Physical Science Notes

Physical Science Notes Chemical Bonds

1. What is a compound?

a. Substances that are made up of the combined atoms of two or more elements

b. Most matter you see is in the form of compounds Example: Do you normally see Na or Mg on the ground?

2. Why do elements combine to form compounds?

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)

3. Are all atoms unstable in elemental form?

a. No: Group 18 (Noble Gases) rarely ever react.

- Why: Because Noble Gases have a complete set of electrons in their valence shell/orbit

4. What does it mean to be stable?

a. To be stable the valance shell must have a complete set of electrons

-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

- 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.

5. How is stability gained?

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?

a. Is the abbreviation for Sodium Chloride b. This abbreviation is better known as a chemical formula

7. How do you write chemical formulas and what exactly do they mean?

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

b. How do you write them? Example: A compound with one calcium atom and two fluorine atoms - CaF2

A compound with two aluminum atoms and three sulfur atoms - Al2S3