Chapter 5 Atoms and Bonding

Valence Electrons Are those electrons that have the highest energy level and are held most loosely. These electrons are the electrons involved in chemical reactions. The number of valence electrons in an atom of an element determines many properties of that element.

Valence Electrons Each element has a specific number of valence electrons ranging from 1 to 8. Electron Dot Diagram- is a representation of the valence electrons in an atom, using dots. Every element wants to fill its outer energy level. Thus, most elements want 8 valence electron.

Electron Dot Diagram Retrieved 1/31/09 from www.green-planet-solar-energy.com/alkane.html

Chemical Reaction Elements with 8 valence electron are less reactive/more stable then elements that want to gain valence electrons. Reactions happen in one of two ways: An element gains electrons to make itself more stable. An element gives up electrons to make itself more stable.

Chemical Bonds A chemical bond is the force of attraction that holds two atoms together as a result of the arrangement of electrons between them. During atom bonding, electrons may be transferred or shared between the atoms. The bonding causes chemical reactions resulting in the form of new substances.

Reading the Periodic Table Periods Valence electrons increase from left to right on the periodic table. Valence electrons of an atom in the next period are of a higher energy level. Columns Valence electron count is the same Elements have similar properties (except elements in groups 3-14 follow a slightly different pattern)

Reading the Periodic Table Noble Gases All have 8 valence electrons Exception is helium with 2 Very stable Do not react easily Halogens Group 17 7 electrons Very reactive with atoms that can give up or share electrons

Reading the Periodic Table Alkali Metals Group 1 Have 1 valence electron Very reactive Want to lose their one electron

Reading the Periodic Table Other metals Groups 2 through 12 Most have 1,2, or 3 valence electrons React by losing electrons (especially with halogens) How reactive a metal is depends on how easily it loses electrons. Usually reactivity of metals decrease from left to right on the periodic table. In group 1 and 2, reactivity increases from top to bottom.

Reading the Periodic Table Nonmetals Most are gases at room temperature. 5 of the nonmetals are solids and one is a liquid. They become stable by gaining or sharing electrons (except for group 18) Nonmetals usually combined with metals by gaining electrons Nonmetals usually combined with other nonmetals by sharing electrons.

Reading the Periodic Table Metalloids Lie on a zigzag line between the metals and nonmetals. 3 to 6 valence electrons Can either lose or share electrons Hydrogen Above group 1 1 valence electron Nonmetals Reactive Quiet different from alkali metals

Ions An ion is an atom or groups of atoms that has an electric charge. When an atom loses an electron, it loses a negative charge and becomes a positive ion. When an atom gains an electron it gains a negative charge and becomes a negative ion.

Polyatomic ions Polyatomic ions are ions that are made of more than one atom They act as a single unit They have overall positive or negative charges.

Ionic Bonds An ionic bond is the attraction between two oppositely charged ions. They form as a result of the attraction between positive and negative ions. They usually form when a metal combines with a nonmetal A compound consisting of positive and negative ions is called ionic compound.

Chemical Formulas Chemical formulas- combinations of symbols that shows the ratio of elements. When ionic compounds form, the ions come together in a way that balances out the charges on the ions. The chemical formula for the compound reflects this balance.

Subscripts Subscripts tells you the ratio of elements in a compound. No subscript is an understood 1 Polyatomic ions are written similarly

Naming Ionic Compounds For an ionic compound The name of the positive ion comes first followed by the negative ion The positive ion is usually a metal Exceptions-some positive polyatomic ions exist If negative item is a single element, the ending of its name changes to –ide If the negative item is a polyatomic ion, its name usually ends in –ate or –ite.

Properties of Ionic Compounds Characteristics of ionic compounds Hard solids Brittle crystals High melting points Conduct electricity when dissolved in water

Ionic Crystals Ions that form and orderly, three- dimensional arrangement are called crystals. Every ion is attracted to ions of opposite charges that surround it. The crystals are hard and brittle because of the attraction among all ions.

High Melting Point Ionic bonds are very strong so it takes a lot of energy to break the bonds. Thus, ionic bonds have high melting points.

Electrical Conductivity When dissolved in water, ions are free to move around and are conductive. Solid ionic compounds are not conductive because ions cannot move so a current can not occur.

Covalent Bonds The chemical bond formed when two atoms share electrons. These types of bonds usually form between atoms of nonmetals.

Electron Sharing The force that holds atoms together in a covalent bond is the attraction of each atom’s nucleus for the shared pair of electrons. A molecule is a neutral group of atoms joined by covalent bonds.

How Many Bonds? The amount of total covalent bonds that a nonmetal can form is equal to the amount of valence electrons an atom needs to fill its highest energy level.

Double and Triple Bonding A double bond is a chemical bond formed when atoms share two pairs of electrons. A triple bond is a chemical bond formed when atoms share three pairs of electrons.

Molecular Compounds A compound that is composed of molecules Characteristics- Lower melting points Lower boiling points Do not conduct electricity when dissolved in water Have a weaker bond

Unequal Sharing of Electrons Atoms of some elements pull more strongly on shared electrons than do atoms of other elements. As a result, the electrons are pulled more toward one atom, causing the bonded atoms to have slight electrical charges.

Polar and Nonpolar Bonds The unequal sharing of electrons can cause one atom in a bond to be positive and the other to be negative. Polar bond-a covalent bond in which electrons are shared unequally Nonpolar bond- a covalent bond in which electrons are shared equally.

Polar Bonds in Molecules Molecules with nonpolar bonds are nonpolar. Molecules with polar bonds can be polar or nonpolar. A nonpolar molecule with polar bonds only occurs if the molecule’s atoms pull with the same amount of force in opposite directions.

Attraction Between Molecules The properties of polar and nonpolar compounds differ because of differences in attractions between their molecules. Polar molecules are strongly attracted to polar molecules. This is why oil (nonpolar) and water (polar) do not mix.

Metals and Alloy Common characteristics of metals Most are solids Can be hammered flat or drawn into a wire (malleability). Conducts electricity well A lot of every day metals are a form of an alloy. An alloy is a mixture made of two or more elements that has the properties of a metal. One element is always a metal. Alloys are generally stronger and less likely to react with air or water than the pure metals from which they are made.

Physical and Chemical Properties Physical Properties Metal alloys are used in many cases over the original metal because the alloys are a stronger than the original metal but still withholds many of the original elements properties. Chemical Properties Many times strong metals are alloyed with other elements so that the object will not react with water (rust).

Metallic Bonding Most metals have 1,2, or 3 valence electrons. These valence electrons are usually lost when bonding with other elements living the metal with a positive charge. Metals exist in crystals which the valence electrons are free to move between the positively charged ions. Metallic bonds- an attraction between a positive metal ion and the electrons surrounding it. A metal or metal alloy consists of positively charged metal ions embedded in a “sea of valence electrons.

Metallic Properties Metallic properties Easily change shape (malleability) Easily conduct electric current Luster Easily conduct heat The “sea of electrons” model of solid metals explains these properties.

Important Links Watch “Physical Science Series: Chemical Bonding” video on ionic bonds. http://player.discoveryeducation.com/inde x.cfm?guidAssetId=547c7d47-0d64-4969- 8eff-ac94d18f1061

Credits Frank, D. V., Jones, G. T., & Little, J. G. (2006). Science Explorer. Boston: Pearson Prentice Hall. Learning, U. (Director). (1998). Physical Science Series: Chemical Bonds [Motion Picture]. The Alkane Series: Building Them From Scratch (2009). Retrieved 1/31/09 from www.green-planet-solar- energy.com/alkane.html