ES 106 2006 April 7 Matter and atoms I. Matter is composed of atoms—smallest particle that retains properties A. Atoms— 1. composed of ‘subatomic particles— three fundamental ones a. protons 1) have mass ~ 1 atomic mass unit 2) have positive electrical charge b. neutrons 1) have mass ~ 1 atomic mass unit 2) have no electrical charge c. electrons 1) have mass of ~1/2000 atomic mass unit 2) have negative electrical charge 2. structure of atom a. nucleus contains protons and neutrons—subequal numbers 1) number of protons determines the element 2) number of neutrons may be different in different atoms of the same element—leading to different ‘isotopes a) most isotopes are stable b) unstable isotopes are ‘radioactive’, and disintegrate over time i. in a certain length of time, one half of the atoms of an unstable isotope will decay into another substance ii. this length of time is the ‘half-life; of the isotope a. half-life is constant for an isotope b. can be used to determine age of material, by measuring how much of the ‘daughter’ and ‘parent’ b. electrons surrounding in cloud 1) occur on average in more likely positions 2) called ‘shells’, which have energy levels 3) outermost shell are ‘valence electrons, a) responsible for reactions with other atoms b) full shells are not reactive i. first shell can contain 2 electrons ii. successive outer shells can contain 8 electrons c. atom is electrically neutral when it has the same number of electrons and protonsII. Periodic table of elements A. Each atom is represented by a letter symbol— 1. one or two letters a. capitalize the first letter, b. do NOT capitalize a second letter if present 2. letters often initials in a foreign or even obsolete language, for the element or a major substance that contains the element B. Arranged in rows, or ‘periods’ according to atomic number, increasing mass in each succeeding row C. Columns are called ‘groups’ 1. determined by number of valence electrons a. same number of valence electrons results in similar properties b. full shells of electrons are not reactive with other elements c. most atoms lose, gain or share electrons with other atoms in order to attain a ‘full-shell’ electron configuration 1) elements gaining or losing electrons become ‘ions’ a) gaining electrons results in overall negative charge—‘anion’ b) losing electrons results in overall positive charge—‘cation’ 2. groups a. far right—Noble Gases have full electron shells b. next to far right—Halogens: missing one electron of a full shell c. far left—Alkali Metals: single electron in outer shell d. next to far left—Alkaline Earth Metals: two electrons in outer shell D. Elemental structure can be shown with diagrams of electron shells 1. ‘Bohr diagrams’ named after Niels Bohr, who discovered numerous elements a few hundred years ago, and presented the hypothesis of electrons filling shells 2. Show element with its symbol, and arcs indicating the electron shells a. First shell filled with two electrons—e2- b. Successive shells filled with eight electrons c. Number of arcs corresponds to the row number of element in the periodic table E. Electron dot-diagrams are useful for predicting bonding of elements—dots surrounding the element represent its valence electrons 1. Show how elements bond by filling or emptying dot shell 2. become ions with dots, charges and electrons a. Ca-->Ca2+ + 2e- b. Br + e- -->Br- c. Show ionic bonding reactions by transfer of electrons only 3. Covalent bonds share electrons a. Show unfilled shells on left b. Show sharing of electrons on right c. Cl+Cl-->Cl2 d. Electrons shared equally by same type of atom—nonpolar e. Different types of atoms bonded covalently have one element ‘hog’ the electrons, resulting in a molecule that has ‘polarity’, or is ‘polar’ (like a magnet has poles)III. Elements in Earth’s crust A. Most common eight 1. Oxygen a. 47% by weight b. 63% by number of atoms c. 94% by volume 2. Silicon a. 28% by weight b. 21% by number of atoms c. 1% by volume 3. Aluminum a. 8% by weight b. 6% by number of atoms c. ½% by volume 4. Iron—5% wt, 2 % #, ½% by volume 5. Calcium—4% wt, 2% #, 1% by volume 6. Sodium—3% wt, 3% #, 1% by volume 7. Potassium—3% wt, 1% #, 2% by volume 8. Magnesium—2% wt, 2% #, ¼% by volume B. Minerals are composed of bonded elements 1. naturally occurring inorganic solid with an orderly internal arrangement of atoms (crystalline structure) and a definite chemical composition (that can vary within limits) 2. most minerals in Earth’s crust are silicates— a. compounds containing oxygen and silicon b. building block is the ‘silica tetrahedron’—one oxygen, four
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