1. Introduction to Atomic Theory

2. History of the atom Democritus (400 BC) suggested that the material world was made up of tiny, indivisible particles atomos, Greek for “uncuttable” Aristotle believed that all matter was made up of 4 elements, combined in different proportions Fire - Hot Earth - Cool, heavy Water - Wet Air - Light The “atomic” view of matter faded for centuries, until early scientists attempted to explain the properties of gases

3. Re-emergence of Atomic Theory John Dalton postulated that: 1. All matter is composed of extremely small, indivisible particles called atoms 2. All atoms of a given element are identical (same properties); the atoms of different elements are different

4. 3. Atoms are neither created nor destroyed in chemical reactions, only rearranged 4. Compounds are formed when atoms of more than one element combine A given compound always has the same relative number and kind of atoms

5. Make-up of the Atom By the 1850s, scientists began to realize that the atom was made up of subatomic particles Thought to be positive and negative

6. Cathode Rays and Electrons Mid-1800’s scientists began to study electrical discharge through cathode-ray tubes. Ex: neon signs Partially evacuated tube in which a current passes through Forms a beam of electrons which move from cathode to anode Electrons themselves can’t be seen, but certain materials fluoresce (give off light) when energised

7. Oh there you are! JJ Thompson observed that when a magnetic or electric field are placed near the electron beam, they influence the direction of flow opposite charges attract each other, and like charges repel. The beam is negatively charged so it was repelled by the negative end of the magnet

8. http://www.chem.uiuc.edu/clcwebsite/vide o/Cath.movhttp://www.chem.uiuc.edu/clcwebsite/vide o/Cath.mov Magnetic field forces the beam to bend depending on orientation Thompson concluded that: Cathode rays consist of beams of particles The particles have a negative charge

9. Thompson understood that all matter was inherently neutral, so there must be a counter A positively charged particle, but where to put it It was suggested that the negative charges were balanced by a positive umbrella-charge “Plum pudding model” “chocolate chip cookie model”

10. Rutherford and the Nucleus This theory was replaced with another, more modern one Ernest Rutherford (1910) studied angles at which  particles were scattered as they passed through a thin gold foil http://www.mhhe.com /physsci/chemistry/es sentialchemistry/flas h/ruther14.swfhttp://www.mhhe.com /physsci/chemistry/es sentialchemistry/flas h/ruther14.swf

12. Rutherford expected … Rutherford believed that the mass and positive charge was evenly distributed throughout the atom, allowing the  particles to pass through unhindered  particles

13. Rutherford explained … + Atom is mostly empty space Small, dense, and positive at the center Alpha particles were deflected if they got close enough  particles

14. Nucleus: Containing protons and neutrons, it is the bulk of the atom and has a positive charge associated with it Electron cloud: Responsible for the majority of the volume of the atom, it is here that the electrons can be found orbiting the nucleus (extranuclear) The modern atom is composed of two regions:

15. Major Subatomic Particles Atoms are measured in picometers, 10 -12 meters Hydrogen atom, 32 pm radius Nucleus tiny compared to atom If the atom were a stadium, the nucleus would be a marble Radius of the nucleus is on the order of 10 -15 m Density within the atom is near 10 14 g/cm 3 NameSymbolChargeRelative Mass (amu) Actual Mass (g) Electrone-e- 1/18409.11x10 -28 Protonp+p+ +111.67x10 -24 Neutronnono 011.67x10 -24

16. Elemental Classification Atomic Number (Z) = number of protons (p + ) in the nucleus Determines the type of atom Li atoms always have 3 protons in the nucleus, Hg always 80 Mass Number (A) = number of protons + neutrons [Sum of p + and nº] Electrons have a negligible contribution to overall mass In a neutral atom there is the same number of electrons (e - ) and protons (atomic number)

17. Nuclear Symbols Every element is given a corresponding symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as well as the mass number and atomic number E A Z elemental symbol mass number atomic number

18. Find the Find the number of protons number of protons number of neutrons number of neutrons number of electrons number of electrons atomic number atomic number mass number mass number W 184 74 F 19 9 Br 80 35

19. If an element has an atomic number of 34 and a mass number of 78 what is the: If an element has an atomic number of 34 and a mass number of 78 what is the: number of protons in the atom? number of protons in the atom? number of neutrons in the atom? number of neutrons in the atom? number of electrons in the atom? number of electrons in the atom? complete symbol of the atom? complete symbol of the atom? If an element has 91 protons and 140 neutrons what is the: If an element has 91 protons and 140 neutrons what is the: atomic number? atomic number? mass number? mass number? number of electrons? number of electrons? complete symbol? complete symbol?

20. Isotopes Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers The atoms of the same element that differ in the number of neutrons are called isotopes of that element When naming, write the mass number after the name of the element H 1 1 Hydrogen-1 H 2 1 Hydrogen-2 H 3 1 Hydrogen-3

21. How heavy is an atom of oxygen? There are different kinds of oxygen atoms (different isotopes) 16 O, 17 O, 18 O We are more concerned with average atomic masses, rather than exact ones Based on abundance of each isotope found in nature We can’t use grams as the unit of measure because the numbers would be too small Instead we use Atomic Mass Units (amu) Standard amu is 1/12 the mass of a carbon-12 atom Each isotope has its own atomic mass

22. Calculating Averages Average = (% as decimal) x (mass 1 ) + (% as decimal) x (mass 2 ) + (% as decimal) x (mass 3 ) + …

23. Try Again…THINK!! Your test and quiz mark is 30%. 10% is made up of quiz, 20% is made up of tests If you made 85%, 78% and 79% on your quizzes If you made 87% on your only test… What would be your test quiz mark for your report card?

24. Calculate the atomic mass of copper if copper has two isotopes 69.1% has a mass of 62.93 amu The rest (30.9%) has a mass of 64.93 amu Magnesium has three isotopes 78.99% magnesium 24 with a mass of 23.9850 amu 10.00% magnesium 25 with a mass of 24.9858 amu The rest magnesium 26 with a mass of 25.9826 amu What is the atomic mass of magnesium? Calculating Averages

25. If not told otherwise, the mass of the isotope is the mass number in amu The average atomic masses are not whole numbers because they are an average mass value Remember, the atomic masses are the decimal numbers on the periodic table Average Atomic Masses