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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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O!ecti"es# $alton%s atomic theory
# Atomic structure
# Elemental symols
# Isotopes
# Classification of matter
& Periodic tale of the elements
& Molecular and non'molecular compounds
& Molecular and empirical formulae& Ions
# (amin) compounds '' self study
& *+-
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$alton%s Atomic Theory of
Matter
.(othin) E/ists Per 0e E/cept Atoms
and the 1oid
' Lucretius 3ca 45'67 8C9
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Postulate 7
All matter is composed of indivisible
units called atoms.
An atom retains its identity durin)
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 ori)inal theory, each atom of an element
as thou)ht to ha"e the same physical
properties 3e) mass9 of all the other atoms in a
sample of that element;e no
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Postulate +
A compoundis a type of matter composed of atomsof two or more elements chemically combined in
fixed proportions
Law of Constant Composition
#Is ozone, O, a compound! "xplain.
#Are #$O and #O$the same compound!
#Is air %composed of $&' oxy(en and )&' nitro(enby wei(ht* a compound!
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Postulate 5
A chemical reactionconsists of the
rearran(ement of the atoms present in
the reactin( substances to (ive newchemical combinations present in the
substances formed by the reaction
Is the folloin) a chemical reaction=
> ? Cl>>Cl ? >
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(atural Las Arisin) from
$alton%s Theory# La of conser"ation of matter
& arises from postulates 2 and 5
& atoms can e chan)ed in hi)h'ener)y collisions and
natural decay processes, ut not in chemical reactions# La of constant composition
& arises from postulate +
# La of multiple proportions 3postulate +9
& If elements A and 8 comine to form more than onecompound, the masses of 8 that can comine ith a
)i"en mass of A are in the ratio of small, hole
numers
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La of Multiple ProportionsE/ample: 7@6 ) of nitro)en 3(29 comines ith 72@ ) of
o/y)en 3O29 to form a )as, A nder different
conditions, 7B6 ) of (2comine ith 7@7 ) of O2
to form a )as, 8 0ho that compounds A and 8
oey the la of multiple proportions
A(0: sho that the mass of (2per ) of O2needed to
form 8 is a simple ratio of that of A
+ %-./ (* %-&.- (* 0 -.
A %-&./ (* %-$.& (* 0 &.)/
B A = 1.98 2
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Rutherford old Doil E/periment
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Implications:
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Atomic 0tructureElectron Cloud:
Negatively charged Accounts for most of the volume Chemistry
~2 !"-!"m#
Nucleus not to scale ~ !"-$#: Com%osition: &rotons and neutrons 'ost of the mass of the atom &rotons have %ositive charge
Coulom(ic attractions Elements defined (y no) of %rotons
atomic num(er#
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0uatomic Dacts 0heet
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Elemental 0ymols
*+Cl!,
'ass num(er i)e) thenum(er of %rotonsand neutrons--noteactly the same as
the atomic mass#goes here)
Atomic num(ernum(er of %rotons#goes here although itis not often included#
.onic charge if any#goes here
/he sym(ol is read 0chlorine-*+)1
.t is one of to isoto%es of chlorine3
the other (eing*,
Cl chlorine-*,#)
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Isotopes
# An element1s identity is established by
its atomic number
# $ifferent isotopes of an element differ in
their atomic masses# This is due to "aryin) numers of neutrons
# e) 72C has - protons and - neutrons 7+C
has - protons and B neutrons# An atom of a specific isotope is called a
nuclide
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Atomic and Molecular Masses
The atomic mass unit (amu)
An amu is defined as 7F72the mass of oneatom of caron'72 372C9
7 amu G 7--@65 / 7@'2B
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Conundrum:proton mass G 7@@B+ amu
neutron mass G 7@@HB amu
electron mass G 65H- / 7@'5amu
An atom of 72C has - electrons, - protons, and -
neutrons Comined masses of the particles is:
- / 37@@B+ ? 7@@HB ? 65H- / 7@'59 G 72@44+ amu
+23 - amu 0 -4-$ the mass of an atom of -$C5
8indin) ener)y: EG 3m9c2G 3mp?n?e' matom9c2
The indin) ener)y of the 72C nucleus is:
75H2 / 7@'77
0 ).6$ x -&-$7 mol8-
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A"era)e Atomic Masses
# The atomic masses you see in the periodicchart are ei)hted a"era)es of the masses of
the naturally occurrin) isotopes
# e) In nature, romine is found as B48r
3BH47H++- amu9 and H78r 3H@47-2H4 amu9, in6@-4J and 54+7J aundance, respecti"ely
;hat is the a"era)e atomic ei)ht 3mass9 of
8r=
A;8rG 3@6@-493BH47H++-9?3@54+79
3H@47-2H49
G B44@ amu 35 si)nificant di)its9
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Dindin) the J Relati"e Aundance
e.(. Cl has an avera(e atomic mass of /.9/amu, and is comprised of /Cl %m/ 0 9.6:6amu* and Cl %m 0 :.6:: amu*. ;hat arethe relative abundances of the two isotopes!
A. 3here are $ isotopes, and the abundances%
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The Periodic Tale.4&AC
N)A) 5tandard
Atomic sym(ol
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Molecules, Compounds,
and all that
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Molecules
# A moleculeis a neutral, discrete assemly of toor more atoms that are chemically ound
# The properties of a molecule are "ery different
from those of the atoms from hich it iscomprised
# Most elementsdo not occur in nature in atomic
form e) o/y)en occurs at O2or O+3oKone9
& O(L the nole )ases occur as atoms at roomtemperature and pressure
& This is not to say that most elements occur as
molecules
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If not molecules then hat=# Molecular elements include: >2, (2, O2, 0H,
P5, C-@, D2, Cl2, 8r2, and I2
# The metals and most metalloids occur as
e/tended structures, not as discrete units
# IT I0 I(CORRECT TO TALN A8OT.MOLECLE0 OD ELEME(TAL METAL0
&artial structure of elemental metal
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Molecular Compounds# Molecular compounds are sustances that
consist of discrete units 3molecules9 that containmore than one type of chemically ound
element
& e) ;ater, >2O hydro)en pero/ide, >2O2# Most molecular compounds are comprised of
non'metals, e) or)anic compounds
# Molecules containin) metals are possile e)
De3CO96
# Most metal'containin) compounds are e/tended
structures e) TiO2
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Molecular "s Empirical
Dormulae# Molecular formula)i"es the actual numers
and types of atoms in a molecule
& e) Ethene: C2>5
# Empirical formula)i"es the relati"e numersand types of atoms in a molecule
& e) Ethene: C>2
C6CH
H
H
H
'olecularstructureof ethene
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Ions and Ionic Compounds
# Ions are char)ed atoms or molecules# They can e positi"e 3cations, e) (a?9 or
ne)ati"e 3anions, e) Cl'9
# Atoms form ions in order to complete the nole
)as confi)uration
# Metal atoms tend to lose electrons, non'metals
tend to )ain electrons
& e) (a3(e+s79 (a?3(e9 ? e'& e) Cl3(e+s2+p69 ? e' Cl'3Ar9
# (ote that ionic char)es result from )ain and loss
of electrons, not protons
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Main'roup Monatomic Ions
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Main'roup Monatomic Ions
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Polyatomic Ions
# Atoms form molecules in order to otain
nole )as confi)uration for all atoms
# If chemical ondin) does not satisfy this
reQuirement, char)in) the molecule
mi)ht
# e) (O+"s (O+'
N77
7+
_
8eis structure ofneutral# N7*
97ctet not satisfied
N77
7+
_ _
8eis structure ofanion# N7*
-
e-
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#amin( Inor(anic Compounds and
Acids +ases
%@elf @tudy B.:*Responsile Dor:
# 0imple cations and anions 3e) Cu? G copper3I9 Gcuprous O2' G o/ide9
# Polyatomic o/o'anions and acid forms 3e) (O+'
G nitrate >CO+' G hydro)en caronate9
# Ionic compounds 3e) Cu2O G copper3I9 o/ide G
cuprous o/ide9# Acids 3e) >ClO5 G perchloric acid9
# 8inary molecular compounds 3e) (O2 G nitro)endio/ide9
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Chemical Reactions
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!# ;o the sim%lest thing first: Balanceelements hich a%%ear in the feestchemical s%ecies in the e7H? O2 CO2? >2O> ?2+@2
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Dractions or ;hole (umers=# Dractions are acceptale ecause the
coefficients refer to molesof sustance
# If you liH!> # 2+@272 g#> C72 g# ? H27 #
x 2
2 C>H!> # 2+ 72 g#! C72 g# !> H27
# (ote: se loest hole'numer coeff%s
3e) 5, 6@, +2, +- ould not e
acceptale9
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Physical 0tates
The physical states of the reactants and productsare often indicated parenthetically:
CH>7H 39? 26F2O2 3g9H CO2 3g9? 4 >2O 39
Other conditions, such as the temperature, sol"ent,
and hether heat 39 has een added may eindicated ao"e or elo the arro
298 K
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0elected Chemical Reactions
Combustion: a rapid reaction eteen a
fuel and O2in air that produces a flame
e)
C+>H ? O2 CO2 ? >2Og# g# g# g#* $
* Cs here Need * Cs here> Hs here Need >Hs here 7s$ 7s
Need !"7s here
+
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Alkali metals with water: al2O NO> ? >2l#s# g#aq#1/2
BalancedBalancedNot (alancedAll alali metals (ehave similarly)
/his is %art of the utility of the %eriodic ta(le)
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Combination reactions: to or more
sustances react to form one product
Li ? (2 Li+(s# g# s#2
2 Ns ! N! 8i 8is
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Decomposition reactions: One reactant
forms to or more products
NClO+ NCl ? O2s# s# g#*@2
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0ummary
# Atoms are arran)ed on the periodic taleaccordin) to their chemical properties
# The identity of an element is determined y the
numer of protons in the nucleus
# The masses of isotopes "ary accordin) to thenumer of neutrons
# Electrons are responsile for chemistry
# Chemical compounds can e molecular, ionic, orco"alent, e/tended structures
# $ri"in) force ehind chemistry is formation of
stale octet%