Chapter 5 Section 1 Key concepts: how is the reactivity of elements related to valence electrons in atoms? What does the periodic table tell you about atoms and the properties of elements? Key terms: valence electrons, electron dot diagrams, chemical bonds

Valence Electrons and Bonding Valence electrons – electrons that have the highest energy level and are held most loosely (meaning they will be given up soonest in a chemical bond) The number of valence electrons in an atom of an element determines many properties, including the ways in which the atom can bond with other atoms (as in what other elements it will bond with and in what configuration) Each element has a specific number of valence electrons – between 1 and 8.

Whoa wait, what? “I thought electron number was equal to proton number which is equal to atomic number.” Yessss you are right. The term “valence electrons” refers to the electrons that are closest to the outer edge of that electron cloud, so they are the electrons that elements tend to share or give up when near other elements.

Valence Electrons Atoms are most stable when they have 8 valence electrons. For example, Neon, Argon, Krypton, and Xenon (Noble gases) have 8 valence electrons and are less likely to react. Some small atoms, like Helium, are stable with just 2 valence electrons.

Electron dot diagrams – illustrate the valence electrons by showing them as dots around the chemical symbol. Each dot is one valence electron

Chemical Bonds Atoms usually react to make themselves more stable. The atom can bond with another atom’s valence electrons or give them up A chemical bond is the force of attraction that holds two atoms together Electrons may be given up or shared

The periodic table As atomic number increases, electron number increases The elements in each group have similar properties – including the same number of valence electrons

For example Group 1 elements have 1 valence electron, group 2 have 2. Elements in group 13 have 3, group 14 have 4, and so on. What about 3-12?

Other Metals Groups 3 through 12 – most have 1, 2, or 3 valence electrons. How reactive a metal is depends on how much it wants to give up an electron. Among groups 1 and 2, reactivity decreases from top to bottom. Reactivity decreases from left to right (as you remember from last chapter)

Noble gases Group 18 are the noble gases. Atoms of these elements have 8 valence electrons, except for He, which has 2. Atoms with 8 are unlikely to react because they don’t need more electrons.

Reactive nonmetals and metals The elements in group 17, the halogens, have atoms with 7 valence electrons. A gain of just one more electron gives these atoms the stable number of 8, as a result, the halogens react easily with other elements whose atoms can give up or share electrons.

Alkali Metals Alkali metals - group 1 – have only 1 valence electron. If they give up the electron, they will go down to their next set of electrons, which is a stable 8 (except for Lithium, which goes down to 2, and it is stable at 2). They would LIKE 8 electrons, which is why alkali metals are so reactive and always give up an electron. Ridding themselves of that electron makes them more stable.

Other nonmetals All of the nonmetals have four or more valence electrons. They are stable when they gain electrons.

Metalloids From 3 to 6 valence electrons. They can either lose or share electrons when they combine with other elements.

Hydrogen Has only 1 valence electron, considered a nonmetal.

http://www.youtube.com/watch?v=C_aK8zTB55g http://www.youtube.com/watch?v=r-6Muq8jEAM