1. 2.1 Atoms, Ions, and Molecules KEY CONCEPT All living things are based on atoms and their interactions.

2. 2.1 Atoms, Ions, and Molecules 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 Hydrogen atom (H) Oxygen atom (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…

3. 2.1 Atoms, Ions, and Molecules –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 (-) An atom has a nucleus and electrons.

4. 2.1 Atoms, Ions, and Molecules Representing Atoms Apply: How many electrons would need to be added to fill the outermost energy level of hydrogen? Of oxygen? 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.

5. 2.1 Atoms, Ions, and Molecules –water (H 2 O) O HH _ ++ A compound is made of atoms of different elements bonded together.

6. 2.1 Atoms, Ions, and Molecules –carbon dioxide (CO 2 ) A compound is made of atoms of different elements bonded together. –water (H 2 O)

7. 2.1 Atoms, Ions, and Molecules –many other carbon-based compounds in living things A compound is made of atoms of different elements bonded together. –water (H 2 O) –carbon dioxide (CO 2 ) Contrast: How are elements different from compounds?

8. 2.1 Atoms, Ions, and Molecules 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. 2.1 Atoms, Ions, and Molecules Check for understanding: 1.Why is the sodium ion shown smaller than the sodium atom? 2.How does the size of the chlorine atom compare with the size of the chloride ion? 3.What determines whether an atoms becomes a positive or a negative ion?

10. 2.1 Atoms, Ions, and Molecules 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. covalent bonds Oxygen atom (O)Carbon atom (C)Oxygen atom (O) Carbon dioxide (CO 2 ) –multiple covalent bonds –diatomic molecules (there are 7): H, N, O, F, Cl, Br, I

11. 2.1 Atoms, Ions, and Molecules Check your understanding: 1.How many electrons does a single oxygen atom have in its outer energy level? A carbon atom? 2.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? 3.What happens to electrons in outer energy levels when two atoms form a covalent bond?