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Atomic Structure. What is a theory?. a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena;

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what is a theory
What is a theory?
  • a well-substantiated explanation of some aspect of the natural world;
  • an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena;
  • "theories can incorporate facts and laws and tested hypotheses"
early theories
Early Theories
  • Democritus: 4 B.C.: “atom”
    • Believed there were 4 elements:
    • Fire, Air, Water, Earth
dalton 1766 1844
Dalton: 1766-1844

>All elements composed of tiny particles

called atoms

>Atoms of same element are identical;

atoms of different elements are different

>Atoms of different elements can physically

mix together or chemically combine to form

compounds

>Chemical reactions cannot change atoms

of one type of element to another

thomson 1856 1940
Thomson: 1856-1940
  • >discovered electrons in 1897
  • >used a cathode ray tube
  • >the ray produced was deflected by an electrical field (showed that atoms had particles with (-) charge)
cathode ray tubes
Cathode Ray Tubes
  • A cathode ray tube or CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface.
  • TVs, PCs, ATMs, video games, video cameras, and monitors all contain cathode-ray tubes.
  • Displays millions of colors.
rutherford 1871 1937
Rutherford: 1871-1937

>Gold Foil Experiment

>Discovered the nucleus

slide9

Rutherford’s Gold Foil Experiment

Experiment

Shot positively charged alpha particles at gold foil

  • Results
  • Most particles passed through the foil
  • A few were deflected
rutherford s gold foil experiment
Rutherford’s Gold Foil Experiment

Conclusions

  • small, dense, positively charged core (nucleus)
  • the rest of the atom is empty space
modern theories
Modern Theories

Bohr  planetary model

  • electrons arranged in concentric circular patterns
  • paths or orbits around nucleus (energy level)

Wave-Mechanical Model  Electron Cloud Model

  • based on the ideas that orbitals are the area of highest probability where an electron will be found.
  • Orbitals have a variety of shapes and names (s, p, d, f)
example wave mechanical model
Example: Wave Mechanical Model
  • Ψ2 (psi2) is a calculation that can predict the probability of finding an electron in a given area.
summary atomic models
Summary- Atomic Models

Dalton’s Cannonball

atomic number
Atomic Number
  • Equal to the number of protons
  • Every element has its own atomic number
  • See Periodic Table

C

6

mass number
Mass Number
  • Equal to the sum of the protons and the neutrons (whole number)
  • Can be written as carbon-12

C

12

to find
To find:

# of protons 

look up atomic number on Periodic Table

to find1
To find:

# of electrons 

in a neutral atom, it is equal to the number of protons

to find2
To find:

# of neutrons 

if protons + neutrons = mass then,

# of neutrons = mass # - # protons

practice
Practice

20

40

20

20

20

12

24

12

12

12

11

23

11

12

11

2

4

2

2

2

slide26
Ions
  • Defined as “charged particles”
  • Ions are formed when the number of electrons changes.
  • If a (+) ion is formed, electrons are lost (called cations).
  • If a (-) ion is formed, electrons are gained (called anions).
examples
Examples
  • Ca2+

A Ca atom has 20 protons and 20 electrons.

A Ca2+ ion has lost two electrons to have 18.

examples1
Examples
  • Cl-

A Cl atom has 17 protons and 17 electrons.

A Cl-ion has gained one electron to have 18.

practice1
Practice

30

30

65

30

35

26

56

26

30

23

9

19

9

10

9

127

53

74

54

53

3

4

3

7

2

isotopes
Isotopes
  • Definition: elements that have the same atomic number but different mass (different # of neutrons)
isotopic symbols
Isotopic Symbols
  • Must write isotopic symbol to show mass
  • Same atomic #, different mass #

X

Mass #

Atomic #

why is atomic mass not a whole number
Why is atomic mass not a whole number?
  • The atomic mass on the periodic table is a weighted average of the isotopes of the elements.
  • The weighted atomic mass takes into account the relative abundances of all the naturally occurring isotopes.
example of a general weighted average
Example of a general weighted average
  • Your grade in chemistry
  • 60% exams 85
  • 15% quizzes 100
  • 15% labs 95
  • 10% HW/CW 80

88.25

(0.60)85 + (0.15)100 + (0.15)95 + (0.10)80 =

example 1
Example 1:
  • Determine weighted atomic mass
  • Boron-10 19.78% 10.013 amu
  • Boron-11 80.22% 11.009 amu

10.812 amu

(0.1978) 10.013 + (.8022) 11.009 =

example 2
Example 2
  • Determine weighted atomic mass
  • Potassium-39 93.12% 38.964 amu
  • Potassium-41 6.88% 40.962 amu

39.101 amu

(0.9312) 38.964 + (0.0688) 40.962 =

bohr models
Bohr models

How do electrons “orbit” the nucleus?

Each principal energy level …

  • is a fixed distance from the nucleus
  • can hold a specific number of electrons
  • has a definite amount of energy
slide41
The greater the distance from the nucleus…the greater the energy of the electrons in it.
  • The orbits are called principal energy levels or shells.
energy levels or shells
Energy levels or shells

energy level number of e-

1 2

2 8

3 18

4 32

Increasing distance from nucleus

Increasing energy

bohr models examples
Bohr models: examples

-energy levels

and total number

of electrons

8 e-

P+

n0

nucleus---

# protons

And neutrons

2 e-

Electron configuration: element’s symbol and number of

electrons in each orbit; LOOK UNDER ATOMIC NUMBER

ON PT of E

try these
TRY THESE

12 p+

12 n0

Mg

2 e- 8 e- 2 e-

Electron configuration (bottom left corner on PT): Mg 2-8-2

H Na F C

answers
answers

H Na

H 1 Na 2-8-1

F C

F 2-7 C 2-4

11 p+

12 n0

1p+

1 e-

2 e- 8e-1e-

9 p+

10 n0

6 p+

6 n0

2 e- 7 e-

2 e- 4 e-

lewis dot diagrams
Lewis Dot Diagrams

Valence shell: outer most shell of an atom that contains electrons

Valence electrons: electrons that occupy the valence shell (last number in electron configuration)

Electron dot diagrams or Lewis dot diagrams:

show only the valence shell of the atom

Ex: Lewis dot for nitrogen: N

try these1
TRY THESE

O F C

Ne I K

slide48
Ions

For ions: remember that ions have gained or lost electrons.

(+)  indicate charge

(-)  use brackets and charge

slide49
Ca  Ca+2

Cl  [ Cl ]-1

ground state vs excited state
Ground State vs. Excited State
  • When all electrons in an atom occupy the lowest available orbitals, it is said to be in the ground state.
  • When electron(s) absorb energy, they have the ability to jump to higher energy levels.
  • The excited state is when electrons have absorbed energy and no longer occupy the lowest available energy levels.
absorption
Absorption
  • When an electron “jumps” to a higher energy level it absorbs energy.
  • The excited state is a temporary state.

Excited State

(i.e. energy level 2)

e-

Ground State

(i.e. Energy level 1)

emission
Emission
  • The electron then falls back down to the ground state, emitting energy.
  • The energy is in the form of light.
slide54
This radiant energy has a characteristic color and wavelength that can be determined.
  • Every electron transition produces a specific wavelength of light and all transitions for an element blend together.
  • This light can be separated through a prism into its various wavelength components.
  • Every element has its own unique bright line spectrum that can be used to help identify the presence of that element.
  • Ex: elements in a star, forensic analysis, flame tests, spectroscopy
light and atomic spectra bright line spectra
Light and Atomic Spectra (bright line spectra)

Electromagnetic spectrum consists of light that exists as waves.

sunlight and prisms
Sunlight and prisms
  • Sunlight produces a continuous range of wavelengths and frequencies that can be separated into all the colors of the rainbow.
  • R O Y G B I V.
slide57
Atomic emission spectra produce narrow lines of color called bright line spectra.
  • Each line corresponds to an exact wavelength.
experiments flame tests
Experiments – Flame Tests
  • Flame Tests – demonstrates the emission spectrum of a substance.
  • Completed by heating elements to high temperatures so they may enter excited state.
  • Characteristic color will be emitted as excited electrons return to ground state.
  • Used to determine metal ion presence in unknown substance.
experiments spectroscopy
Experiments – Spectroscopy
  • Spectroscopy – used to view the bright line spectra for given gases.
  • Completed by viewing a gas tube through which an electric current is passed.
  • Use an instrument called a spectroscope, contains a prism to separate emitted light into line spectra.
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