1. Chemistry Ms. Pollock 2013 - 2014

2. Introduction  Dalton’s atomic theory very good but not entirely correct  Atoms able to be broken into smaller particles

3. Electrons, Protons, and Neutrons  Electrons  Negative charge  Repulsed by negative objects  Attracted to positive objects  Able to “stake out” territory and “defend” that territory from other electrons

4. Electrons, Protons, and Neutrons  Protons  Positive charge  Attracted to positive objects  Repulsed by negative objects  Forced to group together into one big clump – strong nuclear forces  Form dense, positively charged center

5. Electrons, Protons, and Neutrons  Neutrons  Proposed by Rutherford  Neutral  Neither attracted nor repelled  Bound in nucleus by strong nuclear forces

6. Electrons, Protons, and Neutrons  No real interaction of neutrons  Protons and electrons interacting  Expected attraction between protons and electrons  Electrons much smaller than neutrons and protons  Most atom mass in nucleus

7. Electrons, Protons, and Neutrons ParticleRelative Mass (amu) Mass in Grams (g) Electric Charge Location Electron1/18409.11 X 10 -28 Outside nucleus Proton11.67 X 10 -24 +1Nucleus Neutron11.67 X 10 -24 0nucleus  Atomic mass and charge for subatomic particles in table  Atomic mass unit – 1/12 mass of carbon atom; mass of proton and neutron one unit; also called a dalton One proton for every electron in neutral atom Two protons = two electrons Ten protons = ten electrons

8. Atomic Number and Mass Number  Different elements distinguished by number of protons  Atomic number – number of protons in nuclei of any atom of an element; symbol Z  Also number of electrons for neutral atoms

9. Atomic Number and Mass Number  Mass number – total number of protons and neutrons in nucleus; symbol A  No electrons in mass number due to tiny mass  Mass number A = (number of protons) – (number of neutrons)

10. Atomic Mass Calculation Example  What is the mass number of an atom that contains 3 protons and 4 neutrons?  (number of protons) = 3  (number of neutrons) = 4  mass number A = (number of protons) + (number of neutrons)  mass number A = (3) + (4) = 7

11. Atomic Mass Calculation Example  What is the mass number of an atom of helium that contains 2 neutrons?  (number of protons) = 2 (Remember that an atom of helium always has 2 protons.)  (number of neutrons) = 2  mass number A = (number of protons) + (number of neutrons)  mass number A = (2) + (2) = 4

12. Isotopes and Atomic Mass  Number of neutrons possibly different  Isotopes – atoms of same element with different numbers of neutrons  Protons not varied – same atomic number  Neutrons varied – different mass number

13. Isotope Example  What is the atomic number (Z) and the mass number (A) of an isotope of lithium containing 3 neutrons? A lithium  atom contains 3 protons in its nucleus.  atomic number Z = number of protons = 3  number of neutrons = 3  mass number A = (number of protons) + (number of neutrons)  mass number A = (3) + (3) = 6  What is the atomic number (Z) and the mass number (A) of an isotope of lithium containing 4 neutrons? A lithium  atom contains 3 protons in its nucleus.  atomic number Z = number of protons = 3  number of neutrons = 4  mass number A = (number of protons) + (number of neutrons)  mass number A = (3) + (4) = 7

14. Isotopes and Atomic Mass  Isotopes part of Dalton’s theory that has been disproved – not all atoms of an element identical  Natural samples of elements combinations of different isotopes  Isotope abundance important to calculation of atomic mass  Periodic table mass weighted average mass of atoms in naturally occurring sample

15. Average Atomic Mass Example  Boron has two naturally occurring isotopes. In a sample of boron, 20% of the atoms are B- 10, which is an isotope of boron with 5 neutrons and a mass of 10 amu. The other 80% of the atoms are B-11, which is an isotope of boron with 6 neutrons and a mass of 11 amu. What is the atomic mass of boron?  0.2(10) + 0.80(11) = 10.8 amu

16. Average Atomic Mass Example  Neon has three naturally occurring isotopes. In a sample of neon, 90.48% of the atoms are Ne-20, which is an isotope of neon with 10 neutrons and a mass of 19.99 amu. Another 0.27% of the atoms are Ne-21, which is an isotope of neon with 11 neutrons and a mass of 20.99 amu. The final 9.25% of the atoms are Ne-22, which is an isotope of neon with 12 neutrons and a mass of 21.99 amu. What is the atomic mass of neon?  0.9048(19.99) + 0.0027(20.99) + 0.0925(21.99) = 20.20 amu

17. Isotopes and Atomic Mass