1. A TOMS & Atomic Theory

2. D EFINING THE A TOM An atom is the smallest particle of an element that retains its identity in a reaction. The basic building blocks of matter that make-up everyday objects.

3. A TOMIC T HEORY Democritus was an early Greek Scholar. He was the first to suggest the existence of atoms Democritus believed that atoms were indivisible and indestructible. He never developed a theory because he did not have experimental support nor did he explain chemical behavior. It took 2000 years after Democritus for the real nature of atoms and events at the atomic level to be established

4. D ALTON ’ S A TOMIC T HEORY Using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory 1. All matter is composed of tiny indivisible particles called atoms 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms from different elements can physically mix together or can chemically combine in simple whole- number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

5. D ALTON ’ S T HEORY R EVISED Most of Dalton’s theory is still accepted today EXCEPT that atoms are known to be divisible. Atoms can be broken down into 3 subatomic particles: electrons, protons and neutrons.

6. J.J. T HOMSON Used a cathode ray tube to prove the smallest particles present must have a negative charge. He discovered the ELECTRON!

7. J.J. T HOMSON Atoms were electrically neutral, so there must be a + particle to cancel out the – charge from the electron. Developed the Plum Pudding Model (positive ball containing scattered electrons)

8. R UTHERFORD ’ S G OLD -F OIL E XPERIMENT Former student of Thomson, disproved the Plum Pudding Model of the atom. The Gold Foil Experiment: Sent a beam of + charges (alpha particles) through a piece of very thin gold foil. Angles of deflection were measured.

9. R UTHERFORD Results: Most of the alpha particles passed straight through, most of the foil must be regions of “empty” space – not a + sphere like Thomson believed. + charges and the atoms mass must be found in the center  discovered the nucleus

10. R UTHERFORD ’ S A TOMIC M ODEL In Rutherford’s atomic model, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atoms.

11. ATOMIC STRUCTURE Particle proton neutron electron Charge + ve charge -ve charge No charge 1 1 nil Mass

12. ATOMIC STRUCTURE the number of protons in an atom the number of protons and neutrons in an atom He 2 4 Atomic mass Atomic number number of electrons = number of protons

13. HELIUM ATOM + N N + - - proton electron neutron Shell Are atoms electrically neutral? Why?

14. C OMPLETE THE FOLLOWING TABLE IN YOUR NOTES Atomic # Mass # # of Protons # of Neutron s # of Electro ns 910 1415 4722 5525

15. A TOMIC N UMBER Elements are different because they contain different numbers of protons Atomic Number : The number of protons in the nucleus of an atom of that element. This is used to identify an element. Example: Carbon’s atomic number is 6 because there are 6 neutrons in each Carbon atom’s nucleus For each element the number of protons equals the number of electrons. Atoms are electronically neutral, so the negative charge must equal the positive charge.

16. M ASS N UMBER The total number of protons and neutrons in an atom If you know the atomic number and mass number of an atom of any element, you can determine the atom’s composition. The number of neutrons in an atom is the difference between the mass number and atomic number. Number of neutrons= mass # - atomic # The composition of any atom can be represented in shorthand notation using atomic number and mass number

17. I SOTOPES Atoms that have the same number of protons but different numbers of neutrons

18. I SOTOPES Because isotopes have a different number of neutrons, they also have different mass numbers. Chemically alike because they have identical numbers of protons and electrons, which are responsible for chemical behavior. Ex: Three known isotopes for H H: Hydrogen (no neutrons, mass # of 1) H-2: Deuterium (one neutron, mass # of 2) H-3: Tritium (two neutrons, mass # of 3)

19. C ALCULATING A VERAGE A TOMIC M ASS OF I SOTOPES In nature, isotopes occur in various percentages. The higher the percent the more abundant. In order to figure out the average mass of each element the percent abundance and mass of each isotope need to be considered We can calculate average atomic mass in much the same way as we calculate your grade in this class…

20. C ALCULATING A VERAGE A TOMIC M ASS 1. Divide the percent abundances by 100. (natural occurrence) 2. Multiply each isotope mass by its natural occurrence. (maintain sigfigs)* 3. Add up all the masses (maintain place values) 4. Include a unit (amu)

21. W HAT ARE THE DIFFERENT CATEGORIES THAT YOU ARE GRADED ON IN THIS CLASS ? Classwork: 79pts Practice: 12pts Final: 14pts What would your semester grade be if you received an 81% for classwork, 52% for practice, and 73% on your final? 0.81 x 79 = 64 0.52 x 12 = 6.2 0.73 x 14 = 10 Add all answers together to get % semester grade 63.2 + 3.5 + 10.1 = 80.2  80 % (a B)

22. N OW LETS TRY WITH AN ELEMENT ! Copper has two isotopes: copper-63 and copper- 65. The relative abundances of these isotopes are 69.2% and 30.8% respectively. Calculate the average atomic mass of copper. 0.692 x 63 = 43.6 0.308 x 65 = 20.0 43.6 + 20.0 = 63.6

23. O NE MORE EXAMPLE … Uranium has three naturally occurring isotopes with the following percent abundances: U-234 (0.0058%), U-235 (0.71%), and U-238 (99.23%). What do you expect the average atomic mass to be and why? What is the average atomic mass? 237.9

24. SUMMARY 1. The Atomic Number of an atom = number of protons in the nucleus. 2. The Atomic Mass of an atom = number of Protons + Neutrons in the nucleus. 3. The number of Protons = Number of Electrons. 4. Electrons orbit the nucleus in shells. 5. Each shell can only carry a set number of electrons.

25. A TOMIC M ASS Since the 1920’s the atomic mass has been able to be determined by using a mass spectrometer Because the actual masses of individual atoms are so small the atomic mass unit was developed Atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom. It is more useful to compare the relative masses of atoms using a reference isotope (carbon-12) as a standard. This isotope was assigned a mass of exactly 12 amu Example: He-4 with a mass of 4.0026amu has about one-third the mass of a carbon-12 atom whereas nickel-60 has about 5 times the mass of a carbon-12 atom.

26. A TOMIC M ASS C ONTINUED … A carbon-12 atom has 6 protons and 6 neutrons in its nucleus, and a mass set at 12 amu Since the protons and neutrons account for nearly all of this mass a single proton or neutron is about 1 amu In nature most elements occur in two or more isotopes The atomic mass of an element is a weighted average of the atoms in a naturally occurring sample of the element. A weighted average reflects both the mass and the relative abundance of the isotopes as they occur in nature.

27. C ALCULATING A TOMIC M ASS FOR AN E LEMENT Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. Example: Element X has two natural isotopes. The isotope with a mass of 10.012amu has relative abundance 19.91%. The isotope with a mass of 11.009amu has a relative abundance of 80.09% Calculate the atomic mass of this element.

28. C ALCULATING A TOMIC M ASS F OR A N E LEMENT Solution: Knowns: Isotope 10 X has mass=10.012 amu abundance: 19.91%=0.1991 Isotope 11 X has mass=11.009 abundance: 80.09%=0.8009 For 10 X 10.012amu X 0.1991 = 1.993 For 11 X 11.009 X 0.8009 = 8.817 For element X 1.993+8.817 = 10.810 amu The calculated mass value is closer to the mass of the more abundant isotope, which is what you would expect

29. T HE P ERIODIC T ABLE A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements) Period - Each horizontal row of the periodic table. The properties of the elements vary as you move across it from element to element. Group (or family )- Each vertical column of the periodic table. Elements within a group have similar chemical and physical properties.

30. T HE P ERIODIC T ABLE