Life Science

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Life Science

• Ch 4.2
• Chemical Properties are based on the structure of
  atoms

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What are atoms?

• Gets its name from Greek word atomos
• Meaning invisible
• It is the smallest particle of an element
• Example atom of carbon or oxygen or gold

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Atoms of all elements are made of..
SOURCEThomas Jefferson National Accelerator
Facility - Office of Science Education
http//education.jlab.org/faq/index.html
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Even smaller particles
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What is the nucleus?

• The central core
• Protons and neutrons packed tightly together

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http//education.jlab.org/atom tour/fact1.html
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What about electrons?

• Electrons have much less mass than neutrons and
  protons
• The move continuously about the outside of the
  nucleus
• The attraction between the negatively charged
  electrons and the positively charged protons
  keeps the electrons close to the nucleus

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Figure 4.4 from text

• Helium has 2 protons and 2 neutrons
• This model represents the moving electrons as a
  spherical cloud
• In a real atom, this cloud is much larger than
  the nucleus
• See page 75

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What is Atomic Number?

• The difference among elements is the number of
  protons in their atoms
• All atoms of a particular element have the same
  atomic number (number of protons)
• The number of protons determine the atoms
  properties and behavior

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IMPORTANT

• Individual atoms do not have melting or boiling
  points, density, color, or any other observable
  properties
• Individual atoms do not look like the element
  that is made from these atoms.
• Because of their unique atomic number (number of
  protons), the atoms shows special characteristics
  associated with that number of proton.

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What are isotopes?

• Isotopes of an element have the same number of
  protons in their atoms but have different
  numbers of neutrons
• Some isotopes are stable (do not change over
  time)
• Some isotopes are unstable and break down, giving
  off radiation radioactive isotopes

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Atoms of three isotopes of carbon (figure 4.5
text)
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Notice the number of electrons is always 6
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Electrons and Reactivity

• Electrons differ in the amount of energy they
  have and how tightly they are held by the protons
  of the nucleus
• Chemists describe this as energy levels

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Energy Levels

• First or lowest energy level (nearest the
  nucleus) can hold 2 electrons
• Second energy level can hold 8

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Reactivity

• For example
• hydrogen atom has 1 electron
• Since the electrons fill the lowest energy level
  first, hydrogens electron occupies the first
  energy level

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Energy Levels continued

• Carbon has 2 electrons in its lowest energy level
  and 4 more in the second level
• Nitrogen has 2 electrons in its lower energy
  level and 7 more in the second level
• Because more electrons are needed to fill the
  second level, this conditions makes these atoms
  chemically reactive

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Energy levels

• Atoms such as helium and argon have no
  partly-filled energy levels
• They are said to be inert- do not react

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www.colorado.edu/physics/2000/applets/a2.html
Periodic Table
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Homework

• Page 77
• Questions 1 - 4