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. Millions of atoms would fit into a space the size of a period at the end of this sentence.
An element is one type of atom. H O
The human body has 6 major elements: C, H, O, N, Ca, P and 22 other elements in trace amounts: K, Na, Cl, Mg, S, B, Cr, Co, F, I, Fe etc…
Hydrogen atom (H)
Oxygen atom (O)

3. An atom has a nucleus and electrons.
The nucleus has protons and neutrons.
Electrons are in energy levels outside nucleus.
Electrons are arranged in energy levels based on the octet rule.
Oxygen atom (O)
Nucleus: 8 protons (+) 8 neutrons
outermost energy level: 6 electrons (-)
inner energy level: 2 electrons (-)

4. Representing Atoms
The exact positions of the electrons cannot be known. They are somewhere in a three-dimensional electron cloud around the nucleus similar to the model shown at the right.
Apply: How many electrons would need to be added to fill the outermost energy level of hydrogen? Of oxygen?

5. O A compound is made of atoms of different elements bonded together.
water (H2O) O H _ +

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

7. A compound is made of atoms of different elements bonded together.
water (H2O)
carbon dioxide (CO2)
many other carbon-based compounds in living things
Contrast: How are elements different from compounds?

8. Ions form when atoms gain or lose electrons.
An ion is an atom that has gained or lost one or more electrons.
positive ions – called cations
negative ions – called anions
Ionic bonds form between oppositely charged ions. This bond forms between a metal and a non-metal.
Sodium atom (Na)
Chlorine atom (CI)
Sodium ion (Na+)
Chloride ion (CI-)
Na loses an electron to CI
ionic bond
gained electron

9. Check for understanding:
Why is the sodium ion shown smaller than the sodium atom?
How does the size of the chlorine atom compare with the size of the chloride ion?
What determines whether an atoms becomes a positive or a negative ion?

10. Atoms share pairs of electrons in covalent bonds.
A covalent bond forms when atoms share a pair of electrons. These form when a non-metal is bonded to a non-metal.
multiple covalent bonds
diatomic molecules (there are 7): H, N, O, F, Cl, Br, I
covalent bonds
Oxygen atom (O)
Carbon atom (C)
Carbon dioxide (CO2 )
Not all atoms easily gain or lose electrons. Rather, the atoms of many elements share pairs of electrons. The shared pairs of electrons fill the outermost energy levels of the bonded atoms. A covalent bond forms when atoms share a pair of electrons. Covalent bonds are generally very strong, and depending on how many electrons an atom has, two atoms may form several covalent bonds to share several pairs of electrons. FIGURE 1.4 illustrates how atoms of carbon
and oxygen share pairs of electrons in covalent bonds. All three atoms in a molecule of carbon dioxide (CO2) have full outer energy levels.
A molecule is two or more atoms held together by covalent bonds. In the compound carbon dioxide, each oxygen atom shares two pairs of electrons (four electrons) with the carbon atom. Some elements occur naturally in the form of diatomic, or “two-atom,” molecules. For example, a molecule of oxygen (O2) consists of two oxygen atoms that share two pairs of electrons. Almost all of the substances that make up organisms, from lipids to nucleic acids to water, are molecules held together by covalent bonds.
What happens to electrons in outer energy levels when two atoms form a covalent bond?

11. Check your understanding:
How many electrons does a single oxygen atom have in its outer energy level? A carbon atom?
What is the significance of the number of electrons in the outer most energy levels of oxygen and carbon in a molecule of carbon dioxide?
What happens to electrons in outer energy levels when two atoms form a covalent bond?