Combining Atoms and Molecules

1
Combining Atoms and Molecules

• 5-1 How Atoms Form Compounds

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Vocabulary

• compound
• chemical formula
• molecule
• ionic bond
• chemical bond
• Lewis dot diagram
• valence
• covalent bond
• organic compound

3
What is a Compound?

• A compound is a pure substance that contains two
  or more elements.
• Compounds are chemical combinations of elements
  with properties that are different from the
  elements that formed them.
• Table salt is formed when the elements sodium and
  chlorine combine (NaCl) sodium is a soft metal,
  chlorine is a greenish yellow gas.

4
What is a Compound?

• A chemical formula is an ingredient list for a
  compound that uses atomic symbols and subscripts.
• The chemical formula for sucrose (sugar) is
  C12H22O11
• A neutral particle that forms as a result of
  electron sharing is a molecule.

5
Ionic Bonds and Ionic Compounds

• The force that holds atoms together in a
  compound is called a chemical bond.
• An ionic bond is an electrical attraction between
  positively and negatively charged ions in an
  ionic compound.
• An ionic compound is one in which two or more
  elements or compounds gain or lose electrons and
  form ionic bonds.

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Ionic Bonds and Ionic Compounds (contd)

• The positive ion is usually a metal (like
  lithium)
• The negative ion is usually a non-metal (like
  fluorine)

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Ionic Bonds and Ionic Compounds (contd)

• Lithium loses its outer electron and becomes a
  positively charged ion.
• Fluorine gains the electron from Lithium and
  becomes negatively charged.

8
Ionic Bonds and Ionic Compounds (contd)

• Because the Lithium and Fluoride ions now have an
  equally opposite charge (1 and 1-) they are
  attracted to each other and form an ionic
  compound, Lithium fluoride (LiF)
• Lithium fluoride is the simplest type of compound
  made only of two elements and known as a binary
  compound.

9
Ionic Bonds and Ionic Compounds (contd)

• Properties of Ionic Compounds include
• Usually solids at room temperature
• Brittle and break apart easily
• High melting and boiling points
• Many dissolve in water becoming good conductors
  of electricity.

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Ionic Bonds and Ionic Compounds (contd)

• Lewis dot diagrams are one method for using
  atomic symbols and dots representing the valence
  electrons to help predict how compounds will
  form.
• The number of electrons in an atoms outermost
  energy level are its valence electrons.

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Ionic Bonds and Ionic Compounds (contd)

• Chlorine has 7 valence electrons and needs to
  gain one more to become more stable. It will
  then have a 1- charge.
• Argon is stable with a full valence shell and
  will neither gain or lose electrons.

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Concepts in Motion- Electron Transfer and Sharing
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Covalent Bonds- Sharing Electrons

• A covalent bond is a chemical bond formed when
  atoms share electrons.
• All organic compounds are covalent compounds
  based on carbon atoms.
• Covalent compounds (molecular compounds) can be
  solids, liquids, or gases at room temperature.

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Covalent Bonds- Sharing Electrons (contd)

• Atoms that have unpaired electrons can form
  compounds if they share electrons, as it requires
  too much energy to gain or lose a large number of
  electrons.
• Carbon has four unpaired electrons, and can
  therefore form four covalent bonds.

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Concepts in Motion Electron Sharing
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Covalent Bonds- Sharing Electrons (contd)

• Properties of Covalent Compounds include
• Can be solids, liquids, or gases at room
  temperature
• Usually have lower melting and boiling points
  than ionic compounds
• Do not usually separate in water
• Most do not conduct electricity

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Covalent Bonds- Sharing Electrons (contd)

• Covalent bonds can consist of single bonds (one
  pair of electrons share), double bonds (two pairs
  of electrons shared- stronger than single bonds),
  and triple bonds (three pairs of electrons
  shared- stronger than double bonds).

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5-1 Lesson Review Questions

• Bromine is in Group 17. How many electrons does
  bromine need to gain or lose to obtain a noble
  gas structure?
• A gain one electron
• B gain two electrons
• C lose one electron
• D lose two electrons

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5-1 Lesson Review Questions

• Which element can form a negative ion?
• A carbon
• B magnesium
• C chlorine
• D lithium

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5-1 Lesson Review Questions

• What holds two elements together in an ionic
  bond?
• A covalent bonds from shared electrons
• B electron clouds combining
• C unpaired electrons attracting each other
• D opposite charges on negative and positive ions

21
Homework

• Answer the three multiple choice lesson review
  questions.
• Write a Step-Up-To-Writing style paragraph
  regarding the following Suppose we were limited
  to the 100 elements. How would the world be
  different if atoms did not combine to form
  molecules of different compounds?