http://www.ck12.org Chapter 2. Matter
- The octet rule states that atoms react to form compounds that allow them to have eight valence electrons like
the noble gases, which are the least reactive elements. - Noble gases are used for balloons, light bulbs, and lighted signs.
Vocabulary
- noble gas: Nonmetal in group 18 of the periodic table that has eight valence electrons (or two in the case of
helium) and is unreactive.
Explore More
Watch the video about noble gases at the following URL, and then answer the questions below.
http://www.open.edu/openlearn/science-maths-technology/science/chemistry/noble-gases
- Compare and contrast the reactivity of the elements helium, neon, argon, krypton, and xenon.
- Compare the density of these noble gases to the density of air.
- Describe how the mass of noble gases changes from the top to the bottom of group 18.
Review
- What are noble gases?
- Explain why noble gases are almost completely nonreactive.
- What is the octet rule? How is it related to noble gases?
- Hydrogen (H) atoms have one electron and exist as diatomic (“two-atom”) molecules (H 2 ). Helium atoms
have two electrons and exist only as single helium atoms. Explain why hydrogen and helium differ in this
way.
Summary
This unit defines matter and describes its properties. It compares elements, compounds, and mixtures. It explains
how matter changes state and identifies special properties of gases. The unit also delves into atoms and elements,
including how elements are classified.