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Transcript
Chemistry primer Atom = the smallest unit of an element Element determined by the number of protons in the nucleus E- is an electron, P+ is a proton, N is a neutron Carbon atom Electron cloud Nucleus Carbon has 6 electrons… …and a nucleus of 6 protons … …and 6 neutro ns. Electron (–) Proton ( +) Neutron 1 Atoms have the same number of e- and P+ Ion: in an ion the number of e- is not equal to the number of P+ Charge = number of P+ minus number of eAtomic weight is equal to the number of protons plus the number of neutrons Atomic number si equal to the number protons Atomic number determines the identity of the atom or ion. In other words atomic number determines which element the ion or atom is. Columns indicate ionic charge Columns indicate number of e- in outer shell Row indicates number of electron shells 2 The charge of an ion determines which elements (and how many ions) will combine together. The size of the ion determines how the ions are arranged Chemical Bonds • Covalent Bonds: ions share electrons • Ionic Bonds: One ion takes electron from the other ion and a weaker electrical attraction hold ions together • Metalic Bond: free electron sharing across many ions • Van Der Wal Bond: a weak electronic attraction between polarized regions of chemical compounds. 3 Salt Ionic bond one ion takes away the other ions electron Diamond Covalent bond sharing of electrons Van Der Wal Bond C C + C C C + - C + C C Carbon nucleus This is a + polarized area Due to the presence of protons C C + C C + C - C C Covalent bond this is a negatively polarized area due to the presence of electrons 4 Mineral: an inorganic, naturally occurring, crystalline solid with a narrowly defined chemical formula Crystalline: an ordered repeating pattern within a crystal structure Crystal is the outward expression of this crystalline structure 5 Physical Properties of mineral related to crystalline structure and composition (the more reliable ones for mineral identification) Hardness: Resistance to scratching. Greater hardness due to: strong bonds, tight structure, small ions Cleavage: Planes of weakness along which a mineral breaks. Reflect crystalline structure, Cleavage occurs along planes of weakness between atoms. Week bond have better cleavage. Density: Mass of mineral divided by volume. This is related to chemical formula, (size of ion and atomic weight) and tightness of packing (crystalline structure and bond type) Crystal habit: This is the shape of the crystal, It reflects internal structure. This is reliable but seldom found and can be confused with cleavage. 6 Cleavage 7 Crystal Habit Crystals rarely have room to grow 8 Physical Properties of mineral related to crystalline structure and composit ion (The less reliable ones for mineral identification) Luster: How the mineral reflects light. Streak: Color in power form. This is more reliable than color due to uniformit y of grains. Color: Determined by the composition (chemical formula) but it can also be affected by impurities. This is why color is a poor mineral identifier. Fracture: How a mineral breaks across cleavage planes. Related to strength and arrangement of bonds 9 Color Color can be unreliable due to impurit ies 10 Streak is more reliable Ionic compounds certain combinations of ions occur in many minerals with other elements Silicate (SiO 4) contains one silicon and four oxygen which leaves a -4 ionic charge for this compound. 11 Carbonate Quartz structure Silicate ion ( SiO44–) The silicate ion forms tetrahedra. Quartz is a silicate polymor ph. Oxygen io ns (O2–) Silicon io n (Si4+) Tetrahedra can arra nge i n other ways. 12 Mineral Chemical fo rmula Cleavage planes and numbe r of cleavage directions 1 plane O livine ( Mg, Fe) 2SiO 4 Pyroxene ( Mg, Fe) SiO 3 Amphibole Ca2( Mg, Fe) 5Si 8O 22( O H ) 2 Micas Muscovite: KAl2( Al Si3O 1 0) ( O H ) 2 2 planes at 90° Silicate structu re Specimen I solated tetra hedra Single chains 2 planes at 60° and 120° Double chains 1 plane Sheets 2 planes at 90° Three-di mensional framewo rk Biotite: K( Mg, Fe) 3AlSi 3O 10( O H ) 2 Feldspars O rthoclase feldspar : KAlSi 3O 8 Plagioclase feldspar: ( Ca, N a) AlSi3O 8 Silicate layer Alu minu m hydroxide layer “Sandwich” Alu minu m a tom Silicate layer Potass iu m ion s Cleavage occu rs between layers. “Sandwich” 13