Ch. 3: Atomic StructureThe Theory of the Atom

• ________________, a famous Greek teacher who lived in the 4th Century B.C., first suggested the idea of the atom.

• ________ __________ came up with his atomic theory based on the results of his experiments. (See Figure 5.1)

The Atom• The smallest particle of an ________________ is an atom.

• The atom is made up of three ________________ particles. (1)The electron was discovered in _______ by J. J. Thomson. The

electron has a _______ charge. It’s mass is much smaller than the other 2 subatomic particles, therefore it’s mass is usually ______________. Like a Nutrition label at Old Country Buffet.

(2) The proton has a ______ charge, and it was discovered in _________ by E. Goldstein.

Democritus

John Dalton

element

subatomic

1897(-)

ignored

(+)1886

(3) The neutron does not have a charge. In other words, it is ________. It was discovered in ____ by James Chadwick. The neutron has about the same _________ as the proton.

• These three particles make up all the ____________________

in the Universe!

Nuclear Atomic Structure

• The atom is made up of 2 parts/sections:

(1) The ______________ --- (in the center of the atom)

(2) The ____________ _________ --- (surrounds the nucleus)

neutral 1932mass

visible matter

nucleuselectron cloud

nucleus (p+ & n0)

e- cloud

The Nucleus

• Discovered by Ernest ________________ in ________.

• He shot a beam of positively charged “alpha particles”, which are ___________ nuclei), at a thin sheet of ______ _____.

•

• 99.9% of the particles went right on through to the ______________.

• Some were slightly deflected. Some even ____________ ________ towards the source!

• This would be like shooting a cannon ball at a piece of tissue paper and having it bounce off.

•Or.....

Rutherford 1911

helium gold foil

detector

bounced back

Conclusions about the Nucleus

(1) Most of the atom is more or less _________ ___________.

(2) The nucleus is very _________. (Stadium Analogy)

(3) The nucleus is very ___________. (Large Mass ÷ Small Volume)

(4) The nucleus is ______________ charged.

empty space

densetiny

positively

Counting Subatomic Particles in an Atom

• The atomic # of an element equals the number of ____________ in the

nucleus.

• The mass # of an element equals the sum of the _____________ and

______________ in the nucleus.

• In a neutral atom, the # of protons = # of ______________.

• To calculate the # of neutrons in the nucleus, ______________ the

___________ # from the __________ #.

protons

protons

neutrons

electrons

subtract

atomic mass

How to Use the Periodic Table

5

B10.61

Symbol

Atomic Mass

Atomic Number

Practice Problems• Find the # of e-, p+ and n0 for sodium. (mass # = 23)

• Find the # of e-, p+ and n0 for uranium. (mass # = 238)

3) What is the atomic # and mass # for the following atom? # e- = 15; # n0 = 16

Atomic # = 11 11 electrons11 protons

# neutrons = 23-11 = 12

Atomic # = 92 92 electrons92 protons

# neutrons = 238-92 = 146

Atomic # = 15 = # e- = # p+ Mass # = p+ + n0 = 15+16 =31

The element is phosphorus!

Isotopes• An isotope refers to atoms that have the same # of ___________,

but they have a different # of ___________.

• Because of this, they have different _________ #’s (or simply, different ___________.)

• Isotopes are the same element, but the atoms weigh a different amount because of the # of ______________.

Examples---> (1) Carbon-12 & Carbon-13

(2) Chlorine-35 & Chlorine-37

(The # shown after the name is the mass #.)

• For each example, the elements have identical ___________ #’s, (# of p+) but different _________ #’s, (# of n0).

• Another way to write the isotopes in shorthand is as follows:

C Cl12

6

35

17

The top number is the ________ #, and the bottom # is the __________ number. Calculating the # n0 can be found by _____________ the #’s!

protons neutrons

massmasses

neutrons

atomic mass

mass atomic subtracting

More Practice Problems• Find the # e-, p+ and n0 for Xe-131.

• Find the # e-, p+ and n0 for

3) Write a shorthand way to represent the following isotope:

# e- = 1 # n0 = 0 # p+ = 1

Cu63

29

Atomic # = 54 n0 = 131-54 = 77Electrons = 54

Protons = 54

Atomic # = 29 = p+ = e- n0 = 63-29 = 34

Atomic # = p+ = e- = 1 mass # = n0 + p+ = 1+ 0 = 1

H-1 or H1

1

Atomic Mass• Based on the relative mass of Carbon-12 which is exactly

_______.

• 1 p+ ≈ __ atomic mass unit (amu) 1 n0 ≈ __ amu 1e- ≈ __ amu

• The atomic masses listed in the Periodic Table are a “weighted average” of all the isotopes of the element.

12

1 1 0

Weighted AveragePractice Problems: (1) Señorita Schwartz's Spanish 1 semester grades are calculated using a

weighted average of three category scores: Major Grades= 60% of your gradeMinor Grades= 30% of your gradeSemester Exam=10% of your grade

• If a student had the following scores, what would they receive for the semester?

Major= 80 (B-)Minor= 60 (D-)

Semester Exam=65 (D)

Weighted AverageStep (1): Multiply each score by the % that it is weighted.

Step (2): Add these products up, and that is the weighted average!

60% x 80 = 48.0

30% x 60 = 18.0

10% x 65 = 6.5

Add them up!!

A “normal average” would be calculated by simply adding the raw scores together and dividing by 3…

80 + 60 + 65 = 205 ÷ 3 = 68.3 = D

+

72.5 (C)

Weighted Average

Practice Problems:

(2) In chemistry, chlorine has 2 isotopes:

Cl-35 (75.8% abundance) Cl-37 (24.23 % abundance)

What is the weighted average atomic mass of chlorine?

35 x 0.758 = 26.53

37 x 0.2423 = 8.9651

Add them up!!!

(3) Oxygen has 3 isotopes:

O-16 (99.76%) O-17 (0.037%) O-18 (0.2%)

Estimate oxygen’s average atomic mass.

Barely over 16.0 amu.

+

35.4951 amu