Focus on the WHITE words 22.1 Notes Part B Focus on the WHITE words

Objectives Describe how the properties of compounds differ from those of their constituent atoms. Explain what a chemical formula represents. Discuss why chemical bonding occurs.

Compounds When elements chemically combine Often have chemical and physical properties that aren’t anything like those of the individual elements

Example NaCl = Table Salt How are Na and Cl different… Na = shiny, soft metal that reacts violently with water Cl = poisonous greenish yellow gas How are Na and Cl different… Physically? Chemically? http://www.youtube.com/watch?v=9bAhCHedVB4

Chemical Formula Tells what elements a compound contains and the exact number of the atoms of each element in a unit of that compound Shows the kinds and numbers of atoms in the smallest representative unit If only one atom of an element, you only write the symbol If more than one atom, a number is used as a subscript

Atomic Stability Atoms form compounds because electric forces between electrons and protons attract and hold atoms and molecules together Atoms/Elements are stable when they have a full valence shell Gain, lose, or share electrons to fill valence shell

Noble gases Unusually stable 8 outer electrons Don’t readily form compounds or bonds with other elements

Electrons and Energy Levels Period = how many energy levels Group = how many valence electrons

How do elements fill outer energy levels? They do this by combining with other atoms that also have partially complete outer energy levels As a result, each achieves stability. Group 1 and 17 (7A) atoms combine easily. Why?

This illustration shows electron dot diagrams for sodium and chlorine. When they combine, sodium loses one electron and chlorine gains one electron. Na had only one electron in its outer energy level, which is lost to Cl as they combine to form sodium chloride. Na drops down to a new outer energy level, and it is stable with eight electrons.

Chemical bond Attraction between atoms as they gain, lose, or share electrons, pulling them together to form a compound. A chemical bond is the force that holds atoms together in a compound

In-Class Assignment/Homework 22.1 Reinforcement WKT