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Earth Chemistry • Objectives – Compare chemical properties and physical properties of matter – Describe the basic structure of an atom – Compare atomic number, mass number, and atomic mass – Describe arrangement of elements in periodic table – Define isotope, compound, molecule – Interpret chemical formulas – Describe how electrons form chemical bonds between atoms – Explain the differences between compounds and mixtures Matter • What is matter? • Physical Characteristics – Density, color, hardness, freezing pt, boiling pt, ability to conduct electric current • Chemical Characteristics – How a substance reacts w/ other substances to produce different substances – Ex. Iron-oxygen=rust, helium reacts w/ nothing Atoms • What is an atom? – Smallest unit of an element that has chemical properties of that element – Can it be broken down any farther? How big is an atom? • Atomic Structure • What makes up the atomic structure? – Protons= + charge, nucleus, dense – Neutrons= neutral charge, nucleus, dense – Electrons= negative charge, electron cloud. Travel at high speeds as they orbit the nucleus. Do not travel in same plane • Nucleus – Made up of closely packed neutrons and protons. + charge • Electron Cloud – Surrounds nucleus, made up of electrons. Why are electrons attracted to atom Elements • Substance that cannot be broken down into anything simpler, stable. • More than 90 elements occur naturally on Earth – Examples • 2 dozen created in lab • 8 elements make up 98% of Earth’s crust Atomic and Mass Number • Atomic Number – Number of protons in nucleus of atom – # of protons equals the # of electrons • Periodic table- system for classifying elements, arranged according to atomic # – Same column, similar arrangements of electrons in atoms • Atomic Mass – Sum of number of protons and neutrons in atom=mass number – Measurement read in atomic mass unit (amu) • Isotopes – Atom that has same # protons but differs in neutron # of other atoms of that element. – Have slightly different properties Arrangement of Periodic Table • Elements arranged in columns=groups • Atom’s chemical properties determined by # of electrons in outer energy level – Outermost electrons found in energy levels=valence electrons – First energy level can hold 2 electrons, all after that can hold 8 electrons • Groups 1 and 2, same # of electrons as group #, 312 have 2 or more, 13-18 same as group # -10 except for helium (only has 2) • Metals – Alkali, alkaline-earth, transition, others • Nonmetals – Halogens, Noble gases, others • Semiconductors and Hydrogen • Welcome to Discovery Education Player Sec 2-Combination of Atoms • Compound-Substance made up of 2 or more elements joined by chemical bonds between atoms of their elements – Valence electrons are involved in chemical bonding – Octet rule-atoms combine to form compounds and molecules in order to obtain the stable electron configuration like noble gases • Molecule-smallest unit of matter that can exist by itself and retain all of substance’s chemical properties Chemical Formulas • What is a chemical formula? – Combination of letters and numbers that shows which elements make up compound – Also shows # of atoms of each element to make up molecule of compound – Examples Chemical Equations • Reaction of elements and compounds that are described in formula – Reactants=left side of arrow – Products=right hand side – Arrow means gives or yields • Why use chemical equations? – To show the types and amounts of the products that could form from a reactant – Atoms must be equal on each side – How do you do this? – Coefficient multiples subscript Chemical Bonds • Forces that hold together atoms in molecules, form because of the attraction between positive and negative charges – How do they form bonds? • Share or transfer valence electrons from one atom to another • Ions-Particle (atom or molecule) that carries a charge – Electrons are transferred, atoms have electrical charge because of the unequal # of electrons and protons – NaCl- Sodium(11 protons/11 electrons), Chlorine (17 protons/17electrons). Sodium gives up electron, now a positive charge. Chlorine gains electron now a negative charge Types of bonds • Ionic bond – Attractive force between oppositely charge ions that result from transfer of electrons from one atom to another • Covalent bond – Bond formed by attraction between atoms that share electrons – + nucleus attracted to – electron. The force keeps atoms joined – Ex. Water • Polar covalent bond – Covalent bond in which the bonded atoms have unequal attraction for shared electrons Mixtures • Combination of 2 or more substances that are not chemically combined, substances keep individual properties – Mixtures can be separated into parts by physical means • Heterogeneous – Mixtures in which 2 or more substances are not uniformly distributed – Ex. Igneous rock=granite + quartz and feldspar • Homogeneous – Having same composition and properties throughout – Solution- 2 or more substances uniformly dispersed throughout mixture is a solution – Ex. Sea water Significant Figures • Significant figures-digits necessary to express the results of a measurement to the precision with which it was made – Ex. Reading a thermometer • Precision-how often a particular measurement will repeat itself in series of measurements • Accuracy-tells how close the measured value is to a known or standard accepted value of the same measurement – Measurements might show high degree of precision but might not always reflect a high degree of accuracy or vice versus • When making measurements, it is important to determine the # of significant digits for results to be meaningful • Rules must be followed – All nonzero digits (digits from 1 to 9) are significant • 254 contains three significant figures • 4.55 contains three significant figures • 129.454 contains six significant figures – Zero digits that occur between nonzero digits are significant • 202 contains three s.f. • 450.5 contains four s.f. – Zeros at the beginning of a number are considered to be placeholders and are not significant • 0.00078 contains two s.f. • 0.00205 contains three s.f. • Zeros that occur at the end of a number that include an expressed decimal point are significant. Decimal pt is taken as an indication that the measurement is exact to the places indicated – 57500. contains five s.f. – 34.00 contains four s.f. • Zeros that occur at the end of a number without an expressed decimal pt are not considered to be significant – 2000 contains one s.f. – 40620 contains four s.f. Rounding Off Numbers • When dealing with scientific figures, it is often necessary to round off numbers in order to keep the results of calculations significant – – – – – – Ex. Round 64.82 to three s.f.= 64.8 75.52 to three s.f.= 75.5 9.08352 to two decimal places= 9.08 1345.54 to a whole number= 1346 7962400 to three s.f.= 7960000 0.000275 to two s.f.= 0.00028 • Scientific Notation with significant figures