Atomic Structure

1
Atomic Structure

• Chapter 4

Figure https//i.ytimg.com/vi/Qi_EMo9nGOw/hqdefa
ult.jpg
2
4.1 Defining the Atom

• Democritus (460 BC 370 BC)
• Had an atomic philosophy
• Matter was made of small invisible particles
  called atoms
• No scientific evidence, but it was logical
• Could not explain chemical behavior

3
Daltons Atomic Theory

• John Dalton (1766-1844) was an English chemist
  and school teacher
• Daltons theory was the first scientific theory
  of the atom

4
John Dalton looks like another famous John
5
Daltons Atomic Theory
6
Daltons Atomic Theory

1. Matter is composed of tiny indivisible atoms
2. All atoms of a given element are exactly the same
3. Different elements are made of different atoms
4. Atoms of different elements can combine in simple
   whole number ratios to form compounds
5. Chemical reactions involve separating, combining,
   or rearranging atoms.

7
How big are atoms?

• Pretty small
• Just 4 grams of helium (He) contains 6.02 x 1023
  atoms!
• Most atoms have a radius in the range of 5 x
  10-11 m to 2 x 10-10 m
• Atomic sizes are in the pico- and nanometer range

8
Can we see atoms?

• Amazingly, yes!
• STM
• Scanning tunneling microscope has made it
  possible to see atoms.
• These are gold atoms
• They are only 2.3 Å apart!

9
STM of Silicon
http//www.andrew.cmu.edu/user/feenstra/stm/
10
Here is a picture of a mixture of tin, lead, and
silicon atoms!
atoms of tin (blue) and lead (green) deposited on
a silicon substrate (red). (Credit Oscar
Custance) physicsworld.com/cws/article/news/27208
The colors are added to help distinguish the atoms
Nature video
11
4.2 Structure of the Nuclear Atom

• When do I know I have learned this section?
  When
• I can describe 3 types of subatomic particles
• I can describe the structure of the nuclear atom
• I can explain Thomsons discovery
• experiment and model of the atom
• I can explain Millikans discovery
• I can explain Rutherfords discovery
• experiment and model of the atom

12
Subatomic Particles
4.2

• A cathode ray is deflected by electrically
  charged plates.

13
Subatomic Particles

• In Daltons theory, atoms had no charge
• Thomson studied cathode rays (the cathode ray
  experiment)
• Resulted in discovery of the electron
• Electrons are negatively charged particles
• Discovered the charge to mass ratio of electron

14
JJ Thomsons Plum Pudding Model

• Atoms were electrically neutral
• (-) charged electrons embedded in a () charged
  matrix
• Like a chocolate chip cookie

15
Millikans Oil Drop Experiment

• Discovered the actual quantity of charge for the
  electron
• Using Thomsons charge/mass ratio, Millikan was
  also able to determine the mass of the electron
• Mass of electron is 1/1840 the mass of a hydrogen
  atom.
• Millikan's Oil Drop Experiment

16
Protons

• If H atoms are neutral, and they give off
  negatively charged electrons, what must be left?
• Positive charge
• Proton discovered by Eugene Goldstein noticed a
  ray traveling in opposite direction of the
  electrons in the cathode ray tube
• Toward the negative end (cathode)

17
Neutrons

• Discovered by James Chadwick

18
Quarks

• Quarks are subnuclear particles
• Protons and neutrons are composed of quarks

19
Rutherfords Gold Foil Experiment

• Conducted Hans Geiger and Ernest Marsden in 1909
• Interpreted by Ernest Rutherford in 1911Source
  Boundless. The Thomson Model. Boundless
  Physics. Boundless, 19 Sep. 2016. Retrieved 13
  Oct. 2016 from https//www.boundless.com/physics/t
  extbooks/boundless-physics-textbook/atomic-physics
  -29/the-early-atom-185/the-thomson-model-685-6307/

20
The Atomic Nucleus

• Ernest Rutherford discovered the nucleus
• Gold foil experiment

21
Interpretation of Rutherfords Findings
22
Rutherfords Conclusions

• All positive charge and nearly all the mass of
  the atom are concentrated in a very small nucleus
• Atom contains mostly empty space
• If the nucleus was the diameter of a basketball,
  the atom would have a diameter of 15 miles!!

23
Summary of Atomic Models

• Democritus Invisible, indivisible particles
• Dalton Tiny homogeneous sphere
• 5 (4) statements
• Thomson Plum pudding
• Rutherford Nuclear atom

24
Daltons Atomic Theory

1. Matter is composed of tiny indivisible atoms
2. All atoms of a given element are exactly the same
3. Different elements are made of different atoms
4. Atoms of different elements can combine in simple
   whole number ratios to form compounds
5. Chemical reactions involve separating, combining,
   or rearranging atoms.

25
Thomsons Model of the Atom

• Plum Pudding
• Negatively charged electrons embedded in a
  positive matrix

26
Rutherfords Model

• Nuclear Atom
• Most of the volume of the atom is empty space
• All positive charge most mass (99.95) in a
  small nucleus
• Nucleus contains protons and neutrons
• Electrons occupy space outside the nucleus

27
4.3 Distinguishing Among Atoms

• Atomic Number
• number of protons in nucleus
• Each element has unique atomic number
• Mass Number
• number of protons and neutrons in nucleus
• neutrons mass - atomic

28
Practice
29
Symbols of Elements

• Elements are symbolized by one or two letters.
• First letter is always capitalized.
• Second letter, if there is one, is always lower
  case.

30
Atomic Number

• All atoms of the same element have the same
  number of protons
• The atomic number (Z)

31
Mass Number

• The mass of an atom in atomic mass units (amu)
  is the total number of protons and neutrons in
  the atom.

32
Isotopes

• Isotopes are atoms of the same element with
  different masses.
• Isotopes have different numbers of neutrons.

33
Different Ways of Indicating Isotopes

• Chemical symbols
• 168O 188O
• Symbol and mass number
• O-16 O-18
• Name and mass number
• oxygen-16 oxygen-18

34
Isotopes of Neon
Atomic Mass
35
Atomic Mass (Atomic Weight)

• Measured in amus (atomic mass units) because
  their mass is so small
• Definition one atom of carbon-12 equals exactly
  12 amus
• 1 amu exactly 1/12 the mass of a C-12 atom
• 1 amu 1.6605 x 10-24g

36
Different Isotopes Have Different Abundance
Atomic Masses
Isotope Abundance Atomic mass (amu)
Chlorine-35 75.77 34.969
Chlorine-37 24.23 36.966
What is the atomic mass of chlorine on the
periodic table?
37
What is the Average Mass of a Group of Chickens?
38
What is the Average Mass of a Group of Chickens?
Is the simple average represent the true average
mass of the group?
39
What is the Average Mass of a Group of Chickens?
What if you were to determine the average based
upon total mass?
40
What is the Average Mass of a Group of Chickens?
We can arrive at the same average by multiplying
the mass of each times its relative abundance and
adding the results.
41
Average Atomic Mass

• Atoms are too small and numerous to count
• Average atomic mass can be calculated in the same
  way
• Multiply the mass of each isotope by its relative
  abundance, and add them
• This will give you the weighted average atomic
  mass, often called atomic weight

42
Average Atomic Mass

• Determining abundance of isotopes when average
  atomic mass is known

• Multiply the mass of each isotope by its relative
  abundance (decimal equivalent) and add the
  products

Isotope Atomic Mass Abundance
R-125 125 ________ ________
R-120 120 ________ ________
124
43
Periodic Table - Preview

• Allows you to easily compare and predict
  properties of elements
• Groups Columns
• Numbered 1-18 or
• 1A-8A (representative elements)
• 1B-8B (transition metals)
• Periods Rows
• Numbered 1-7