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 e- Atomic weight is equal to the number of protons plus the number of neutrons Atomic number is 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 number of e- in outer shell Row indicates number of electron shells Columns indicate ionic charge 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 WalBond C C C C C + + + + + - - - - C C C C C C C C C C Carbon nucleus This is a + polarized area Due to the presence of protons 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 composition (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 uniformity 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 impurities 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 Silicate ion ( SiO 4 4 ) The silicate ion forms tetrahedra. Quartz structure Quartz is a silicate polymor ph. Oxygen io ns (O 2 ) Silicon io n (Si 4+ ) Tetrahedra can arra nge i n other ways. 12
Cleavage planes Mineral Chemical fo rmula and numbe r of cleavage directions Silicate structu re Specimen 1 plane Isolated tetra hedra O livine (Mg, Fe) 2 SiO 4 2 planes at 90 Single chains Pyroxene (Mg, Fe)SiO 3 2 planes at 60 and 120 Double chains Amphibole Ca 2 (Mg, Fe) 5 Si 8 O 22 (OH) 2 1 plane Sheets Micas Muscovite: KAl 2 (Al Si 3 O 10 )(OH) 2 Biotite: K( Mg, Fe) 3 AlSi 3 O 10 (OH) 2 Orthoclase feldspar : 2 planes at 90 Three-di mensional framewo rk Feldspars KAlSi 3 O 8 Plagioclase feldspar: (Ca, Na) AlSi 3 O 8 Silicate layer Aluminum hydroxide layer Aluminum atom Silicate layer Potass iu m ion s Sandwich Cleavage occurs between layers. Sandwich 13