Chapter 2 Atoms and Elements Visualizing Atoms Binnig

Chapter 2 Atoms and Elements

Visualizing Atoms Binnig & Rohrer – development of the Scanning Tunneling Microscope (STM). Produce images on the atomic level. Iodine atoms on the surface of platinum metal

Modern Atomic Theory Law of Conservation of Mass Antoine Lavoisier – 1789 In a chemical reaction, matter is neither created nor destroyed. 7. 7 g Na + 11. 9 g Cl 2 19. 6 g Na. Cl

Modern Atomic Theory Law of Definite Proportions Proust – 1797 All samples of a given compound will have the same proportions of their constituent elements. Mass of Water Mass of Oxygen Mass of Hydrogen Ratio O: H 18. 0 g 16. 0 g 2. 0 g 8: 1 45. 0 g 40. 0 g 5. 0 g 8: 1

Modern Atomic Theory The atomic theory of matter Dalton, 1808 Four postulates (main themes) 1. Each element is composed of tiny, indestructable particles called atoms. 2. All atoms of a given element are identical; The atoms of different elements are different and have different properties.

Modern Atomic Theory 3. Atoms combine in simple, whole number ratios to form compounds. 4. Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms only change the way they are bound together. The evidence for the existence of atoms is overwhelming!

Discovery of Atomic Structure Discovery of the electron. J. J. Thompson ‑ cathode ray tube ‑ 1897 – determined that an electron was negatively charged.

Discovery of Atomic Structure Robert Milliken ‑ oil drop ‑ 1909 – determined the charge value of an electron as well as its mass.

Oil Drop Experiment

Discovery of Atomic Structure Radioactivity Curie – 1900 Rutherford – 1905 Three main types of radioactive particles: Alpha, Beta, and Gamma. Alpha particles Essentially a helium nuclei. Sources – many, but Curie’s used radium.

Nuclear Model of the Atom Ernest Rutherford ‑ 1911. Gold foil experiment. Gold can be smashed into very thin sheets that are only a few atoms thick. Alpha particles, from an alpha source, beamed at the gold foil. Photographic paper placed as a detector in front and behind alpha source.

Gold Foil Experiment

Gold Foil Experiment

Nuclear Model of the Atom Only 1 in 8000 alpha particles is scattered. Scattering occurs when an alpha particle encounters a massive gold nuclei. Rutherford proposed that: 1. Most of the atom’s mass and all of its positive charge were found in the small core of the atom called the nucleus. 2. Most of the volume of the atom is empty space.

Nuclear Model of the Atom Rutherford’s model still had one problem. H = 1 proton in nucleus. He = 2 protons in nucleus. He mass 4 x mass of H mass. Final piece of the puzzle is the neutron. Neutrons have no charge and a mass of 1 amu. Discovered in 1932 by James Chadwick.

Modern View of Atomic Structure Three subatomic particles exist in an atom. Protons, neutrons, and electrons.

Modern View of Atomic Structure Electrons have a negative charge of 1. 602 x 10 -19 C and a negligible mass. Protons and neutrons reside in the nucleus which is extremely small. Protons have a positive charge, equal in magnitude to an electron and a mass of about 1 amu. Neutrons have no charge and a mass of 1 amu. Over 99. 9% of the mass of an atom resides in the nucleus.

Modern View of Atomic Structure The atom is 100, 000 times larger than the nucleus. If a golf ball represented the size of the nucleus, the atom would be about 3 miles in diameter. Diameter of nucleus 10 -15 m. Diameter of atom 10 -10 m. The density of the nucleus is roughly 1013 to 1014 g/cm 3.

Atomic and Mass Numbers The Atomic Number (Z) is equal to the number of protons in the nucleus. Each element has a unique atomic number and hence a unique number of protons. The Mass Number (A) is the sum of the protons and neutrons found in the nucleus.

Isotopes & Symbols Isotopes for an element occur when they have more than one mass number. Isotopes of an element have the same number of protons, but a different number of neutrons. An isotope can be designated by its mass number in the upper left corner – AX. Or it can be designated after the symbol – X–A. Example - 12 C or C-12

Ions Atoms quite often will gain or lose electrons when forming compounds. When Lithium metal reacts, it loses one electron forming a +1 charge. Li+1. When Fluorine gas reacts, it gains one electron forming a -1 charge. F-. Positive ions are called cations. Negative ions are called anions.

Periodic Law Mendeleev (1869) – first to group the elements by similar properties. First, he listed them in order of increasing atomic mass. He then started a new row when elements had similar properties. Thus, they are arranged according to horizontal rows which highlight the repeating properties of the elements in the vertical columns. periods ‑ the elements in a horizontal row constitutes a period. groups ‑ the elements in a vertical column constitutes a group.

The Periodic Table Groups are numbered and labeled with A and B's. Different conventions of numbering are used, however, we will use the traditional N. A. method with the A’s for the first two and last six groups.

The Periodic Table The A groups are called main group elements and the B groups are called transition elements. Some groups have special names: Group 1 A = Alkali metals Group 2 A = Alkaline Earth metals Group 7 A = Halogens Group 8 A = Noble gases or inert gases.

The Periodic Table

The Periodic Table Metals ‑ a substance that has a characteristic of luster or shine, and is a good conductor of heat and electricity. Metals are solids at room temperature and tend to lose electrons easily. Metals are found to the left and below the diagonal line that runs through the right side of the main group elements. The greatest majority of elements are metals.

The Periodic Table Non‑metals ‑ either a gas at room temperature or a brittle solid, they are non‑conductors of heat and electricity. Non‑metals tend to gain electrons easily. Non-metals are found to the right and above the diagonal line. Metalloids ‑ are elements that have the properties of both metals and non‑metals. These fall along the diagonal line and include B, Si, Ge, As, Sb, and Te Hydrogen – the one oddball on the periodic table.

Describing An Element How would you describe the element: C Al Sr Fe

Ions Metals form cations. Ex) Na Na+ + 1 e Non-metals form anions. Ex) Cl + 1 e- Cl Predicting the charge of a species based on the periodic table.

Ions and Ionic Compounds Main group elements usually form only one charge (valence) Transition metals usually form many different charges. Polyatomic ions are special groups of atoms that chemically combine to form a charged species.

Atomic Weights The Atomic Mass Unit Scale is based on the 12 C atom. 1 amu = 1/12 the mass of the 12 C atom. Average atomic masses are based on the masses of each type of isotope a well as their abundance in nature. Relationship of an amu to grams: 1 amu = 1. 66054 x 10 -24 g 1 g = 6. 012214 x 1023 amu

Determining An Average Mass A. W. = S(fract. abundance) x (isotopic mass) Example Chlorine has two isotopes, Cl-35 and Cl-37. Cl-35 has a mass of 34. 969 amu and an abundance of 75. 78% Cl-37 has a mass of 36. 966 amu and an abundance of 24. 22%. What is the atomic weight of Chlorine?

Determining An Average Mass A. W. = (34. 969 amu) x (0. 7578) + (36. 966 amu) x (0. 2422) A. W. = 35. 45 amu

Molar Mass The Mole Unit of quantity used in Chemistry. Not convenient to count atoms. 1 atom of C-12 = 12 amu (exact) One mole of C-12 = 12 grams (exact) Avogadro’s Number Represents the number of C-12 atoms in 12 grams. =6. 02 x 1023 atoms.