<p> Physical Science Notes Chemical Bonds</p><p>1. What is a compound?</p><p> a. Substances that are made up of the combined atoms of two or more elements</p><p> b. Most matter you see is in the form of compounds Example: Do you normally see Na or Mg on the ground?</p><p>2. Why do elements combine to form compounds?</p><p> a. When atoms are in elemental they are very unstable and react quite easily Example: Na and Cl in elemental form are very dangerous; however when Na and Cl combine they form a stable compound NaCl (table salt)</p><p>3. Are all atoms unstable in elemental form?</p><p> a. No: Group 18 (Noble Gases) rarely ever react.</p><p>- Why: Because Noble Gases have a complete set of electrons in their valence shell/orbit</p><p>4. What does it mean to be stable?</p><p> a. To be stable the valance shell must have a complete set of electrons</p><p>-Example: Argon: Argon being in period 3 has three energy levels. The first energy level can only have 2 electrons. Energy level 2 and 3 can each have 8 electrons. The reactivity of an element is due to its valence elctrons. Since Argon has a complete set it is considered chemically stable</p><p>- Question: Helium is in Group 18 but does not have 8 valence electrons it has 2. How is it stable. Remember the first energy level can only have 2 electrons. Helium has 2 so it has a complete set.</p><p>5. How is stability gained?</p><p> a. Through chemical bonds - In chemical bonding an atom either gains or gives up or shares an electron to get enough electrons to fill the valence shell. 6. What is NaCl?</p><p> a. Is the abbreviation for Sodium Chloride b. This abbreviation is better known as a chemical formula</p><p>7. How do you write chemical formulas and what exactly do they mean?</p><p> a. Gives you the ratio/number of each element that makes up a compound. Example: NaCl = 1 Na to 1 Cl Total of 2 atoms H2O = 2 H to 1 O Total of 3 atoms CO2 = 1 C to 2 O Total of 3 atoms Mg(OH)2 = 1 Mg to 2 OH Total of 5 atoms (NH4)3 = 3 N to 12 H Total of 15 atoms</p><p> b. How do you write them? Example: A compound with one calcium atom and two fluorine atoms - CaF2</p><p>A compound with two aluminum atoms and three sulfur atoms - Al2S3</p>