Atomic Structure

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Unit 02

• Atomic Structure

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Just How Small is an Atom?

• You dont need to write.
• A speck 0.1 mm in diameter (about half the size
  of a period at the end of the sentence) requires
  one million atoms.
• It would require a million atoms, edge to edge,
  to match the thickness of a page of paper.

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Can you see an atom?

• Technically, you cannot "see" anything smaller
  than the shortest wavelength of light that you
  can see it with.
• But there are ways to "visualize" it, like Atomic
  Force Microscopy. But these are all just
  measurements converted to computer images, and
  are not in any real sense "seeing" the atom.
• You can't see atoms in any normal sense of using
  an optical microscope.
• You don't get an optical image, but it does allow
  you to map out an image of the atoms of a
  molecule. To do this you use a metallic tip which
  interacts with the atoms you want to image. As
  you move the tip over the atoms, you pass a
  current, called a tunneling current, between the
  tip and the atom. This current is extremely
  sensitive to the distance between the atom and
  the tip.

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- REMEMBER FROM Elements, Mixtures, and
Compounds -
Element - a pure substance made up of one
type of atom. - organized on periodic
table - each element has a unique
number of protonsits atomic number
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Atomic Structure Atoms contain three subatomic
particles 1. Protons 2. Neutrons 3.
Electrons
positive charge
These are located in NUCLEUS!
neutral charge
negative charge
Electrons surround the nucleus in orbitals
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Atomic Structure

• An atom is considered electrically neutral.
• Electrically neutral means the number of protons
  () the number of electrons (-)

4 red protons 4 blue electrons
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Properties of Subatomic Particles
Particle Symbol Relative electrical charge Relative mass Actual mass (g)
Electron e- 1- 1/1840 9.11 10-28
Proton p 1 1 1.67 10-24
Neutron n0 0 1 1.67 10-24
Protons and Neutrons have the same mass.
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A. Discovery of the Atom

• Ernest Rutherford discovered the nucleus by
  shooting alpha particles (have a positive charge)
  at a very thin piece of gold foil.

He predicted that the particles would go right
through the foil at some small angle.
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Rutherfords Gold Foil Experiment
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Rutherfords Gold Foil Experiment

• some particles (1/8000) bounced back from the
  foil
• this meant there must be a powerful force in
  the foil to hit particle back

Predicted Results Actual Results
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Discovery of the Atom

• Purpose
• The students will find the shape of different
  items and relate this to the early scientist that
  made discoveries about the shape and size of the
  atom.
• Procedure
• 1. Title the left side of your spiral Discovery
  of the Atom.
• 2. For each item you will write the letters then
  draw your predicted shape of the item.
• 3. Then you will write 1 sentence describing why
  they think your prediction is the shape of the
  item.
• A Item in brown bag Use your hands to feel the
  shape of the item.
• B Item in clay Using the toothpicks provided
  find the shape of the object enclosed in the
  modeling clay.
• C Black box Maneuver the black box with a
  marble inside to discover the shape of the object
  enclosed.

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B. Models of the Atom
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J.J. Thomson

• Plum pudding atom negatively charged e- stuck
  into a lump of positively charged material
• similar to chocolate chip cookies

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• Ernest Rutherford
• In Rutherfords gold foil experiment he
  discovered electrons surround a dense positive
  nucleus

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• Bohr Model
• electrons are arranged in fixed orbits around the
  nucleus.

ex. Orbits gum
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Quantum Mechanical Model

• Quantum mechanics was developed by Erwin
  Schrodinger
• Estimates the probability of finding an e- in a
  certain position
• Electrons are found in an electron cloud

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I. Nuclear Symbols
B
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5
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A. Mass Number

• mass protons neutrons

• always a whole number
• NOT on the Periodic Table!

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B. Isotopes

• Atoms of the same element with different mass
  numbers. (different number of neutrons)

• Nuclear symbol

• Hyphen notation carbon-12

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B. Isotopes
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You must know how to find
C. Nuclear Symbols

• of protons atomic number
• mass of n0 of p (atomic )
• Whats in the nucleus of the atom
• of electrons of protons (in a neutral atom)

Boron 5 B 10.811
atomic number
(Not the same as the mass )
(average) atomic mass
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• How to write a Nuclear Symbol

Charge if ion
Element Symbol
B
Mass Number p n0
-3
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5
Atomic Number p
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C. Nuclear Symbols

• Chlorine-37
• atomic
• mass
• of protons
• of electrons
• of neutrons

• 17
• 37
• 17
• 17
• 20

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Nuclear Symbol Examples
Cl
35
17
Number of Neutrons
Number of Electrons
Number of Protons
Mass Number
Atomic Number
17
17
35
18
17

• Mg

27
12
Number of Neutrons
Number of Electrons
Number of Protons
Mass Number
Atomic Number
12
27
12
15
12
25
D. Relative Atomic Mass

• 12C atom 1.992 10-23 g

• atomic mass unit (amu)
• 1 amu 1/12 the mass of a 12C atom

• 1 p 1.007276 amu1 n 1.008665 amu1 e-
  0.0005486 amu

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E. Average Atomic Mass

• weighted average of all naturally occuring
  isotopes
• on the Periodic Table
• round to 2 decimal places

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E. Average Atomic Mass

• EX Calculate the avg. atomic mass of oxygen if
  its abundance in nature is 99.76 16O, 0.04 17O,
  and 0.20 18O.

16.00 amu
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E. Average Atomic Mass

• EX Find chlorines average atomic mass if
  approximately 8 of every 10 atoms are chlorine-35
  and 2 are chlorine-37.

35.40 amu
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Example A sample of cesium is 75 133Cs,
20 132Cs and 5 134Cs. What is the average
atomic mass? Answer .75 x 133 99.75 .20 x
132 26.4 .05 x 134 6.7 132.85
average atomic mass
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• II. The Periodic Table

Periodic Law properties of elements can be
predicted by their position on the periodic table
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A. History of the Periodic Table

• Dmitri Mendeleev (1871)
• Developed the first periodic table
• It was arranged by atomic mass because atomic
  number had not been discovered
• He was able to predict properties of elements

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A. History of the Periodic Table

• Henry Moseley (1913)
• - developed the modern periodic table
• - arranged in order of increasing atomic number

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B. Organization of the Periodic Table

• Period
• horizontal rows numbered 1 - 7
• Energies of outermost electrons are similar

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B. Organization of the Periodic Table

• Groups/ Families
• vertical columns
• have similar chemical physical properties

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Group 1 (IA) Alkali Metals
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Group 2 (IIA) Alkaline Earth Metals
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Group 18 (VIIIA) Noble Gases
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Group 17 (VIIA) - Halogens
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B. Organization of the Periodic Table
Left of stair step
Metals- Nonmetals- Metalloids-
Right of stair step
On the stair step
NONMETALS
METALLOIDS
METALS
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B. Organization of the Periodic Table
Group A- Representative Group B - Transition

• REPRESENTATIVE
• Group A

TRANSITION Group B
INNER TRANSITION
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The Extended Periodic Table