Atomic Theories

Atomic timeline• Your poster should contain:

– The date the theory was developed– A picture of the ‘model’– The name of the main contributor– A short summary of the theory

Atomic Timeline

450 BC

1803

1896

1910

1911

450 BC - Democritus• Said that matter is composed of tiny,

indivisible particles called “atomos”

• Atom (2006): Smallest part of an element that still has the property of that element

I was close enough. You

little punks living in your AD don’t know how good

you have it.

1803 – John Dalton• The theory that atoms are the fundamental

building blocks of matter reemerged in the early 19th century

• John Dalton: School teacher

• Based on observations of how elements combine

1803 – John Dalton• Basically summarized what was already said

in his Top Five Descriptions of Matter

Haha, that’s like my top ten list that

I do everynight..except

it’s the top 5 – and its about

matter. I get it.

• 1. Elements are composed of extremely small particles called atoms

• 2. All atoms of an element are identical in mass & properties

• 3. Atoms of one element are different from atoms of another element

• 4. Atoms of an element are not changed into atoms of another element; nor are they created or destroyed in chemical reactions

• 5. Compounds are formed when atoms combine; a given compound always has the same relative number & kind of atoms

Daltons Table of Elements

Dalton’s Postulates…• Law of Constant Composition (was known at the time)

– In a compound, the relative #’s and kinds of atoms stay the same

5. Compounds are formed when atoms combine; a given compound always has the same relative number & kind of atoms

• Law of Conservation of Mass (was known at the time)– The total mass of materials present after a chemical reaction is

the same as the total mass present before

Massreactants = MassProducts4. Atoms of an element are not changed into atoms of another element; nor are they created or destroyed in chemical reactions

• Law of Multiple Proportions (Dalton predicted!)– If two elements A and B combine to form more than one

compound, they do so in simple whole number ratios

J.J. Thomson – The Cathode Ray Tube

• Discovered negatively charged part of an atom

• This was the first Sub Atomic particle

• Was able to use his cathode ray tube to determine Charge:Mass ratio

J.J. Thomson’s Model

•Electrons were small mass, so they must be small size compared to the atom

•Explain where the electrons were located in his Plum Pudding Model

•Uniform positive sphere with electrons embedded

Millikan’s Oil Drop Experiment

• Robert Millikan– Determined the charge of an electron– Used this & Thomson’s charge:mass ratio to

determine the mass– Mass e- = 9.10 x 10-28 g

Rutherford’s Gold Foil

Rutherford (and his gold foil)• Rutherford

– Gold Foil Experiment– Atom is mostly empty space (how empty?)– Nucleus: Contains all of the mass (neutrons and

protons)– Electrons: Account for most

of the volume

Questions• Why do things have different mass?

• Why does this happen?

Modern Atomic Theory2 Problems with Dalton’s

Theory

1. Atoms are divisible

2. Atoms of an element can be different

Subatomic Particles• Proton: Discovered by Rutherford in 1919

– Charge: 1.602 x 10-19 C

• Neutron: Discovered by James Chadwick in 1932– Charge: 0

• Electron: (J.J.) Charge: -1.602 x 10-19 C1 amu = 1.66054 x 10-24 g

Identifying Atoms• Atomic number:

– # of protons in the nucleus– “Address” of the atom – identifies what element it

is

• Atomic Weight:– Average mass of the atom

• Mass Number– The number of protons + neutrons in the nucleus– Whole #

Why don’t we

count electrons

?

1836 Electrons = mass of 1 proton

Using these numbers…• Atomic # = # of protons = # of electrons

• Mass # = protons + neutrons

• Neutrons = Mass # - protons

Example’sDetermine the number of P+, No, and E- in…

1) MgAtomic # = 12 12protons, 12electronsMass # = 24 12 neutrons

2) TcAtomic # = 43 43protons, 43electronsMass # = 99 56 neutrons

3) MnAtomic # = 25 25protons, 25electronsMass # = 55 30 neutrons

4) SnAtomic # = 50 50protons, 50electronsMass # = 119 69 neutrons

Chemical Symbols

a.

1. Cannot determine number of electrons without additional information.

2. The atom has 30 electrons.

3. The atoms has 15 electrons.

4. The atom has no electrons unless it is charged.

a.

1. Cannot determine number of electrons without additional information.

2. The atom has 30 electrons.

3. The atoms has 15 electrons.

4. The atom has no electrons unless it is charged.

b.

1. The protons reside in the nucleus of the atom.

2. The protons are evenly distributed throughout the atom.

3. The protons are dispersed with the electrons around the nucleus.

4. The protons reside in a shell just outside the nucleus.

b.

1. The protons reside in the nucleus of the atom.

2. The protons are evenly distributed throughout the atom.

3. The protons are dispersed with the electrons around the nucleus.

4. The protons reside in a shell just outside the nucleus.

Modern Atomic Theory2 Problems with Dalton’s

Theory

1. Atoms are divisible

2. Atoms of an element can be different

Isotopes• Atoms of the same element with different #’s of

neutrons

D:\Chapter_02\Present\eMedia_Library\HydrogenIsotopes\HydrogenIsotopes.html

116C

126C

136C

146C

Average Atomic Mass• Isotopes in Banana’s

19

K39.098

This number represents the average of all the naturally occurring isotopes of

Potassium

Relative Abundance• To calculate average mass, we must know

how much each isotope occurs

• Calculating Average:(mass isotope 1 x %) + (mass isotope 2 x %) +….

A hypothetical element has two isotopes. One of the isotopes has an abundance of 75.0% and a relative mass of 12.0 amu, while the other has an abundance of 25.0% and a relative mass of 14.0 amu. The atomic mass

of this hypothetical element is

1. 12.5 amu.2. 13.0 amu.

3.13.5 amu.4.13.8 amu.

Correct Answer:

Atomic mass = (0.750)(12.0 amu) + (0.250)(14.0 amu)

Atomic mass = 9.0 amu + 3.5 amu = 12.5 amu

1. 12.5 amu.2. 13.0 amu.

3.13.5 amu.4.13.8 amu.

SAMPLE EXERCISE 2.4 Calculating the Atomic Weight of an Element from Isotopic Abundances

Naturally occurring chlorine is 75.78% 35Cl, which has an atomic mass of 34.969 amu, and 24.22% 37Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine.

PRACTICE EXERCISEThree isotopes of silicon occur in nature: 28Si (92.23%), which has an atomic mass of 27.97693 amu; 29Si (4.68%), which has an atomic mass of 28.97649 amu; and 30Si (3.09%), which has an atomic mass of 29.97377 amu. Calculate the atomic weight of silicon.

Answer: 28.09 amu

Solution The average atomic mass is found by multiplying the abundance of each isotope by its atomic mass and summing these products. Because 75.78% = 0.7578 and 24.22% = 0.2422, we have

This answer makes sense: The average atomic mass of Cl is between the masses of the two isotopes and is closer to the value of 35Cl, which is the more abundant isotope.

Ions• Atoms of the same element with different #’s

of electrons

Solution (a) Magnesium has atomic number 12, and so all atoms of magnesium contain 12 protons and 12 electrons. The three isotopes are therefore represented by (b) The number of neutrons in each isotope is the mass number minus the number of protons. The numbers of neutrons in an atom of each isotope are therefore 12, 13, and 14, respectively.

SAMPLE EXERCISE 2.3 Writing Symbols for Atoms

Magnesium has three isotopes, with mass numbers 24, 25, and 26. (a) Write the complete chemical symbol (superscript and subscript) for each of them. (b) How many neutrons are in an atom of each isotope?

PRACTICE EXERCISEGive the complete chemical symbol for the atom that contains 82 protons, 82 electrons, and 126 neutrons.

How many neutrons are there in an atom of 14C?

1. 62. 83. 124. 14

Correct Answer:

Atomic numberMass number 14

6 CThe difference between the mass number and the atomic number is the number of neutrons (14 6) = 8.

1.6

2.8

3.12

4.14

An isotope of Cr3+ ion containing 24 protons and 27 neutrons would contain ___ electrons.

1. 21

2. 24

3. 27

4. 3

Correct Answer:

The +3 charge indicates there are 3 more protons than electrons:

24 3 = 21

1. 21

2. 24

3. 27

4. 3