SNC 1D1 – Putting Atoms Together (Synthesis) Molecules most substances are not made up of individual atoms. they are made up of molecules. a molecule is a group of atoms that are chemically joined together (oxygen, nitrogen, carbon dioxide and water) oxygen molecules make up 21% of the air. each oxygen molecule is made up of two oxygen (O 2 ). the 2 in the oxygen molecule is called the subscript, the 2 tells you there are two atoms of oxygen in each molecule of oxygen gas.

Oxygen gas can also be found as O 3 (ozone). Ozone is toxic and highly reactive. Ozone provides protection from the ultraviolet radiation from space. At ground level ozone is a component of smog. Molecular Elements There are seven elements that form molecules consisting of two atoms: H 2, N 2, O 2, F 2, Cl 2, I 2, Br 2 A molecule that contains two atoms is called diatomic. The prefix “di-” means two.

Compounds Most pure substances contain more than one element. Water is made up of two different elements hydrogen and oxygen. A pure substance containing two or more elements is called a compound. The chemical formula for water is H 2 O Water has 2 hydrogens and 1 oxygen. Other examples of compounds include; CO 2, CH 4, and NO.

Interpreting Formulas: The symbols in the formula tell you the elements in the substance. If there is only one symbol, the substance is an element. If there is more than one symbol the substance is a compound. The subscripts tell you how many atoms of each element are in each molecule of the substance. Ca 3 (PO 4 ) 2 Ca – P - O -

Ionic Compounds Some compounds are not composed of neutral atoms. These compounds are made up of charged particles called ions. An ion forms when an atom gains or loses one or more electrons without changing its number of protons. When atoms gain electrons they become negative, we call these anions. When atoms lose electrons they become positive, we call these cations.

When sodium atoms for example form ions they usually lose 1 electron. This results in a sodium atom with 11 positive charges (protons) and only 10 negative charges (electrons). Since it has more positive charge than negative charge the sodium has an ionic change of +1. Chlorine gains an electron to become an ion and has an ionic charge of -1. By losing the one electron to chlorine both sodium and chlorine become stable. Sodium (Na + )Chloride (Cl - ) Positive charge Negative charge Ionic charge+1- 1

Homework: Page 261 #