Download CHEM121 Lecture Ch2

Did you practice your
conversions today?
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Classifications of Matter
Pure Substances:
Matter that has a fixed or definite composition
– Elements
• Composed of only one type of atom
– Compounds
• Composed of multiple types of atom
• Ex. Water (H2O)
• Always same proportion of hydrogen to oxygen
• Chemical process can break compounds down into
simpler substances
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Example: NaCl
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Mixtures
• Almost everything exists as a mixture (clean air,
water, etc.)
• Heterogeneous Mixture:
– One or more visible boundaries; not uniform
– Example: milk
• Homogeneous Mixture:
– No visible boundaries; individual atoms, ions or
molecules mixed
– Example: sugar water
Mixtures v. Compounds
In what ways are mixtures not compounds?
1. Proportions of the components can vary
2. The individual properties of the components are
observable
3. The components can be separated by physical
means
Pure substance or mixture?
• Salt water
MIXTURE
• Helium gas
PURE / ELEMENT
• Vegetable soup
MIXTURE
• Hydrogen peroxide (H2O2)
PURE / COMPOUND
Are the mixtures homogeneous or heterogeneous?
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Elements and Symbols
• Elements:
– primary substances from which all other things
are built.
– Cannot be broken down into simpler substances
• Chemical symbols
– Each element has its own symbol
– One or two letter abbreviations
– First letter always capital letter
– Second letter always lowercase letter
– Ex. cobalt = Co
nitrogen = N
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Elements
1A
8A
2A
• Elements ordered by atomic number
• “Periodic” trends
• Periodic table by Mendeleev (1871)
3B
4B
5B
6B
7B
8B
1B
2B
3A
4A
5A
6A
7A
Reading the Periodic Table
Period: horizontal row
Group: vertical column
Alkali Metals
soft, shiny metals; good electrical/thermal conductors
react vigorously with water
Alkaline earth metals:
shiny metals, less reactive than alkali metals
Halogens:
strongly reactive (i.e. Cl2, Br2)
Noble Gases:
unreactive
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Metals, Nonmetals, and Metalloids
• Metals:
– Left side of zigzag “staircase”
– Shiny solids (Hg is a liquid!)
– Ductile and malleable
– Good electrical/thermal conductors
• Nonmetals:
– Dull
– Poor electrical/thermal conductors
– Lower melting points and densities
• Metalloids:
– B, Si, Ge, As, Sb, Te, Po, At
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The Atom
Atom: smallest particle of an element that retains the
characteristics of that element.
Dalton’s Atomic Theory (1808)
1. All matter is made up of tiny particles called atoms
2. All atoms of a given element are:
–
–
Similar to one another
Different from atoms of other elements
3. Compounds are combinations of atoms of two or more
different elements. A particular compound always:
–
–
Is made up of same kinds of atoms
Has the same number of each kind of atom
4. Chemical reaction: rearrangement, separation or
combination of atoms
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Structure of an Atom
Rutherford gold foil experiment
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Atoms and Subatomic Particles
Diameter of one atom:
0.1 – 0.5 nm
(Carbon atom: 0.15 nm)
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Atomic Number and Mass Number
• Atomic Number = # of protons in atom
• Atoms are electrically neutral
• # protons = # electrons
• Mass Number = # of protons + # of neutrons
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Isotopes
Isotopes:
atoms of the same element that have
different numbers of neutrons
Atomic Symbol
Mass #
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12
Mg
Atomic #
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Isotopes of Carbon
12C
: 6 protons + 6 neutrons
13C : 6 protons + 7 neutrons
14C : 6 protons + 8 neutrons
98.89 %
1.11 %
<0.01 %
All have nearly identical chemical behavior
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Atomic Mass
• Atomic Mass: weighted average of masses of all
the naturally occurring isotopes of that element
Example: chlorine
Mass (amu)
35Cl
37Cl
34.97
36.97
Contribution to
x Abundance (%) = average Cl atom
x
x
75.78
100
=
24.22
100
=
26.50 amu
+
Atomic mass of Cl
8.954 amu
35.45 amu
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Electromagnetic Spectrum
All waves in the EM spectrum travel at the same speed.
Speed of light:
c = 3.00 x 108 m/s
Longer wavelength  smaller energy
Shorter wavelength  more energy
Properties of Waves
Which wave has the longer wavelength?
Which wave has more energy?
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Carbon atoms
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Atomic Spectra
• When light passes through a prism, it is dispersed
– Splits into specific colors
• Each type of material will
produce different wavelengths
of light when heated.
• These patterns are called atomic spectra
Atomic Spectra
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Energy States
• Each atomic spectrum line is
associated with an electron
changing energy
• Electrons can only have certain
energy levels (quanta)
• Energy levels:
• Principal quantum numbers
• Positive integers
• Smaller numbers
• closer to nucleus, lower energy
• Bigger numbers
Note: biggest “step”
from n=1 to n=2
• Further from nucleus, higher energy
Changes in Energy Levels
• Ground state: lowest level
– n=1
• Excited states: higher levels
– n = 2, 3, 4,…
• Energy absorbed: electron
jumps up to higher E
• Energy released: electron
drops down to lower E
If a photon is emitted, light may be observed.
The color of the light emitted relates to the
wavelength of the photon emitted
Energy Sublevels
• Each level consists of sublevels
• The number of sublevels in each level equals the
principal quantum number
• n = 1  one sublevel
• n = 2  two sublevels
• n = 3  three sublevels
• n = 4  four sublevels
1s
2s & 2p
3s & 3p & 3d
4s & 4p & 4d & 4f
Increasing energy of sublevels within an energy level:
s < p < d < f
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Orbitals
• There is no way to know exactly where an
electron is in an atom
• Orbitals describe the regions with the highest
probability of finding an electron
• Each type of orbital has a given shape:
s orbital
p orbitals
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Orbitals
d orbitals
Each orbital can hold
TWO electrons
• s orbitals: 2 e• p orbitals: 6 e• d orbitals: 10 e-
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Orbital Energy Levels
• Pauli Exclusion Principle:
– Any orbital can hold up to 2 electrons
• Electrons want to sit in lowest energy level
• Which is lower: 2s or 2p orbitals?
• For a given n value:
– Order of sublevel energy: s < p < d < f
• Orbital Diagrams
Electron Configurations
• Periodic Table tells us the number of e- in an atom
Electron Configuration:
tells us which orbitals electrons are in for a given atom
Example: Hydrogen
1s
Atomic Number: 1
(How many electrons?) 1
Orbital Diagram:
Electron Configuration: 1s 1
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Electron Configurations
Example: Helium
Atomic Number: 2
(How many electrons?) 2
1s
Orbital Diagram:
Electron Configuration: 1s 2
Example: Lithium
1s
Atomic Number: 3
2s
(How many electrons?) 3
Orbital Diagram:
Electron Configuration: 1s 2 2s 1
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Electron Configurations
Example: Boron
Atomic Number: 5
(How many electrons?) 5
1s
2s
2p
Orbital Diagram:
Electron Configuration: 1s 2 2s 2 2p1
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Electron Configurations
Example: Oxygen
Atomic Number: 8
(How many electrons?) 8
1s
2s
2p
Orbital Diagram:
Electron Configuration:
1s 2 2s 2 2p4
Abbreviated Configuration:
[He] 2s 2 2p4
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Electron Configurations relate
to the Periodic Table!
• For s block elements: electron configuration
should end with electrons in s orbitals
– Ex. Na: 1s2 2s2 2p6 3s1
• For p block elements: electron configurations
should end with electrons in p orbitals
– Ex. P: 1s2 2s2 2p6 3s2 3p3
• For d block elements: electron configurations
should end with electrons in d orbitals
– Ex. Ni: 1s2 2s2 2p6 3s2 3p6 4s2 3d8
Note: 4s orbital comes before 3d orbital
Sublevel Order
Exceptions in sublevel order:
Cr: 1s2 2s2 2p6 3s2 3p6 4s2 3d4
Cr: 1s2 2s2 2p6 3s2 3p6 4s1 3d5
Cu: 1s2 2s2 2p6 3s2 3p6 4s2 3d9
Cu: 1s2 2s2 2p6 3s2 3p6 4s1 3d10
Half-filled or totally filled orbitals are more stable
Periodic Trends
Valence electrons:
– Outermost electrons (last e- in electron config.)
– Responsible for chemical behavior
– Elements in a group have same number of valence e– Elements in a group have similar chemical properties
Electron-Dot Symbols
Boron (B): 1s2 2s2 2p1
How many valence electrons? 3
B
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Periodic Trends
Electron-Dot Symbols
Alumnium (Al): 1s2 2s2 2p6 3s2 3p1
How many valence electrons? 3
Chlorine(Cl):
Al
1s2 2s2 2p6 3s2 3p5
How many valence electrons? 7
Cl
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Periodic Trend: Atomic Size
Atomic radius: distance from nucleus to outermost e• Decreases across a period
(stronger pull from nucleus)
• Increases down a group
(more energy levels)
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Periodic Trend: Ionization Energy
Ionization: removing an e- from an atom
Increases across a period
(stronger pull from nucleus)
Decreases down a group
(further from nucleus)
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A neutral atom has 7 protons and 7 neutrons:
-What is the mass number?
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-How many electrons does this element have? 7
-What is the atomic number? 7
-What Element is this? Nitrogen
-Is the atomic size Larger or Smaller than Boron? Smaller
-Is the atomic size Larger or Smaller than Phosphorus? Smaller
-What is the electron configuration? 1s2 2s2 2p3
-How many valence electrons? 3
-Write the electron dot symbol for this atom . .. .
N
.