How do atoms differ?

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How do atoms differ?

• Section 4.3

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REVIEW THE ATOM
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REVIEW
Particle Location Charge Relative Mass (amu) Actual Mass (g)
Electron In the space surrounding nucleus 1- 0 (1/1840) 9.11 x 10-28
Proton Nucleus 1 1 1.673 x 10-24
Neutron Nucleus 0 1 1.675 x 10-24
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• If all atoms are composed of protons, neutrons
  and electrons, how do elements differ?
• What makes an atom of oxygen an oxygen atom?

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ATOMIC NUMBER (Z)

• An elements atomic number (Z) represents the
  number of protons in its nucleus
• The number of protons in an atoms nucleus
  determines the identity of the atom
• If an atom has 9 protons its fluorine, if it has
  20 its calcium
• In a neutral atom
• number of protons number of electrons, so
• atomic number protons electrons

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THE MODERN PERIODIC TABLE

• Elements are arranged according to atomic number

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• An elements atomic number determines its
  position on the periodic table
• The periodic table is organized left-to-right,
  top-to-bottom by increasing atomic number

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TAKE A LOOK AT A PERIODIC TABLE

• Complete the following table

Element Number of protons Number of electrons
sodium (Na)
Copper (Cu)
Boron (B)
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MASS NUMBER (A)

• An atoms mass number (A) represents the total
  number of neutrons and protons in the nucleus of
  an atom
• Mass number protons neutrons
• or
• Mass number atomic number neutrons
• How do you determine the number of neutrons in an
  atom?
• neutrons mass number (A) - atomic number (Z)

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• It is always true that
• A carbon atom has 6 protons in its nucleus
• A neutral carbon atom has 6 electrons
• A carbon atom also contains neutrons, but unlike
  electrons and protons the number of neutrons can
  change from carbon atom to carbon atom
• Some carbon atoms have 6 neutrons, others have 7
  neutrons and still others have 8 neutrons

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ISOTOPES

• Daltons atomic theory states that all atoms of a
  given element are identical. This is mostly true
• Atoms of the same element can differ in the
  number of neutrons
• most elements have two or more isotopes
• Isotopes are atoms of the same element with
  different numbers of neutrons (and therefore
  different masses)
• atoms have the same atomic number but different
  mass numbers

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IDENTIFYING ISOTOPES

• To distinguish one isotope from another an atoms
  mass number is added after the elements name
• carbon-12 carbon-14 uranium-235
• Remember the mass number of an isotope represents
  the sum of the neutrons and protons in the nucleus

Potassium-39 Potassium-40 Potassium-41
protons 19 19 19
neutrons 20 21 22
electrons 19 19 19
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isotope protons electrons neutrons
Hydrogen1 (protium) 1 1 0
Hydrogen-2 (deuterium) 1 1 1
Hydrogen-3 (tritium) 1 1 2
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• WRITING SYMBOLS FOR ATOMS

A
X
Z
MASS NUMBER PROTONS NEUTRONS
SYMBOL
ATOMIC NUMBER PROTONS
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PRACTICE
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Fe
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26 protons
26 electrons
30 neutrons
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PRACTICE FILL IN THE BLANKS
symbol atomic number mass number number of protons number of electrons number of neutrons
Fe 56
60 144
102 45 45
59 31
Al 27
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used for nuclear reactors and atomic bombs
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TURN TO THE PERSON NEXT TO YOU

• You have an atom each of two isotopes of carbon
• What is the same about the two atoms?
• What is different about the two atoms?
• How can we tell the two atoms apart?

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IMPORTANT TO NOTE

• Isotopes with more neutrons have greater mass,
  but are chemically the same (under normal
  conditions)
• The number of protons determines the identity of
  the element and electrons are responsible for an
  elements chemical properties

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THE MODERN PERIODIC TABLE
Element name
Symbol one or two letters
Average atomic mass weighted average of
elements isotopes masses

• Elements are arranged according to atomic number

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ATOMIC MASS

• The mass of an atom depends on the number of
  electrons, protons and neutrons it contains
• because atoms are extremely small particles (even
  the smallest speck of dust can contain
  10,000,000,000,000,000 atoms) its impractical to
  measure the mass of atoms in grams

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Particle Actual Mass (g)
Electron 9.11 x 10-28
Proton 1.673 x 10-24
Neutron 1.675 x 10-24
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MEASURING ATOMIC MASS

• Because the actual masses of protons and neutrons
  are very small chemists have developed a way to
  measure atoms based on the Carbon-12 atom as the
  standard
• Instead of grams, the unit we use is the Atomic
  Mass Unit (amu)
• Carbon-12 is exactly 12 amu
• 1 amu is exactly 1/12 of the mass of a carbon-12
  atom
• Protons and neutrons have a mass slightly greater
  than 1 amu

Particle Mass (amu)
Electron 0.000549
Proton 1.007276
Neutron 1.008665
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AVERAGE ATOMIC MASS

• When one consults the periodic table, the atomic
  mass of carbon is not 12.00 amu but 12.011 amu
• This is because most elements have more than one
  naturally occurring isotope and the atomic mass
  on the periodic table is the weighted average of
  the mass of each of the elements isotopes

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AVERAGE ATOMIC MASS

• In nature elements exist as a mixture of their
  isotopes, for example chlorine
• 75 chlorine-35
• 25 chlorine-37
• Atomic mass of chlorine is 35.453 amu
• So the average atomic mass is calculated by
  taking the weighted average of the isotopes
  masses
• This is why an elements atomic mass is not a
  whole number

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AVERAGE ATOMIC MASS OF CARBON
Isotope Mass of isotope Abundance of isotope ()
carbon-12 12.00000 amu 98.90
carbon-13 13.00335 amu 1.10
mass x abundance
carbon-12 12.000 x 0.9890 11.868 amu
carbon-13 13.00335 x 0.0110 0.143 amu
12.011 amu
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PRACTICE CALCULATE ATOMIC MASS FOR ELEMENT X.
IDENTIFY THE ELEMENT

Isotope Mass (amu) abundance
X-6 6.015 7.5
X-7 7.016 92.5
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Isotope Mass (amu) abundance
6X 6.015 7.5
7X 7.016 92.5
1. Mass contribution (mass)( abundance) 6X
Mass contribution (6.015)(.075) 0.451 amu 7X
Mass contribution (7.016)(.925) 6.490 amu 2.
Sum of the mass contributions 0.451 amu 6.490
amu 6.941 amu Which element is this?