Chapter 5. Atoms and Molecules

1. Chapter 5.Atoms and Molecules Atoms are the smallest particles of an element that can exist and still have the properties of that element.

2. Atomic TheoryProposed by John Dalton, 1808 All matter is made up of small particles called atoms. All atoms of a given type are similar, and different from atoms of other types. The relative number and arrangement of different types of atoms in a substance determine its identity. Chemical change is a union, separation, or rearrange-ment of atoms to give new substances. Atoms are indivisible, so only whole atoms can participate in or result from any chemical change.

3. Atomic TheoryProposed by John Dalton, 1808 How big are atoms? Not very! Diameter ~ 10-10 m ~ 1 million in diameter of printed period Mass ~ 10-22 g ~ 1025 would weigh 1 pound This was figured out in early 1900's

4. Atomic TheoryProposed by John Dalton, 1808

5. Molecules A molecule is a group of two or more atoms that functions as a unit because the atoms are tightly bound together.

6. Molecules Some elements exist as molecules. Also H2, N2, O2, F2, Br2, I2

7. Molecules In molecules, elements are bound in a specific order and geometry. They act as a unit.

8. Molecules Not all compounds exist as molecules!

9. Chemical Formulas A chemical formula is a notation that shows the chemical symbols of the elements present in a compound, with numerical subscripts to the right of each symbol to show how many atoms of each element are present in the compound. Water H2O Methane CH4Aspirin C9H8O4 Calcium hypochlorite Ca(OCl)2 Tetraethyl lead Pb(C2H5)4

10. Chemical Formulas Writing chemical formulas: Subscripts are omitted if only one atom of that element is present. Elements are usually listed in order from left to right as they appear on the periodic table. Sometimes a group of atoms acts as a unit within a compound. Such groups are en-closed in parentheses, followed by a sub-script to show how many groups are present.

11. Chemical Formulas Types of chemical formulas: Empirical formulas show the smallest whole-number ratio of the elements found in a compound. They are always used for non-molecular compounds. Molecular formulas are usually used for molecular compounds. They are whole-number multiples of empirical formulas.

12. Chemical Formulas How many of each type of atom are found in the following compounds? Na2O C6H12O6 NH3 N2H4 Al(NO3)3 Fe2O3 COCl2 CoCl2 Which of these are empirical formulas? Which are molecular formulas?

13. Subatomic Particles For almost 100 years, atoms were thought to be indivisible. In the late 1800's pieces of them started showing up! By 1935, it was known that atoms are made up of three types of subatomic particles: Electrons Protons Neutrons

14. Subatomic Particles Diagram of a gas discharge tube used to study the behavior of “cathode rays.”

15. Subatomic Particles Particle Mass, Charge Notes amu Electron 0.0005 -1 Exists on periphery of atom Does chemistry Proton 1.007 +1 Exists in nucleus Defines element Participates in nuclear reactions Neutron 1.008 0 "Glue" in nucleus Gives isotopes

16. Subatomic Particles Rutherford’s gold foil experiment

17. Subatomic Particles Where things are:

18. Subatomic Particles Relative sizes: If the nucleus were the size of a pea, placed in the center of a stadium, the electrons would be in the outer seats!

19. Subatomic Particles Fun facts: The atom is mostly empty space! Most of the mass is packed into the nucleus which has a density of ~1014 g/cm3!

20. Complete Atomic Symbols The complete atomic symbol shows the num-ber of protons and neutrons in the nucleus, and the charge (if any) on the atom. MC E Z

21. Complete Atomic Symbols E = Symbol for Element Z = Atomic number = # of protons in nucleus Atomic number defines element Above symbol for element in Periodic Table M = Mass number = sum of protons and neutrons in nucleus C = Charge; Zero for neutral atom, when # of electrons = # of protons

22. Complete Atomic Symbols Reading symbols of neutral atoms: 14 56 238 C Fe U 6 26 92 Alternate notation: U-235 P-31 C-14

23. Isotopes Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Isotopes of an element have the same nor-mal chemistry (that's a job for electrons!) but different nuclear behavior. This can show up in magnetic properties and nuclear reactions.

24. Isotopes Isotopes of some common elements: 1 2 3 H H H 1 1 1 12 13 14 C C C 6 6 6

25. Isotopes Most elements have several isotopes. These are exceptions: 9 19 23 Be F Na 4 9 11 27 31 Al P 13 15

26. Atomic Masses The atomic mass of an element is a weighted average mass of the isotopes of that element. It appears below the element's symbol on the periodic table. It represents the mass one would obtain by measuring the mass of a group of atoms.

27. Atomic Masses Atomic mass units (amu) are used to give masses of atoms. Atomic mass units are defined relative to 12C. One atom of 12C has a mass of exactly 12 amu. 1.00 amu = 1.99 x 10-23 g

28. Atomic Masses Atomic mass of Magnesium Isotope Atom % Mass, Contribution amu to total 24Mg 78.70 23.985 18.88 amu 25Mg 10.13 24.985 2.531 amu 26Mg 11.17 25.983 2.902 amu Total = 24.31 amu

29. Charges MC E Z C = Charge; Zero for neutral atom, when # of electrons = # of protons # of electrons = # of protons - charge

30. Charges Examples: 231+ 35 1- 63 2+ Na Cl Cu 11 17 29 If we're working with a collection of ions, all isotopes are present, and we don't need M (or Z). Simplify: Na1+ Cl1- Cu2+