (1911)

• Discovered the nucleus in his famous “gold foil” experiment.

• Data from his experiments led Rutherford to propose a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge. Only such a concentration of charge could produce the electric field strong enough to cause the heavy deflection of alpha particles observed.


• Henri Mosely (1915)

• Determined the charges on the nuclei of most atoms resulting in a reorganization of the periodic table based upon atomic number instead of atomic mass.

• Ernest Rutherford (1917)

• concluded that hydrogen nuclei were singular particles and a basic constituent of all atomic nuclei. He named such particles protons.

Further developed the

atomic model by theorizing that

alpha and beta radiation results

from the decomposition of a

neutral particle found in the

nucleus, the neutron

• H atoms - 1 p; He atoms - 2 p

• mass He/mass H should = 2

• measured mass He/mass H = 4

James Chadwick (1932)

Subatomic Particles

Atoms contains subatomic particles:

• protons have a positive (+)

charge

• electrons have a negative (-)

charge

• like charges repel and unlike

charges attract

• neutrons are neutral

Like charges repel, and unlike

charges attract.

ATOMIC COMPOSITION

• Protons + electrical charge mass = 1.67262158 x 10-24 g relative mass = 1.0073 (amu)

• Electrons negative electrical charge mass = 9.10938188 x 10-28 g relative mass = 0.0005486 amu

• Neutrons no electrical charge mass = 1.67492716 x 10-24 g mass = 1.0087 amu

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ATOM

COMPOSITION

• 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.

The atom is mostly

empty space

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The atomic number

• is specific for each element

• is the same for all atoms of an element

• is equal to the number of protons in an atom

• appears above the symbol of an element

Atomic Number

11

Na

Atomic Number

Symbol

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

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Examples:

• Hydrogen has atomic number 1, every H atom

has one proton.

• Carbon has atomic number 6, every C atom

has six protons.

• Copper has atomic number 29, every Cu atom

has 29 protons.

• Gold has atomic number 79, every Au atom

has 79 protons.

Atomic Numbers and Protons


22

State the number of protons in each atom.

A. A nitrogen atom

1) 5 protons 2) 7 protons 3) 14 protons

B. A sulfur atom


C. A barium atom


Learning Check


23

An atom of any element

• is electrically neutral; the net charge of an atom is

zero

• the number of protons is equal to the number of

electrons

Example:

An atom of aluminum Group 3A (13) has 13 protons

and 13 electrons. It has a net charge of zero.

13 protons (13 +) + 13 electrons (13 -) = 0

Electrons in an Atom


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Mass Number

The mass number

• represents the number of particles in the nucleus

• is equal to the

Number of protons + Number of neutrons


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25

An atom of zinc has a mass number of 65.

A. How many protons are in this zinc atom?

1) 30 2) 35 3) 65

B. How many neutrons are in the zinc atom?

1) 30 2) 35 3) 65

C. What is the mass number of a zinc atom that

has 37 neutrons?

1) 37 2) 65 3) 67

Learning Check


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An atom has 14 protons and 20 neutrons.

A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is

1) 14 2) 16 3) 34

C. The element is

1) Si 2) Ca 3) Se

Learning Check


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Isotopes

• are atoms of the same element that have

different mass numbers

• have the same number of protons, but different

numbers of neutrons

Isotopes


Element Isotopes

• determined with a mass spectrometer

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A nuclear symbol

• represents a particular atom of an element

• gives the mass number in the upper left corner

and the atomic number in the lower left corner

Example: An atom of magnesium with an atomic

number of 12 and a mass number of 24 has the

following atomic symbol:

Nuclear Symbol


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• The nuclear symbol indicates the number of

protons (p+), neutrons, (n), and electrons (e-) in

a particular atom.

16 31 65 O P Zn 8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n

8 e- 15 e- 30 e-

Information from Nuclear

Symbols


6

31

Naturally occurring carbon consists of three

isotopes, 12C, 13C, and 14C. State the number of

protons, neutrons, and electrons in each of the

following.

12C 13C 14C

6 6 6

protons ______ ______ ______

neutrons ______ ______ ______

electrons ______ ______ ______

Learning Check


32

Write the nuclear symbols for atoms with the following subatomic particles:

A. 8 p+, 8 n, 8 e- ___________

B. 17 p+, 20 n, 17e- ___________

C. 47 p+, 60 n, 47 e- ___________

Learning Check


33

Learning Check

1. Which of the following pairs are isotopes of the

same element?

2. In which of the following pairs do both atoms have

8 neutrons?

A. 15X 15X

8 7

B. 12X 14X

6 6

C. 15X 16X

7 8


Atomic Mass Scale

• Because the masses of atoms are so small, the units of grams is much too large to be used conveniently. Therefore, the Atomic Mass Unit (amu) is used.

• On the atomic mass scale for subatomic particles,

• 1 atomic mass unit (amu) is equal to 1/12 of the mass of the carbon-12 atom

• a proton has a mass of 1.007 amu

• a neutron has a mass of about 1.008 amu

• an electron has a very small mass of 0.00055 amu

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Atomic Mass

The atomic mass of an element

• is listed below the symbol of each

element on the periodic table

• gives the mass of an “average” atom

of each element compared to 12C

• is not the same as the mass number

11

Na

22.99


36

Isotopes of Magnesium

In naturally occurring magnesium,

there are three isotopes.


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37

Examples of Isotopes and Their

Atomic Masses

Most elements

have two or more

isotopes that

contribute to the

atomic mass of

that element.


Average Atomic

Mass

• Because in the real world we use large amounts of

atoms and molecules, we use average masses in

calculations.

• Average mass is calculated from the isotopes of an

element weighted by their relative abundances.

• Boron is 20% 10B and 80% 11B. That is, 11B is 80

percent abundant on earth.

• For boron, its average atomic mass

= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

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Calculating Average Atomic

Mass The calculation for atomic masst requires the

• percent(%) abundance of each isotope

• atomic mass of each isotope of that element

• sum of the weighted averages

mass isotope(1) x (%) + mass isotope(2) x (%) + …

100 100


40

Average Atomic Mass of

Magnesium The average atomic

mass of Mg

• is due to all the Mg isotopes

• is a weighted average

• is not a whole number


Calculating Average Atomic

Mass

Isotope Mass Abundance 24Mg = 23.99 amu x 78.70/100 = 18.88 amu

25Mg = 24.99 amu x 10.13/100 = 2.531 amu

26Mg = 25.98 amu x 11.17/100 = 2.902 amu

Atomic mass (average mass) Mg = 24.31 amu

Mg

24.31 42

Average Atomic Mass for Cl

• The atomic mass of

chlorine is the

weighted average of

two isotopes 35Cl and 37Cl.


Chlorine, with two naturally occurring

isotopes, has an atomic mass of 35.45.

8

43

35Cl has atomic mass 34.97 (75.76%) and 37C has

atomic mass 36.97 (24.24%).

• Use the atomic mass and percent of each

isotope to calculate the contribution of each

isotope to the weighted average.

34.97 x 75.76 = 26.49 amu

100

35.97 x 24.24 = 8.962 amu

100 35.45 amu

• The sum is the weighted average or atomic

mass of Cl. 35.45 amu

Calculating Atomic Mass for

Cl


44

Gallium is an element found in lasers used in

compact disc players. In a sample of gallium,

there is 60.11% of 69Ga (atomic mass 68.93)

atoms and 39.89% of 71Ga (atomic mass 70.92)

atoms.

What is the average atomic mass of gallium?

Learning Check


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