2. Quantum Physics

3. Dalton’s Atomic Theory Dalton’s indivisible atom has not been disregarded—it has been modified to explain new observations. Two important concepts, (1) All matter is composed of atoms and (2) Atoms of any one element differ in properties from atoms of another element, remain unchanged.

4. Electrons Discovered by Joseph John Thompson and Robert A. Millikan Discovered through experiments with cathode-ray tubes. Negative charge Very small mass  9.109 x 10 -31 kg Plum pudding model-now had subatomic particles

5. Discovery of the electron led to 2 other inferences about atomic structure: –Because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons. –Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.

6. Atomic Nucleus Ernest Rutherford, Hans Geiger, and Ernest Marsden Discovered through experiments with alpha particles and gold foil. Except for the nucleus of the hydrogen atom, contains protons and neutrons. –Protons have a positive charge equal to the negative charge of the electron. Atoms are electrically neutral because they contain equal #s of protons and electrons. –Neutrons are electrically neutral.

7. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

8. Atoms Defined as the smallest particle of an element that retains the chemical properties of the element. Composed of: –Nucleus  protons (+) & neutrons (no charge) –Electrons (-) –Collectively referred to as subatomic particles.

9. Proton mass = 1.673 x 10 -27 kg –1,836 times greater than the mass of the electron Neutron mass = 1.675 x 10 -27 kg

10. Nuclear Forces Nuclear forces are short-range proton- neutron, proton-proton, and neutron- neutron forces hold the nuclear particles together.

11. Atomic Number The atomic number of an element is the number of protons in the nucleus of each atom of that element. Determines the identity of the atom. Look at the periodic table, what is the atomic number for hydrogen? Nitrogen? Sodium?

12. Atomic Mass Total number of protons and neutrons Therefore, if you want to know number of neutrons, you will have to subtract atomic # from the mass #. If you were not given a specific mass for your atoms, ONLY THEN should you use the average atomic mass on the periodic table.

13. Atomic # Mass # Symbol Info on the Periodic Table

14. Isotopes Isotopes are atoms of the same element that have different masses. –The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. –Most of the elements consist of mixtures of isotopes. –Although isotopes have different masses, they do not differ significantly in their chemical behavior. –Nomenclature

15. The number of protons defines the element. The number of neutrons varies. H 1 1 H 2 1 H 3 1 Protium Deuterium Tritium Hydrogen-1Hydrogen-2 Hydrogen-3 Isotopes of Hydrogen

16. The total number of protons and neutrons in the nucleus of an atom is equal to the mass number. He 4 2 mass number atomic number Mass Number Or if it is shown as Helium-4 then it is showing element-mass #

17. Quantum numbers-Pauli’s exclusion principle N = energy level (row on periodic table) l = sublevel (s-0, p-1, d-2, f-3) m l = blank in sublevel s = spin (+1/2 for up -1/2 for down)

18. Photoelectric Effect Emission of electrons from a metal when light shines on the metal. This stream of electrons creates an electric current For a given metal, no electrons will be emitted IF the light’s frequency is below a certain minimum (no matter how long the light is shined on it)

20. Planck and Einsten Planck suggested that hot objects release EM energy (light) as small, specific amounts he called quanta Einstein said these particles of EM energy had no mass and carry a specific amount of energy (quantum)

21. Different metals required different minimum frequency of energy E = h v Energy = Planck’s x frequency of constant radiation Joules 6.626 x 10 -34 Hz Js

23. What does this have to do with atoms? When electricity is passed through a light bulb, the atoms are excited This means electrons jump to a higher energy level… but must release energy to return to their ground state If this light is passed through a prism, it separates into specific frequencies of light

24. He 4 2 How many protons are in the nucleus of this helium atom? How many neutrons? Practice 2 protons and 2 neutrons # of protons + # of neutrons = mass number

25. Practice Problems How many protons, electrons, and neutrons are there in an atom of chlorine-37? 17 protons 17 electrons 20 neutrons

26. More Practice How many protons, electrons, and neutrons are in each of the following atoms: –Bromine-90 –Carbon-13 What element contains 15 electrons and 15 neutrons?

27. IONS When an atom (or group of atoms) gains or loses an electron(s), the atom is no longer neutral –Cation-+ charged ion because the atom LOST electron(s) –Anion-(-) charged because the atom GAINED electron(s) –YES, that does seem backwards, but remember electrons are negatively charged