Lecture 2 Atoms & Molecules

7/24/2019 Lecture 2 Atoms & Molecules

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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%

Documents

Lecture 2 Atoms & Molecules