chem - atoms, molecules
TRANSCRIPT
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Lecture 2:
Atoms, Molecules, Ions and
their Role in Chemical
Reactions
Atomic Theory and Trends in the
Properties of the Elements
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Objectives Daltons atomic theory
Atomic structure
Elemental symbols
Isotopes
Classification of matter
Periodic table of the elements
Molecular and non-molecular compounds
Molecular and empirical formulae Ions
Naming compounds -- self study
3.6
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Daltons Atomic Theory of
Matter
Nothing Exists Per Se Except Atoms
and the Void
- Lucretius (ca. 94-51 BC)
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Postulate 1
All matter is composed of indivisible
units called atoms.
An atom retains its identity during
chemical reactions
Cl
H C
Br -
Cl-
H
Br
C
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Postulate 2
An elementis a type of matter composed of
only one kind of atom
In the original theory, each atom of an element
was thought to have the same physical
properties (e.g. mass) of all the other atoms in a
sample of that elementWe now know that an element may have several
isotopes (atoms of different mass)
What determines the identity of an element?
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Postulate 3
A compoundis a type of matter composed ofatoms of two or more elements chemically
combined in fixed proportions
Law of Constant Composition
Is ozone, O3, a compound? Explain.
Are N2O and NO2the same compound?
Is air (composed of 20% oxygen and 80%
nitrogen by weight) a compound?
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Postulate 4
A chem ical react ionconsists of the
rearrangement of the atoms present in
the reacting substances to give newchemical combinations present in the
substances formed by the reaction
Is the following a chemical reaction?
H + ClHHCl + H
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Natural Laws Arising from
Daltons Theory Law of conservation of matter
arises from postulates 2 and 4
atoms can be changed in high-energy collisions and
natural decay processes, but not in chemical reactions Law of constant composition
arises from postulate 3
Law of multiple proportions (postulate 3)
If elements A and B combine to form more than onecompound, the masses of B that can combine with a
given mass of A are in the ratio of small, whole
numbers.
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Law of Multiple ProportionsExample: 10.5 g of nitrogen (N2) combines with 12.0 g of
oxygen (O2) to form a gas, A. Under different
conditions, 17.5 g of N2combine with 10.1 g of O2
to form a gas, B. Show that compounds A and B
obey the law of multiple proportions.
ANS: show that the mass of N2per g of O2needed to
form B is a simple ratio of that of A.
B: (17.5 g) (10.1 g) = 1.73
A: (10.5 g) (12.0 g) = 0.875
B A = 1.98 2
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Implications:
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Subatomic Facts Sheet
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Elemental Symbols
35Cl17
Mass number (i.e. thenumber of protonsand neutrons--notexactly the same as
the atomic mass)goes here.
Atomic number(number of protons)goes here (although itis not often included)
Ionic charge (if any)goes here
The symbol is read chlorine-35.
It is one of two isotopes of chlorine,the other being 37Cl (chlorine-37).
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Isotopes
An elements identity is established by
its atomic number
Different isotopes of an element differ in
their atomic masses
This is due to varying numbers of
neutrons
e.g. 12C has 6 protons and 6 neutrons;13C has 6 protons and 7 neutrons.
An atom of a specific isotope is called a
nuclide
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Atomic and Molecular Masses
The atomic mass unit (amu)
An amu is defined as1
/12the mass of oneatom of carbon-12 (12C)
1 amu = 1.66054 x 10-27kg
1 kg = 6.02214 x 1026amu
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Average Atomic Masses
The atomic masses you see in the periodicchart are weighted averages of the masses
of the naturally occurring isotopes
e.g. In nature, bromine is found as 79Br
(78.918336 amu) and 81Br (80.916289 amu),
in 50.69% and 49.31% abundance,
respectively. What is the average atomic
weight (mass) of Br?AWBr=
(0.5069)(78.918336)+(0.4931)(80.916289)
= 79.90 amu (4 significant digits)
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Finding the % Relative Abundance
e.g. Cl has an average atomic mass of 35.453amu, and is comprised of 35Cl (m35 = 34.969amu) and 37Cl (m37 = 36.966 amu). What arethe relative abundances of the two
isotopes?A. There are 2 isotopes, and the abundances
(X35and X37) must add to 1; therefore, wehave two equations
(1) X37+ X35= 1 and(2) X37m37+ X35m35= 35.453
Solving we find X35 = 0.75764 and X37 = 0.24236
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The Periodic TableIUPAC
N.A. Standard
Atomic symbol
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Molecules
A moleculeis a neutral, discrete assembly oftwo or more atoms that are chemically bound
The properties of a molecule are very different
from those of the atoms from which it iscomprised
Most elementsdo not occur in nature in atomic
form; e.g. oxygen occurs at O2or O3(ozone)
ONLY the noble gases occur as atoms at room
temperature and pressure
This is not to say that most elements occur as
molecules. . .
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If not molecules then what?
Molecular elements include: H2, N2, O2,
S8, P4, C60, F2, Cl2, Br2, and I2
The metals and most metalloids occur as
extended structures, not as discrete units
IT IS INCORRECT TO TALK ABOUTMOLECULES OF ELEMENTAL
METALS
Partial structure of elemental metal
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Molecular Compounds Molecular compounds are substances that
consist of discrete units (molecules) that containmore than one type of chemically bound
element
e.g. Water, H2O; hydrogen peroxide, H2O2 Most molecular compounds are comprised of
non-metals, e.g. organic compounds
Molecules containing metals are possible; e.g.
Fe(CO)5
Most metal-containing compounds are extended
structures; e.g. TiO2
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Molecular vs. Empirical
FormulaeMolecu lar form ulagives the actual numbers
and types of atoms in a molecule
e.g. Ethene: C2H4
Empir ical formulagives the relative numbersand types of atoms in a molecule
e.g. Ethene: CH2
C=CH
H
H
H
Molecularstructureof ethene
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Ions and Ionic Compounds
Ions are charged atoms or molecules They can be positive (cations, e.g. Na+) or
negative (anions, e.g. Cl-)
Atoms form ions in order to complete the noblegas configuration
Metal atoms tend to lose electrons, non-metals
tend to gain electrons
e.g. Na([Ne]3s1) Na+([Ne]) + e-
e.g. Cl([Ne]3s23p5) + e- Cl-([Ar])
Note that ionic charges result from gain and
loss of electrons, not protons
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Main-Group Monatomic Ions
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Polyatomic Ions
Atoms form molecules in order to obtain
noble gas configuration for all atoms
If chemical bonding does not satisfy this
requirement, charging the molecule might
e.g. NO3vs NO3-
NOO
O+
_
Lewis structure of(neutral) NO3
Octet not satisfied
NOO
O+
_ _
Lewis structure of(anion) NO3
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e-
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Naming Inorganic Compounds and
Acids & Bases
(Self Study 3.6)Responsible For:
Simple cations and anions (e.g. Cu+ = copper(I) =cuprous; O2- = oxide)
Polyatomic oxo-anions and acid forms(e.g. NO3
-= nitrate; HCO3- = hydrogen
carbonate)
Ionic compounds (e.g. Cu2O = copper(I) oxide =cuprous oxide)
Acids (e.g. HClO4 = perchloric acid)
Binary molecular compounds
(e.g. NO2 = nitrogen dioxide)
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Chemical Reactions
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Fractions or Whole Numbers?
Fractions are acceptable because the
coefficients refer to molesof substance
If you like whole numbers, simply
multiply by the appropriate constant
C8H18 () +25/2O2 (g)8 CO2 (g) + 9 H2O ()
x 2
2 C8H18 () + 25 O2 (g)16 CO2 (g) + 18 H2O () Note: Use lowest whole-number coeffs
(e.g. 4, 50, 32, 36 would not be
acceptable)
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Physical States
The physical states of the reactants and productsare often indicated parenthetically:
C8H18 ()+25/2O2 (g)8 CO2 (g)+ 9 H2O ()
Other conditions, such as the temperature, solvent,
and whether heat () has been added may beindicated above or below the arrow
298 K
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Alkali metals with water: alkali metalsreact with water to produce hydroxides
and hydrogen
K + H2O KOH + H2(l)(s) (g)(aq)1/2
BalancedBalancedNot balancedAll alkali metals behave similarly.
This is part of the utility of the periodic table.
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Decomposition reactions: One reactant
forms two or more products.
KClO3 KCl + O2(s) (s) (g)3/2
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Summary
Atoms are arranged on the periodic tableaccording to their chemical properties
The identity of an element is determined by the
number of protons in the nucleus
The masses of isotopes vary according to the
number of neutrons
Electrons are responsible for chemistry
Chemical compounds can be molecular, ionic,or covalent, extended structures
Driving force behind chemistry is formation of
t bl t t