Atoms & Molecules

1. Atoms & Molecules

2.   All matter is composed of ELEMENTS •   ELEMENT: A substance with specific properties that cannot be broken down or converted into different substances by ordinary chemical means. These are the basic building blocks for all MATTER (see Periodic Table of the Elements).

3.   ATOM: Smallest unit of an element  ATOM MOLECULE MACROMOLECULE

4. Atoms are composed of sub-atomic particles

5. Atomic Structure • NUCLEUS: Dense core • Contains protons and neutrons •    ELECTRON ORBITALS: • Amorphous "cloud" surrounding the nucleus • Electrostatic forces hold electrons in orbitals •    Number of electrons per orbital: • 1st orbital 2 e- • 2nd orbital 8 e- • 3rd orbital 8 e-

6. Full electron orbitals are stable (chemically inert, non-reactive) • Noble Gasses: Full orbitals (last column of Periodic Table)

7. ATOMIC NUMBER = # of protons in nucleus • ATOMIC WEIGHT (mass)= # protons + # neutrons • "Pure" elements are electrically neutral (# protons = # electrons) • The number of electrons in the outer orbital of an atom determines the chemical properties of the element.

8.   ISOTOPES: • Different forms of an element. • Same chemical properties, different atomic mass. • 14C is an isotope of carbon with an atomic mass of 14. • Isotopes differ in the number of NEUTRONS contained in the nucleus.

9.   Chemical Bonds: • a. ionic • b. hydrogen • c. covalent • d. hydrophobic

10. IONIC BONDS: •   IONS are charged particles. • Some elements lose or gain electrons to acquire full outer orbitals. • Na + Cl Na+ + Cl– • See Figure 2.7A: • These ions are stable; they have filled outer orbitals. • Electrostatic interactions (attraction of opposite charges) keep these ions together. • Reasonably strong bonds--full "plus 1" and "minus 1" charges

11. COVALENT BONDS: • Atoms SHARE electrons. • Electrons orbit around both nuclei involved in making the bond. • See Figure 2.8: • Very strong bonds. • Note the different representations for covalent bonds: • a. Electron configuration • b. Structural formula

12. A covalent bond is formed by a PAIR of electrons. • Two electrons in a pair will tend to repel each other. • Eight electrons (4 pairs) in second orbital, tend to form a tetrahedron. • Carbon can make 4 bonds, includes single, double and triple bonds. • single bonds--tetrahedral • double bonds--distortion to planar • triple bonds--linear

13. WATER: special biological molecule. • water is polar (dipole)

14. HYDROGEN BONDS: • Weak electrostatic bonds

15.   Novel Properties of Water: • Cohesion, surface tension • High heat capacity • Solid (ice) less dense than liquid • POLAR molecules tend to interact strongly with water. • They are HYDROPHILIC. • Polar molecules dissolve in water. •  NON-POLAR molecules do not interact with water. They are HYDROPHOBIC (water-fearing).

16. HYDROPHOBIC BONDS: • Stable interactions between non-polar (hydrophobic) molecules. Sometimes termed hydrophobic effect. • "Like attracts like" • Oil & water don't mix.

17.   HYDROPHOBIC BONDS: • Stable interactions between non-polar (hydrophobic) molecules. Sometimes termed hydrophobic effect. • "Like attracts like" • Oil & water don't mix.

18. WHAT IS pH? • Water can dissociate to form H+ and OH-, (Hydrogen ion, and Hydroxyl ion). •  In pure water, a certain percentage of the water molecules dissociate.  • Each dissociated water molecule forms one H+ and one OH- ion. • In pure water, 1/107 water molecules are dissociated. • Therefore the concentrations H+ and OH- ions is 10-7 • (one part in 10,000,000)

19. WHAT IS pH? • pH is a measure of the H+ concentration (-Log [H+]) • Pure water = pH 7 • As H+ ion concentration increases, the pH decreases. • As the pH decreases, the ACIDITY increases.

20. WHAT IS pH? • ACIDIC----H+ ions predominate in solution • BASIC----OH- ions predominate in solution • Acids tend to release H+ ions into aqueous solutions • Bases tend to remove H+ ions from aqueous solutions (or to release OH- ions into solution).