How Atoms Combine

Compounds Two or more elements joined in definite proportion Atoms join to become stable Created by bonds between elements Bonds represent energy-electrical forces Symbolized by a formula Example: Water, H2O (Two hydrogen atoms and one oxygen atom)

Chemically Stable Atoms Electrons in outer most energy level are called “Valence Electrons” Certain electron arrangements are more stable than others Stable if outer energy level is filled If unfilled then the element will form a chemical bond Bond by gaining, losing, or sharing electrons Chemical Bonding http://www.visionlearning.com/library/module_viewer.php?mid=55

Valence Electrons Lewis Dot Diagrams Examples of Noble Gases Show filled outer energy levels Sodium & Chlorine fill the outer energy levels by transferring electrons.

Bonding Capacity Also called valence Indicate how many electrons an atom may gain, lose, or share when bonding Found in Periodic Table or in charts Used to write formulas for binary compounds

Ionic Bonds Electrons are transferred between atoms One atom gains while the other loses Atoms are no longer neutral and have a charge; + or - Ion—element with a charge Ionic bond—force of attraction between opposite charges

Ionic Bonds Cont’d For example Na can donate 1 electron to Cl Makes Na+1 and Cl-1 Held together by strong electrostatic attractions Ionic solids like salt are soluble in water

Covalent Bonds Electrons are shared between atoms Occurs in atoms that do not easily lose electrons Positive nucleus of each atom has an equal attraction to shared electrons This attraction holds the atoms together Bonding forms molecules

Example of Covalent Bonding Methane makes four covalent bonds. Each diagram represents the same molecule—CH4

Covalent Bonds Cont’d Can occur between atoms of same element Seven diatomic molecules—two atoms Found in nature as two atoms covalently bonded Diatomic Molecules Hydrogen—H2 Nitrogen—N2 Oxygen—O2 Fluorine—F2 Chlorine—Cl2 Bromine—Br2 Iodine—I2

Naming Binary Compounds Write name of first element (more left on periodic table) Write root name of second element Add –ide ending to the root MgCl2 Magnesium Chlor- -ide