2.7 the Periodic Table - Groups 2 and 7

7/25/2019 2.7 the Periodic Table - Groups 2 and 7

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Physical properties

The arrangement of the periodic table is

such that trends can be analsed both

across a period and down a group!

Group 2of the periodic table is shown

here! Trends that can be analseddown the group include atomic

radius" first ionization energyand

melting point!

#lements in the same group also

undergo similar chemical reactions!


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Trend in atomic radius


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Explaining the trend in atomic radius

The atomic radius of the

elements increasesdown

group 2 from berllium to barium!

The increase in radius is due to higherprinciple energ

le%els being filled" whose orbitals are located furtherfrom

the nucleus!

ElementAtomic

radius(nm)

berllium

magnesium

calcium

strontium

barium

0!112

0!14$

0!194

0!219

0!2$3

The number of protons

increases down the group&

howe%er" so does the numberof shielding electrons! #ffecti%e

nuclear charge therefore

remains appro'imatel

constant!


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First ionization energies in group 2


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Trend in melting points


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Explaining the trend in melting points

The melting points of the

elements decrease down

group 2" with the e'ception

of magnesium to calcium! berllium

magnesium

calcium

strontium

barium

ElementMelting

point ()1$(0

923

111$

10$0

1000

+ metal,s melting point depends

on the strength of its metallicbonds! This decreases down the

group because the atomic radius

increases" resulting in a weaker

attraction between the nucleus

and delocali-ed electrons!

The melting point of magnesium is lower than e'pected due

to %ariation in how its atoms pack in the metallic crstal!



Physical properties summary



First ionization energy of group 2 metals


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!olu"ilities of group 2 hydroxides



!olu"ilities of group 2 hydroxides

The solubilit of the

group 2 hdro'ides

increasesdown the

group! .agnesium

hdro'ide is considered

to be sparingl soluble

and the hdro'ides ofthe lower members of

the groups are all

considered to be soluble!

+s the solubilit of the group 2 hdro'ides increases" sodoes the p/ of the solutions formed! This is because the

more of the hdro'ide that dissol%es" the greater the

concentration of hdro'ide ions / in the solution formed!

.g/2

a/2

5r/2

Ba/2

Group 2

hydroxide !olu"ility

sparingl soluble

slightl soluble

soluble

soluble


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Applications of group 2 hydroxides

+ suspension of magnesium hdro'ide is commonl called

milk of magnesia! 6t is used in medicine as a la'ati%e and to

relie%e acid indigestion!

alcium hdro'ide" alsocalled sla#ed lime" is

used in agriculture to raise

the p/ of soils! 5oil p/ is

an important factor inagriculture!


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!olu"ilities of group 2 sulfates


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!olu"ilities of group 2 sulfates

The solubilit of the group 2 sulfates decreasesdown the

group! .agnesium and calcium sulfate are considered to

be soluble" whereas strontium and barium sulfate are

considered to be insoluble!

.g54

a54

5r54

Ba54

Group 2

hydroxide !olu"ility

soluble

slightl soluble

insoluble

insoluble

7ote that this

decrease in solubilit

down the group is

the opposite of the

trend for the

solubilit of thegroup 2 hdro'ides!


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Testing for sulfate ions



Applications of group 2 sulfates

Barium sulfate is used as a

radiocontrast agent to help

take 8ra images of the

digesti%e sstem! 6t is

sometimes known as a

barium meal,!

Barium sulfate is insoluble" so

is not absorbed b the bod

when swallowed! /owe%er"

barium is a %er good absorberof 8ras and it helps to define

structures of the digesti%e

sstem to aid in diagnosis!



Applications of group 2 compounds



$hemical properties summary



%eaction &ith oxygen



Flame tests

:hen group 2 metals are burned in o'gen" coloured

flames are produced! This is due to the presence of metal

ions! Flame tests e'ploit this fact!

magnesium ' "right &hite

calcium ' "ric# redorange

strontium ' redcrimson

"arium ' pale greenyello&green

The presence of certain metal ions can be identified b

noting the characteristic flame colour that results from

burning! The colours for group 2 metal ions are;


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Explaining flame tests

:hen heated" some electrons in an atom or ion are e'cited

to higher energ le%els! :hen the fall back to their initial

le%els" energ is emitted& sometimes seen as %isible light!

#lectrons ma be e'cited b

different amounts intodifferent energ le%els and

drop back at different times!

The colour of the flame is a

combination of all theseenerg emissions!

heatlight

en

er

g

y


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Flame test colours


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% ti ith t


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%eaction &ith &ater

% d ti ith t


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%edox reaction &ith &ater

:hen group 2 metals react with water the form the metal

hdro'ide and hdrogen gas!


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Explaining the trend in reacti1ity

The reacti%it of the elements

down group 2 from berllium to

barium increases!

+lthough increased shielding

cancels the increased nuclear

charge down the group" the

increase in atomic radius resultsin a decrease in the attracti%e

force between the outer

electrons and the nucleus!

This is because it is

successi%el easierto remo%e

electrons to form the 2= ion!

Mg

$a

!r

a

% ti f id ith t


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%eaction of oxides &ith &ater

:hen group 2 metal o'ides react with water the form the

metal hdro'ide!


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3ecomposition of group 2 car"onates

:hen heated" the group 2 metal carbonates decompose

to form the metal o'ide and carbon dio'ide gas! 5plitting

compounds using heat is called thermal decomposition!

magnesium car"onate4 Mg$+5

calcium car"onate4 $a$+5

strontium car"onate4 !r$+5

"arium car"onate4 a$+5

increasingsta"ility

The group 2 carbonates become more stable to thermaldecomposition going down the group;

M$+5(s) M+(s) * $+2(g)

3 iti f 2 it t


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3ecomposition of group 2 nitrates

Thermal decomposition of group 2 metal nitrates forms

the metal o'ide" nitrogen dio'ide and o'gen!

2M(6+5)2(s) 2M+(s) * 76+2(g) * +2(g)

Like the group 2 metal carbonates" the nitrates become

more stable to thermal decomposition down the group!

magnesium nitrate4 Mg(6+5)2

calcium nitrate4 $a(6+5)

2

strontium nitrate4 !r(6+5)2

"arium nitrate4 a(6+5)2

increasing

sta"ility

E l i i th t d i th l t "ilit


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Explaining the trend in thermal sta"ility

.etal ions become larger down group 2 but ha%e the same

charge! This means their charge densityis reduced!

+ metal ion with a high

charge densit has

strong polari-ing power!

6t can therefore polari-ethe carbonate ion"

making it more likel to

split into 2 and 2

when heated! polarization

+ metal ion with a low charge densit has weak polari-ing

power" meaning the carbonate ion is less polari-ed and

therefore more thermall stable!

E0uations for reactions


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E0uations for reactions

!ta"ility of group 2 car"onates


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!ta"ility of group 2 car"onates


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Glossary


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Glossary

8hat9s the #ey&ord:


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8hat9s the #ey&ord:

Multiple choice 0uiz


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Multiplechoice 0uiz


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8hat are the halogens:


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8hat are the halogens:

The halogens are the elements in ?roup ) of the

periodic table!

The name halogen comes from the ?reek

words for saltmaking!

Physical properties of halogens


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Physical properties of halogens

Trends in "oiling point


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Trends in "oiling point

/alogen molecules increase in si-e down the group! This

leads to greater %an der :aals forces between molecules"

increasing the energ needed to separate the moleculesand therefore higher melting and boiling points!

fluorine

atomic radius ; 72 < -,-2m

"oiling point ; --=>$

iodine

atomic radius ; --? < -,-2m

"oiling point ; -=7>$

1an der

8aals forces

Trends in electronegati1ity


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Trends in electronegati1ity

#lectronegati%it of the halogens decreases down the group

due to an increase in atomic radius!

fluorine

atomic radius ; 72 < -,-2m

electronegati1ity ; 7@,

iodine

atomic radius ; --? < -,-2m

electronegati1ity ; 2@?

6ncreased nuclear charge has no significant effect because

there are more electron shells and more shielding! 6odine

atoms therefore attract electron densit in a co%alent bond

less strongl than fluorine!

Astatine


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Astatine

The name astatine comes from the ?reek word for unstable!

6t was first made artificiall in 1940" b bombarding 209Bi with

radiation! :hat do ou predict for these properties of

astatine@

+statine e'ists in nature in onl %er tinamounts! 6t is estimated that onl 30 grams of

astatine e'ist on #arth at an one time! This is

because it is radioacti1e" and its most stable

isotope 2-,At has a halflife of onl * hours!

electronegati%it!

state at room temperature

colour


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%eactions of the halogens


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%eactions of the halogens

/alogens react with metals such as sodium and iron;

The also take part in displacement reactionswith halide

ions" such as the reaction that is used to make bromine from

potassium bromide in seawater;

halogen * hydrogen hydrogen halide

The also react with nonmetals such as hdrogen;

halogen * sodium sodium halide

chlorine *potassium

"romide

potassium

chloride"romine *

%eaction &ith iron


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%eaction &ith iron


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%edox reactions of halogens


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%edox reactions of halogens

8hat is the reacti1ity of the halogens:


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8hat is the reacti1ity of the halogens:

The reactions of the halogens with iron and hdrogen show

that their reacti%it decreasesdown the group!

/ow do ou think fluorineand astatinewould react with

iron wool and hdrogen@

6ron wool burns and

glows brightl!

6ron wool has a %erslight glow!

6ron wool glows but less

brightl than with chlorine!

chlorine

"romine

iodine

.alogen %eaction &ith iron

&ool

%eaction &ith

hydrogen

#'plodes in

sunlight" reacts

slowl in the dark!

>eacts slowl on

heating with catalst!

>eacts partialland %er slowl!


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.alogen displacement reactions


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g p

/alogen displacement reactions are redoxreactions!

$l2* 2r 2$l* r2

To look at the transfer of electrons in this reaction" the

following two half eAuations can be written;

hlorine has gained electrons" so it is reducedto l ions!

:hat has been o'idi-ed and what has been reduced@

2r r2* 2e$l2* 2e

2$l

Bromide ions ha%e lost electrons" so the ha%e been

oxidizedto bromine!

+xidizing a"ility of halogens


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g y g

fluorine

increas

ingo

xidiz

inga

"ility

iodine

"romine

chlorine

6n displacement reactions between

halogens and halides" the halogen

acts as an oxidizing agent!

This means that the halogen;

:hat is the order of o'idi-ing

abilit of the halogens@

is reducedto form the halide ion!

gainselectrons

oxidizesthe halide ion to thehalogen

+xidizing a"ility of halogens


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g y g


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%eaction of chlorine &ith &ater


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hlorination of drinking water raises Auestions about indi%idual

freedom because it makes it difficult for indi%iduals to opt out!

hlorine is used to purif water supplies

because it is to'ic to bacteria" some of

which can cause disease! +dding it towater supplies is therefore beneficial for

the population!

/owe%er" chlorine is also to'ic to humans"so there are risks associated with gas leaks

during the chlorination process! There is

also a risk of the formation of chlorinated

hdrocarbons" which are also to'ic!

leach and the chlorate() ion


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/ousehold bleach commonl contains the chlorate() ionB

$l+" in the form of sodium chlorate6" 7al!

$l+* .2

+ * $l* 2+.

The chlorine has been reduced because it has gained

electrons! 6ts o'idation state has decreased from =1 in

lto 1 in l!

/ow man electrons are needed to balance this eAuation@

The chlorate6 ion beha%es as an oxidizing agent! 6t o'idi-es

the organic compounds in food stains" bacteria and des!

/as the chlorine been o'idi-ed or reduced in the reaction@

2e-


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.alides


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:hen halogens react with metals" the form compounds

called halides! .an naturalloccurring halides ha%e

industrial" household and medical applications!

caesium chloride

sodium

he'afluoroaluminate

titanium6C chloride

lithium iodide

potassium bromide

.alide Formula Cses

sl

7a+l


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dentifying halide ions


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.ydrogen halides


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The hdrogen

halides are

colourless gasesat room

temperature!

/drogen fluoride has an

une'pectedl high boiling point

compared to the otherhdrogen halides! This is due to

hdrogen bonding between the

/< molecules!

.ydrogen halide oiling point (>$)

/

Documents

2.7 the Periodic Table - Groups 2 and 7