Unit 3 Why Atoms Combine

Element = a combination of identical atoms to form a substance we see. Mixture = a combination of two or more elements that can be easily separated. Compound = a combination of two or more elements chemically combined.

Most of the matter around you is in the form of compounds or mixtures of compounds.

Compound Are difficult to separate into their base elements. * water = hydrogen and oxygen * the air we exhale = carbon and oxygen * sugar = carbon, hydrogen and oxygen

combination of the metal; sodium * Common Name = Table Salt Compounds have properties unlike those of the elements that they are made of. * Sodium Chloride = combination of the metal; sodium and the non-metal; chlorine * Common Name = Table Salt

Chemical Formula Tells what elements the compound contains and the ratio of those atoms.

The symbols for the elements that make up a compound are written together to make up the chemical formula. * salt = NaCℓ * battery acid = H2SO4 * water = H2O

Capital letters indicate a new element in a chemical formula. Lower Case letters go with the capital letter in front of them. * to know how many elements are in a compound, just count the capital letters. 1 2 1 2 3 1 2 3 NaCℓ AgNO3 C6H12O6 1 2 1 2 3 4 CO2 NH4C2H3O2

How can you figure out what elements are in a compound? Match up chemical symbols on the Periodic Table. NaCℓ AgNO3 C6H12O6 CO2 NH4C2H3O2

Subscript Means “written below”. * tells how many atoms of that element are in that compound (ratio)

How can you figure out the ratio of atoms in a compound? Look at the subscripts. NaCℓ AgNO3 C6H12O6 1 : 1 1 : 1 : : : CO2 NH4C2H3O2 1 : 1 : : : :

How can you determine the number of atoms in a compound? By adding up all the subscripts. NaCℓ AgNO3 C6H12O6 1 + 1 1 + 1 + + + CO2 NH4C2H3O2 1 + 1 + + + +

Why do elements combine? To become chemically stable. Some elements do not combine (Family 18) because they are already stable.

Chemically Stable The outer energy level is completely filled with electrons. * for most elements that number is 8 * for Helium it is 2

Chemical Bond The force that holds a compound together. * atoms join together to fill their outer energy level with electrons and become chemically stable like the Noble Gases.

3 Ways electrons can help fill outer energy levels. Losing Gaining Sharing

When losing, gaining or sharing electrons occur; ATTRACTIONS form between atoms and compounds are created.

It is the movement of electrons that allows all chemical reactions to occur.

* 4 Dots = could lose, gain or share Using an elements Dot Diagram, you can tell if an atom will gain or lose electrons when it reacts. * 1, 2, or 3 Dots = LOSE * 5, 6, or 7 Dots = GAIN * 4 Dots = could lose, gain or share

The End