Transcript Ch 4 - USD305.com

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
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–
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–
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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