1. UNIT 2 Atoms, Molecules, and Ions

3. 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical. The atoms of different elements are different and have different properties (including different masses). 3. Atoms of an element are not changed into different types of atoms by chemical reactions. Atoms are neither created nor destroyed in chemical reactions. This is the Law of Conservation of Mass. 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. This is the Law of Constant Composition.

4. John Dalton’s Atomic Theory Led him to deduce the Law of Multiple Proportions: When two or more elements combine to form more than one compound, the relative masses of the elements which combine will be in in the ratio of small whole numbers. In carbon monoxide, CO, 12 g carbon combine with 16 g oxygen. C:O ratio is 12:16 or 3:4. In carbon dioxide, CO 2, 12 g carbon combine with 32 g oxygen. C:O ratio is 12:32 or 3:8.

5. Almost right. A good start. Structure of the atom after Dalton (ca. 1810) very small

6. J.J. Thomson: Cathode Rays Atoms subjected to high voltages give off cathode rays.

7. J.J. Thomson: Cathode Rays Cathode rays can be deflected by a magnetic field.

8. J.J. Thomson: Cathode Rays

9. Cathode Rays = ELECTRONS* Cathode rays are electrons. Electrons are in atoms. high voltage + Positive ion Electron (-) (originally known as a cathode ray) Atom *Thomson wanted the name “corpuscle.”

10. J.J. Thomson’s Model of the Atom Structure of the atom after Thomson (ca. 1900) “Plum pudding” model: Negative electrons are embedded in a positively charged mass. Positively charged mass Electrons (-) Unlike electrical charges attract, and that is what holds the atom together.

11. Henri Becquerel – Natural Radioactivity Structure of the atom incorporating radioactivity Positively charged mass Electrons (-) Some atoms naturally emit one or more of the following types of radiation: alpha (α) radiation (later found to be He 2+ - helium nucleus) beta (β) radiation (later found to be electrons) gamma (γ) radiation (high energy light) γ α α γ Alpha particles Somehow gamma radiation is in there, too.

12. Ernest Rutherford – The Nucleus Scattering experiment: firing alpha particles at a gold foil

13. Ernest Rutherford – The Nucleus Some alpha particles bounce off the gold foil. This means the mass of the atom must be concentrated in the center and is positively charged! gold foil (very thin) stream of α particles Scattering experiment: firing alpha particles at a gold foil detector most pass through the foil some are deflected a little some are deflected a LOT

14. Ernest Rutherford – The Nucleus and the Proton Structure of the atom after Rutherford (1910) The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus. Electrons (-) are now outside the nucleus. The positively charged particles in the nucleus are protons.

15. James Chadwick – The Neutron Structure of the atom after Chadwick (1932) In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons. Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics) The positively charged particles in the nucleus are protons. n n +

16. Structure of the Atom proton (+) neutron nucleus - responsible for the mass of the atom, positively charged electrons - responsible for the volume and size of the atom, negatively charged 10 -14 m 10 -10 m Electrons occupy the sphere.

17. FeatureSizeMass 1 amu = 1 atomic mass unit = 1.66054 x 10 -24 g Electrons are outside the nucleus in quantized energy states called orbitals. proton (+) 10 -15 m1.0073 amu neutron (0) 10 -15 m1.0087 amu electron (-) 10 -18 m ??? 0.0006 amu + n n

18. Atomic Number  The number of protons in the nucleus is called the atomic number Z.  Z determines the identity of an element.  Saying “the atomic number of an element is 6” is the same as saying “carbon.”  The number of electrons in the atom is also Z (because atoms have no net electric charge).  How many neutrons are in C? Carbon atom - proton - neutron

19. 12 C - proton - neutron Isotopes The number of protons and neutrons (nucleons) in an element is called the mass number A. How many neutrons are in C? The answer is “it depends on the isotope.” An element may have different numbers of neutrons but NOT different numbers of protons. Atoms of an element with different numbers of neutrons are called isotopes of that element. A = Z + number of neutrons. A 6 Z

20. Isotopes number of protons (Z) number of neutrons 66 68 88 92146 mass number (A) number of electrons symbol 12 12 C or C-12 14 C or C-14 16 O or O-16 238 U or U-238 6 146 168 23892 6 6 8

21. Atomic Masses Atomic masses are based on 12 C. The mass of 12 C (or C-12) is defined to be exactly 12 amu.

22. Atomic Masses The mass (weight) shown in the periodic table is the mass of the element as its occurs naturally. If the element has more than one isotope, the mass shown is the weighted average of the masses of the isotopes. Mg has 3 isotopes. 24 Mg 78.99% 23.985 amu 25 Mg 10.00% 24.986 amu 26 Mg 11.01% 25.983 amu weighted average of Mg: 0.7899x23.985 18.946 0.1000x24.986 2.499 0.1101x25.983+2.861 24.31 amu atomic weight of Mg based on natural abundance: 24.31 amu

23. Isotopes Are there isotopes in the food you eat? the air you breathe? Are all isotopes radioactive?