Unit 2 Atomic Theory

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Unit 2Atomic Theory
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Matter

• All matter is made of atoms
• Alone as elements
• Au, Na, O, He
• In combination of elements as compounds
• H2O, NaCl, LiO2

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Early Theories of Matter

• Democritus (460-370 B.C.) proposed believed
  that
• Matter was not infinitely divisible
• Made up of tiny particles called atomos
• Atoms could not be created, destroyed, or further
  divided

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John Dalton, 19th Century

• Dalton revised Democritus's ideas based upon the
  results of scientific research he conducted
• Daltons atomic theory
• Not totally correct

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• Daltons First Atomic Theory
• Elements are made up of small indivisible
  particles called atoms
• Atoms of the same element are identical,
  different elements are different
• - (Same size, mass and chemical properties)
• Atoms are not created or destroyed in a chemical
  reaction
• A compound always has the same relative numbers
  and kinds of atoms

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What is an atom?

• An atom is the smallest particle of an element
  that retains the properties of the element

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Atomic Models

• Thomson
• Plum Pudding Model

• Rutherford
• Electron Cloud Model

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Rutherford The Nuclear Atom

• His model consisted of the following ideas
• an atom consists mostly of empty space through
  which electrons move
• electrons are held within the atom by their
  attraction to the positively charged nucleus
• small, dense, positive charged nucleus

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Bohr Model

• Based on Rutherfords model
• Adds idea of quantized energy levels

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Subatomic Particles

• Protons
• Positive charge
• Protons charge equal to but opposite that of an
  electron
• Mass similar to a neutron
• Neutrons
• Neutron has no charge
• Neutron mass nearly equal to that of proton
• Electrons
• Negatively charged
• Move through empty space in atom
• Very, very small mass

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Inside an atom subatomic particles

• These particles have mass and charge
• Nucleus holds protons neutrons
• Protons charge
• Neutrons Ø charge
• Outside of nucleus is the electron cloud
• Electron - charge

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Parts of the Atom
Name Symbol Charge Mass Location
proton p 1 1 amu nucleus
neutron n 0 1 amu nucleus
electron e- -1 0 amu outside nucleus
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Atomic Mass Unit (amu)

• Small mass s are not easy to work with, so the
  atomic mass unit (amu) was developed
• One atomic mass unit (amu) is equal to 1/12 the
  mass of a carbon-12 atom
• The mass of 1 amu is nearly equal to the mass of
  one proton or neutron

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Reading the Periodic Table
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Atomic Number

• The number of protons determine the element
  (ALWAYS!!!)
• Number of protons are unique to each element
• Examples
• Carbon (C) has 6 protons
• Atomic number is 6
• Copper has 29 protons
• Atomic number is 29

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Atomic Number

• In uncharged atoms, atomic number is also the
  number of electrons
• Why?
• If an atom is charged, then it is an ion
• Uncharged atom
• Atomic number of protons of electrons

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Mass Number

• To find the Mass number
• protons neutrons mass number
• To find neutrons
• mass number proton (or atomic number)
  neutrons
• Mass numbers are always WHOLE s!!

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Symbols for Atoms

• X symbol of element
• A mass number
• Z number of protons
• X or X
• also can be written element - A
• i.e. carbon - 12

A
A Z
19
Mg-25 Zn atomic proton neut
ron electron mass
70 30
12
30
12
30
13
30
12
30
25
70
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Isotopes and Mass Number

• C C
• carbon-12 carbon-13
• Isotopes are atoms with the same number of
  protons but different number of neutrons

12 6
13 6
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Isotopes and Mass Number

• Example
• 3 types of Potassium
• All 3 types contain 19 protons and __ electrons
• of Protons of Neutrons Mass Number
• 19 20
• 19 21
• 19 22

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39
40
41
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Whats the difference between mass number and
average atomic mass(weight)?

• C
• carbon-12
• But if you look on the periodic table, the number
  states 12.01
• Atomic Mass (atomic weight) a weighted average
  of the masses of all of the isotopes of that
  element. It is not the same as the mass number.

12 6
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Whats the difference between mass number and
average atomic mass?

• Mass number- specifically about one isotope
• Average atomic mass- includes the masses of all
  the different isotopes for that atom