The periodic table includes four types of elements (5-1). • Noble Gases – The most stable atoms (column 18). • All have the maximum number of valence electrons • All have 8 valence e-, except Helium with 2. • All are very inactive and are so stable they do not form bonds with other atoms. • Metals – to the left of the “staircase” or “zigzag line.” • The most reactive group is Column 1, the alkali metals with one valence e-. • Nonmetals - to the right of the “staircase.” • The most reactive group is Column 17, the halogens with seven valence e-. • Metalloids – the eight elements next to the “staircase.” • Have characteristics of both metals and nonmetals.
All atoms bond to become more chemically stable. • The most stable atoms have eight valence electrons. • The number of valence electrons determines how one atom bonds to another. • An atom may have no more than 8 valence e-. • Valence e- are the farthest away from the nucleus.
A chemical bond is formed when two or more atoms give away, take, or share valence electrons • The Goals Are: • 8 e- in their outermost energy level • Become more stable • A lower amount of stored energy.
Electron Dot Notation • To solve bonding problems we use electron dot notation. • A diagram that shows the element abbreviation with the number of valence electrons around it in groups of two. • Examples:
Ionic Bonds (5-2) • When an atom gives or takes one or more e- to another atom. • Atoms will gain or lose e- so they can have 8 e- in their outermost energy level just like a noble gas does. • The physical properties of ionic compounds include crystal shape, high melting points, and electrical conductivity. • An “ion” is an atom or group of atoms that has become charged. • When an atom loses an electron, it loses a negative charge and becomes a positive ion. • Metals become positively charged, because they give away electrons. • When an atom gains an electron, it gains a negative charge and becomes a negative ion. • Nonmetals become negatively charged, because they gain electrons.
Covalent Bonds (5-3) • When two or more atoms of non-metals share 1 or more pairs of electrons. • e- travel close to the speed of light so both atoms act like they have 8 valance e- • Ex. Chlorine gas - Cl2 • Properties of molecular compounds: • low melting and boiling points, brittle, non-malleable and poor electrical conductivity.
Electron Dot Notation for Covalent Bonds • Draw valence electrons around element abbreviation in a circle. • This shows how many are available to bond • Venn diagrams help show how it works • Examples:
Double and Triple Bonds • Elements can share more than one pair of valence electrons • Sharing one pair is a single bond • Sharing two pairs is a double bond • Sharing three pairs is a triple bond (the max) • Examples:
Unequal Sharing • Sometimes valence electrons aren’t shared equally, so the molecule becomes polar, or electrically charged • Polar molecules are shared unequally (i.e. water) • Example: • Non-polar molecules are shared equally (i.e. chlorine gas) • Example:
Properties of Polar and Non-Polar Molecules • “Like Dissolves Like” • Polar molecules will dissolve polar molecules and ionic compounds (i.e. water will dissolve ammonia and salt (NaCl) • Example: • Non-polar molecules will dissolve non-polar molecules (nitrogen gas will dissolve oxygen, gasoline will dissolve styrofoam) • Example:
Describing Chemical Reactions(6-2) • Chemistry is the study of how elements bond • Chemical formulas aide in that study • Using Chemical Formulas • Reactant + Reactant = Product + Product • Example: • Conservation of Matter • Matter is neither created or destroyed • The mass of the reactants must equal the mass of the products • Example: • Balancing Equations • The number of atoms of an element must be the same on both sides of the equation • Examples:
Reaction Energy (6-3) • Exothermic reactions are chemical reactions that release energy. (“exo” like Exit -> energy goes out = feels hot) • Endothermic reactions are chemical reactions that take in energy. (“endo” means in -> energy goes in = feels cold) • Activation Energy: the energy needed to get a reaction started (the friction in getting a match to light / sunlight to get photosynthesis started). • Four things that affect the rate of a reaction: • Concentration • Surface area • Temperature • Catalyst- Enzymes