1. KEY CONCEPT All living things are based on atoms and their interactions.

2. Living things consist of atoms of different elements.
An atom is the smallest basic unit of matter.
An element is one type of atom.
Hydrogen atom (H) H O
Oxygen atom (O)

3. Where electrons are found Outermost energy levels
The outer electrons that bond to other atoms—magic number is 8 electrons to be STABLE!
An atom has different parts:
1. Electrons
2. Nucleus composed of:
3. Protons
4. Neutrons. .
Oxygen atom (O)
Nucleus: 8 protons (+) 8 neutrons
outermost energy level: 6 electrons (-)
inner energy level: 2 electrons (-)

4. A compound is made of atoms of different elements bonded together.
water (H2O)
carbon dioxide (CO2)

5. Ions form when atoms gain or lose electrons to make stable .
Ionic bonds form between oppositely charged ions.
An ion can be:
positive ions: Na+ (sodium) or K+ (potassium), Ca+ (calcium)
negative ions: Cl- (chlorine)
Sodium atom (Na)
Chlorine atom (CI)
Sodium ion (Na+)
Chloride ion (CI-)
Na loses an electron to CI
ionic bond
gained electron

6. Atoms share pairs of electrons in covalent bonds.
A covalent bond forms when atoms share a pair of electrons.
Can have multiple covalent bonds
Molecules: are stable compounds
covalent bonds
Oxygen atom (O)
Carbon atom (C)
Carbon dioxide (CO2 )