Atomic Number and Atomic Mass

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Atomic NumberandAtomic Mass

• Mr. Perez
• Grade 6 Science

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What Is the Atomic Number?

• The Atomic Number is the number of protons in the
  nucleus
• The atomic number is found above the elements
  chemical symbol
• The elements in the Periodic Table are listed in
  increasing order by their atomic numbers
  (starting with Hydrogen, which has an atomic
  number of one)
• Elements are different because of the their
  different atomic numbers. Each atomic number is
  different.
• Aluminum has an atomic number of 13. This means
  that aluminum has 13 protons in its nucleus.
• Lead has a larger atomic number than hydrogen and
  aluminum. Lead has an atomic number of 82. This
  means that lead has 82 protons in its nucleus.

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The Atomic Number - Finding the Number of
Electrons

• We just discussed that the atomic number is the
  number of protons in the nucleus
• We also know that an atom must be neutral,
  meaning that the number of protons ( charge)
  must equal the number of electrons (- charge)
• To find the number of electrons in an atom,
  simply use the atomic number
• Why? The number of electrons and protons must be
  the same to cancel each other to become neutral
• Calcium has an atomic number of 20. This means
  it has 20 protons. It also means it has 20
  electrons.

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The Atomic Mass

• The atomic mass is the total mass of protons and
  neutrons in an atom
• The atomic mass can be found below the elements
  symbol. If the atomic mass has a decimal, you
  should round it (up or down)
• Scientists cannot measure the mass of an atom in
  grams because it is too big (Think of measuring
  your agenda book in miles - the unit is TOO BIG!)
• Scientists measure the mass of an atom in a
  special unit called the atomic mass unit (amu)
• The mass of one proton is 1 amu, and the mass of
  one neutron is also 1 amu (Electrons have an amu
  less than one)
• What is the atomic mass, in amu, of an atom with
  one proton and two neutrons?
• 3 amu (one proton two neutrons 3 amu)

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The Atomic Mass (or Mass Number

• The atomic mass (or mass number) is the number of
  protons AND neutrons in the nucleus of an atom
• How do you find the number of neutrons in an
  atom?
• Locate the atomic number of the element on the
  Periodic Table. This will tell you the number of
  protons in the nucleus.
• Locate the atomic mass (or mass number) of the
  element on the Periodic Table. This will tell
  you the number of protons and neutrons in the
  atom.
• Subtract the atomic number from the atomic mass.
  This will tell you the number of neutrons.

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The Atomic Mass -Finding the Number of Neutrons

• How can you find the number of neutrons in the
  element, Lithium?
• Find the atomic number. Lithium has an atomic
  number of 3 (so it has 3 protons)
• Find the atomic mass (or mass number). Lithium
  has an atomic mass of 7 (so it has 7 protons AND
  neutrons)
• Subtract the atomic number from the atomic mass.
• 7 - 3 4
• Answer Lithium has 4 neutrons
• 7 protons and neutrons - 3 protons 4 neutrons

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Composition of Atoms

• What is the atomic number of this element?
• The atomic number is 5. Why? There are 5
  electrons (-) and 5 protons ().
• What is the atomic mass?
• The atomic mass is 10. There are 5 protons and 5
  neutrons in the nucleus.
• What is the element shown in this model?
• An isotope of Boron

• Protons have a positive charge
• Neutrons have a no charge
• Electrons have a negative charge

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Structure of the Atom

• The atoms of an element interact with other atoms
  so they can fill their outermost energy level
  with eight electrons
• Hydrogen has only 1 electron in the outer energy
  level
• It must get one more electron to fill the first
  energy level
• It can do this by
• Acquiring more electrons from another atom
• Losing electrons to another atom
• Sharing electrons with another atom

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Water Hydrogen and Oxygen

• In this example, the two Hydrogen atoms are using
  one of their own electrons in the first energy
  level AND one of the electrons from the second
  energy level of oxygen in fill the first energy
  level with two electrons.

• The Oxygen atom is using six of its own electrons
  and one electron from each of the Hydrogen atoms
  to fill its second energy level with eight
  electrons

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Electrons

• The inner energy level of electrons can only hold
  two electrons
• The second energy level can hold eight electrons
• Sodium has eleven electrons, so what do you think
  its arrangement of electrons looks like?
• It needs a third energy level (2 electrons in the
  first energy level, 8 electrons in the second
  energy level, and 1 electron in the third energy)
• The third energy level can be full when an atom
  has eight electrons (but some elements can have
  up to 18 electrons in the third energy level)
• The first 18 elements follow the 2-8-8 pattern

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