Transcript CHEM 1360 CHS 1

The Study of Chemistry
The Atomic and Molecular Perspective of
Chemistry
• Matter is the physical material of the universe.
• Matter is made up of relatively few (ca. 100) elements.
• Elements are the building blocks of matter.
• On the nano (ultramicroscopic) level, matter consists of
atoms. An atom is a “nano-basketball” -- nano = 10 -9.
• Atoms usually are found in the combined state,
either molecules, salts, or alloys.
• Molecules may consist of the same type of atoms
or different types of atoms.
Classification of Matter
Elements
The next five elements are:
Na 2%, K 2%, Mg 2%,
H 1%, Ti 0.5%.
The next six elements are:
N 3%, Ca 1.5%, P 1%,
K,S,Na 0.75%
Elements in the Human Body –
including trace elements
The Periodic Table
Bring your Periodic Table
to each class!
Properties of Matter
Physical and Chemical Changes
When a substance undergoes a physical
change, its physical appearance changes, but
its chemical nature does not.
Example: the melting of ice (physical change) results in a
solid being converted into a liquid, but it is still water.
Physical changes do not result in a change of composition.
When a substance changes its composition, it
undergoes a chemical change
Example: when pure hydrogen and pure oxygen react
completely, they form pure water. In the flask containing
water, there is no oxygen or hydrogen left over.
Units of Measurement
Powers of ten are used for convenience with smaller or
larger units in the SI system.
What is a GigaByte?
Units of Measurement - Temperature
There are three temperature scales:
Kelvin Scale (used in science)
Same temperature increment as Celsius scale.
Lowest temperature possible (absolute zero) is zero Kelvin.
Absolute zero: 0 K = -273.15oC.
Celsius Scale (used in science)
Also used in science.
Water freezes at 0oC and boils at 100oC.
To convert: K = oC + 273.15.
Fahrenheit Scale (used in US engineering and commerce)
Water freezes at 32oF and boils at 212oF.
To convert:
5
9
C 
9
F - 32
F 
5
C  32
Units of Measurement - Temperature
Units of Measurement - Temperature
A user-friendly way to view the Celsius Scale:
0° - Cold! (coat)
10° - Cool (sweat shirt)
20° - Pleasant (long sleeves)
25° - Room temperature (short sleeves)
30° - Very warm (T-shirt)
40° - Hot! (swimming pool!)
Units of Measurement - Density
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•
•
•
Used to characterize substances.
Defined as: density = mass /volume.
Units: g/cm3, also known as specific gravity.
Originally based on mass -- the density was
defined as the mass of 1.00 g of pure water.
The Atomic Theory of Matter
• John Dalton: Elements are composed of atoms.
– All atoms of an element are identical (chemically).
(Dalton stressed “identical in weight” but he didn’t know
about isotopes)
– In chemical reactions, the atoms are not changed.
– Compounds are formed when atoms of more than one
element combine.
(e.g., H2O, C6H6, C12H22O11 but not H2, Cl2)
The Discovery of Atomic Structure
The ancient Greeks were the first to postulate that
matter consists of indivisible constituents.
Later scientists realized that the atom consisted of
charged (+ or -) entities.
A charged particle will have its path bend in either an
electric or magnetic field.
Cathode Rays and Electrons
A cathode ray tube (CRT) is a hollow vessel with
an electrode at either end. A high voltage is applied
across the electrodes.
The Discovery of Atomic Structure
The ancient Greeks were the first to postulate that
matter consists of indivisible constituents.
Later scientists realized that the atom consisted of
charged (+ or -) entities.
A charged particle will have its path bend in either an
electric or magnetic field.
Cathode Rays and Electrons
A cathode ray tube (CRT) is a hollow vessel with
an electrode at either end. A high voltage is applied
across the electrodes.
The Discovery of Atomic Structure
Three spots are noted on the detector:
•a spot which is not affected by the electric field,
•a spot in the direction of the positive (+) plate,
•a spot in the direction of the negative (-) plate.
b-radiation: Large deflection toward the positive plate
corresponding to radiation which is negatively charged and of
low mass. These b particles are light and of low mass. b
particles are electrons.
g-radiation: No deflection; neutral (zero charge) radiation.
a-radiation: Small deflection toward the negative plate
corresponding to high mass, positively charged radiation.
The Discovery of Atomic Structure
The Nuclear Atom
From the separation of radiation
we conclude that the atom
consists of neutral, positively, and
negatively charged entities.
J. J. Thomson assumed all these
charged species were found in a
sphere.
The Discovery of Atomic Structure
The Nuclear Atom
Rutherford’s a-particle experiment:
The Discovery of Atomic Structure
The Nuclear Atom
In order to get the majority of a-particles through a
piece of foil to be undeflected, the majority of the atom
must consist of a low mass, diffuse negative charge - the
electron.
To account for the small number of high deflections
of the a-particles, the center or nucleus of the atom
must consist of a dense positive charge.
The Discovery of Atomic Structure
The Nuclear Atom
Rutherford modified Thomson’s
model as follows:
assume the atom is spherical but
a massive positive charge must be
located at the center, with a
diffuse light negative charge
surrounding it.
The Modern View of Atomic Structure
The atom consists of positive, negative, and neutral
entities (protons, electrons, and neutrons).
Protons and neutrons are located in the nucleus
of the atom, which is small. Most of the mass of the
atom is due to the nucleus.
Electrons are located outside of the nucleus. Most of
the volume of the atom is due to electrons.
The Nucleus
Ångstrom unit: (1Å = 10-8cm =10-10 m)
The Periodic Table
Columns in the periodic table are called groups
(numbered from 1A to 8A or 1 to 18).
Rows in the periodic table are called periods.
Metals are located on the left hand side of the periodic
table (most of the elements are metals).
Non-metals are located in the top right hand side of the
periodic table.
Elements with properties similar to both metals and
non-metals are called metalloids and are located at the
interface between the metals and non-metals.
Some elements occur naturally as
diatomic molecules
(Most elements can be viewed as uniatomic; but there
are unusual elemental molecules, e.g., P4, S8, C60.)
The Periodic Table
Some of the groups in the periodic table are given
special names.
These names indicate the similarities between
group members:
Group 1A: Alkali metals - “al kali” = “the ashes” (of a
fire)
Group 2A: Alkaline earth metals (“earths” historically were
oxides that were difficult to reduce to the metal).
Group 6A: Chalcogens - “ore formers”
Group 7A: Halogens - “salt formers”
Group 8A: Noble gases - “unreactive” gases
At the bottom are the lanthanides (“rare earths”) and the
actinides.
The Periodic Table
Metals
Metalloids
“semiconductors”
Non-Metals
COMPARISON OF METALS AND NONMETALS
PROPERTY
METALS
Appearance
Conductivity of electricity
Conductivity of heat
Change of form
Affinity for electrons
Oxide chemistry
Examples:
Metallic luster
High
High
Malleable
Loses electrons
Oxide forms bases
MO + H2O → MOH
Na2O + H2O → 2NaOH
NONMETALS
Flat, matte appearance
Low
Low
Brittle
Gains electrons
Oxide forms acids
XO + H2O → HXO
SO3 + H2O → H2SO4
Navigating the Periodic Table
Alkaline
Earths
Halogens
Chalcogens
Transition Metals
Alkali
Metals
Lanthanides
(rare earths)
Actinides
Noble or
Inert
Gases
Different Kinds of Compounds
A salt, formed by ionic bonding,
is formed between a metal and
a nonmetal, (e.g., NaCl, Ag2O).
Different Kinds of Compounds
A molecule, formed by covalent
bonding, is formed between a nonmetal
and a nonmetal, (e.g., CO2, PBr3, H2O).
Different Kinds of Compounds
An alloy, formed by metallic
bonding, is formed between a metal
and a metal, (e.g., brass or nickel-steel)
The Wave Nature of Light
Bohr’s Model of the Hydrogen Atom
Line Spectra
Colors from excited gases arise because electrons
move between energy states in the atom. These are
called line spectra.
Na
H
Bohr’s Model of the Hydrogen Atom
Absorption
Emission
E4
E3
E2
E1
The Mole
The mole connects the visible with the invisible.
A fluorine molecule (F2) weighs 38.000 amu.
A mole of fluorine molecules weighs 38.000 grams.
The number of fluorine molecules in a mole is
an incredibly large number, called Avogadro’s
Number, N, which is 6.022 x 1023.
We will be using the mole concept very often.
Amedeo Avogadro
1776-1856
The Mole
Examples:
A mole of H is 1.008 grams.
A mole of H2 is 2.016 grams.
A mole of CO2 is 44.011 grams.
A mole of CO is 28.01 grams.
A mole of octane (C8H18) is 114.22 grams.
A mole of copper (Cu) is 63.54 grams.
A mole of table salt (NaCl) is 58.44 grams.
A mole of sodium bicarbonate (NaHCO3) is 84.01 grams.
A mole of Ag2O is 231.74 grams.
A mole of glucose (C6H12O6) is 180.16 grams.
A mole of chlorophyll (C55H72MgN4O5) is 893.51 grams.
Molecules and Molecular Compounds
Picturing Molecules
Ions and Ionic Compounds
When an atom or molecule loses electrons, it becomes
positively charged.
•For example, when Na loses an electron, it becomes Na+.
Positively charged ions are called cations.
Ions and Ionic Compounds
When an atom or molecule gains electrons, it becomes
negatively charged.
For example when Cl gains an electron it becomes Cl-.
Negatively charged ions are called anions.
An atom or molecule can lose more than one electron.
Ions and Ionic Compounds
When an atom or molecule loses two electrons, it
becomes doubly positively charged.
For example, when Mg loses two electrons, it becomes Mg2+
12p+
12 e-
Mg atom
“magnesium atom”
lose 2 e-
12p+
10 e-
2+
Mg
ion
“magnesium ion”
Ions and Ionic Compounds
When an atom or molecule gains two electrons, it
becomes doubly negatively charged.
For example, when S gains 2 electrons, it becomes S2-
16 p+
16 e-
S atom
“sulfur atom”
gain 2 e-
16p+
2S
18 e-
ion
“sulfide ion”
Ions and Ionic Compounds
Important: note that there are no easily identified NaCl
molecules in the ionic lattice. Therefore, we cannot use
molecular formulas to describe ionic substances.
Ions and Ionic Compounds
Ionic Compounds
I.e., 3Mg atoms need to form 3Mg2+ ions (total 3x2+
charges) and 2 N atoms need to form 2N3- ions (total
2x3- charges).
Therefore, the formula is Mg3N2.
2+
Mg
Mg 3 N 2
3N
MgO
Be careful! what’s the ionic compound formed between
magnesium and oxygen?)
Chemical Equations
• Lavoisier: mass is conserved in a chemical
reaction.
• Chemical equations: descriptions of chemical
reactions.
• Two parts to an equation: reactants and
products:
2H2 + O2
2H2O
Reactants
Product
H2
O2
H2O
• Stoichiometric coefficients: numbers in front
of the chemical formulas give numbers of
molecules or atoms reacting (and numbers
being produced).
Law of Conservation of Mass:
All reactions must be balanced
CH4 + O2
CO2 + H2O
is not balanced. (Why?)
Count atoms:
Reactants:
1C
4H
2O
Products:
1C
2H
3O
Balance reactions only by changing coefficients,
not by altering chemical formula