Keep It Simple Science 2 - Metals

8/13/2019 Keep It Simple Science 2 - Metals

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Patterns of the Periodic Table

In Mendeleev's day no-one could explain why these patterns existed.

However, when scientists see patterns in nature like this, they know there must be underlying "r ules" or

"laws of nature" causing and controlling the patterns.

Perhaps Mendeleev's great contribution was not just the Periodic Table itself,

but the stimulus it gave other scientists to investigate the reasons behind the patterns.

Within 40 years Science had unravelled the secrets of atomic structure, the electron energy levels, and more.

At this sta e our task is to lear n s ome of th e atter ns.

Electrical Conductivity

As you go across any row ("period") of the table, you will

move through anumber of metals, then one or two semi-

metals, then into the non-metals.

Therefore, the conductivity will start out high, but r apidly

decrease as you encounter a semi-metal, and becomeextremely low at the non-metals.

r!, I

CLJ\ I i

llj~H~~tJTEI1:1

Boiling Points

follow a similar pattern to

Melting Points

Valencies are the same

down each group

Melting Point

You learned in topic 1 how melting point is determined by the

bonding within a substance.

At the left side of the table are the very active metals of d1e

Activity Series. They are also usually soft, and have relatively low

(for metals) melting points.

Moving to the right across a period you enter d1e "Transition

Block" containing typical hard, high melting point metals, held

strongly together by "metallic bonding".

Further right you hit d1e Semi-Metals. These often have very high

melting points because of their covalent lattice structure.

Then you enter d1e Non-Metals which have covalent molecular

structures and quite low mp's. At the far right column, each period

ends with an Inert Gas which are all s ingle-atom molecules, and

have d1elowest mp of each period.

This pattern repeats itself along each period.

Melting Points of ElementsI

I

I

I

Peaks are Transi ti on M etals

or Semi-Metals

Sketch Graph.ooo

N

Go~ooo... -

c-

oc..01

0 Na

c

Chemical Bonding, Valency & Reactivity

What you've already learnt about the Activity Series, Ionic and Covalent Bonding and Valency

will help you make sense of the following: ("" G 8 I Group nert ases

No chemical reactions,

no bonding

Activity of Non-Metals

Most active at top-right(FIuori ne)

Activity (generally)decreases downwardsand to the left.


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Atomic Radius

The s ize o f an atom is the distance across its outer electron shell.

You might think that the atoms along each period would be the same size,

because it's the same orbit being added to.

However, the increasing amount of positive charge in the nucleus pulls that

orbit inwards closer and closer to the centre.

1

0.1

~I

Li 0,1

0152~:

01

"IOJ

.~ 1

~I~Iu

Na .~: MQ AI Si P

O]lO 600------------------------------------~Radius decreasing across a periodCa197

o

Be112

o

K231

The Syllabus requires that you

produce a table and a graph of

the changes in a property

across a period,

and down a group

When you do, you can clearly see

how the Periodic Table got its

name.

"Periodic" means "recurring at

regular intervals".

This graph shows what aspreadsheet plot gives for the radii

of the first 37 elements.

o tice h ow the s ame g raphical

pattern keeps recurring ... it is a

periodic pattern.

The following diagramsare to scale and show therelative sizes of the first

20 elements

The numbers given are the atomic radii in picometres.

1 picometre = 1xl 0-12

metre

He50

oNe70

o o o o ooS102

Ar94

o o oDown each group the radius increases.

This is because, as you go down a group, you have added an entire

electron shell to the outside of the previous layer

Spreadsheet Plot of Atomic Radii0

aJ 0

m.....-Q)

E0 0u 0C. N.....,III

::J

"'00ro0a::

u

E0...-

0

Kr

Ar -----------~Ne ------------. Tfend

He _-------- Increasing up_------ down a gfO

20

Atomic Number

There are a number of irregularities and "glitches"

apparent on the graph. It is beyond the scope of

this course (and way beyond the K.ISS. Principle)

to attempt an explanation of these.


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Highest Value Inert gases

..._....., Fluorine ~ot included

'21 I I r-1,~ 1 1; :::::::::__ ._ EI .. V 1 I , Io. ( J.' ectronegatlvlty a ues r..--.!-....~.."'-"-T-"-'''['-'-''~-''-j

~: ~~ ~~ of selected elements 1~~1~~':~~::~';~~Or._j~IL_:..J..~_~-------------t-t- -1- [3.0! i...1 I r 1r. (values decrease to left) -,r---T-_ -+-~--+l~ ,0.81 ! I" I 2 8\

-0 1 I' -----{ t---,-.- ..t-+-l'

l-",-1- :1-..-1'.----.1.---1~--f- -1-.- -:..l .._~

VlI 0.8 Ii : I ' i j, I !2.5 i~:ro~)--lr--1----:-- ..I..--.. --- ---+-- -"II--r+-+-1+1;:;1--,~tr;:;-~-'-lr--r-'-+-,-- _+_~ L....JL.J..~L---,--_..1..._J

L__1.._..!I '--_L-.I _..I-_ J

keep it simple science

Ionization Energy

The meaning of tlle "1st Ionization Energy" was explained

previously in relation to the Activity Series of Metals.

+A (g)

where "A" stands for any atom

in the gas state

Any atom can lose an electron if enough energy is

supplied... even atoms which do not normally lose

electrons.

The Periodic Trend in 1st Ionization Energy

You should remember that the very active metals are the

ones with low 1st ionization energies. They easilylose their

outer electron(s) and so react readily.

Explanations:

1st I.E. increases to the right because each atom across a

period has more and more (+ve) nuclear charge attractingand holding electrons in the orbit concerned. Therefore, it

requires more energy to remove an electron.

1st I.E. decreases down each group because, at each step

down, anextra whole layer of electrons has been added to the

outside of tlle atom. The outer shell is further away from the

nucleus, and is partially "shielded" from nuclear attraction by

the layers of electrons underneath it. Therefore, it becomes

easier and easier to remove an electron.

Electronegativityis avalue assigned to each element to describe tlle

power of an atom to attract electrons to itself.

Atoms with a tendency to gain electrons and

form negative ions have high values.

Atoms with a tendency to lose electrons easily

Qow 1st I.E.) and form (+ve) ions have very low

values.

Once again, there is a pattern in these values in

the Periodic Table.

Northmead High School SL#603217

Successive Ionization Energies

Having added the energy o f 1st I.E. an d r emoved an

electron from any atom, it is then possible to add more

energy and remove a2nd electron, and a 3rd, and s o on.

A(g)

+1st I.E. A (g) + e

2nd I.E. A + ----.. A+2 + e

(g) (g)

3rd I.E. A+2 ~ A+3 + e

(g) (g)

...and so on,

according to how many electrons

the atom has

Once the f irst electr on is removed, tl1e remaining electrons

are pulled in t ighter to the nucleus. Each one experiences

increased force of attraction, so it requires more energy to

remove the next electron.

Once the entire outer orbit has been stripped away,the next

ionization must remove an electron from an underlying

orbit, which requires ahuge increase in the next ionization

energy. This results in an interesting pattern:

Patterns in Successive Ionization Energy Data

(values shown are energy measurements)

Successive Elements on Period 3

Element Electron 1st 2nd 3rd 4th

Config. I.E. I.E. I.E. I.E.

Sodium 2.8.1 0.5 4.5 6.9 9.6

Magnesium 2.8.2 0.7 1.4 7.7 10.5

Aluminium 2.8.3 0.6 1.8 2.8 11.6

Notice how the values "jump" (underlined data) as the next

ionization has t o remo ve an electron from the next lower

orbit.


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As early as 1830, the German a) .

noticed patterns in the properties of the elements. In 1860,

the English scientist b) proposed a

"Law of c) " d es cr ibing the

repeating pattern of properties.

It was the Russian d) who invented

the e) , in more or less its

modern form. He realized that there were probably many

elements that had not f ) ,

so he g) in his table for later

additions . By s tudying the details of k no wn elements, he

w as able to h) very precisely the

properties of the missing elements.

Sure enough when discovered, the missing elements were

found to have properties i) .

Conductivity, which generally j) to

the right, as you go from metals to k) .

and .

Melting Points: tend to l) to about the

mid dle of each p eriod, then m)............................. The

highest value is usually a n) metal or

o ne of the 0) elements. The

lowest value on each period is always the

p)................................ gas member on the extreme

g) (right/left)

Valencies are r) down each vertical

group. Bonding follows the pattern of the main categories

of elements. s) form t) .

bonds when they lose electrons and become u) .

ions. The Semi-metal elements form only v) .

bonds. The Non-metals can bond w) .

or can x) electrons to form y) .

l0ns.

Chemical Reactivity is different for metals and n on -

metals. The most ac tive metals are located at the lef t

z)................................... (top/bottom) of the table.

Generally, activity decreases aa) and to tile

ab) The Inert Gases show no chemical

activity. Apart from them, the most active non-metals are

located on the r ig ht ac) (top/bottom)

of the table. Activity generally decreases as you move

ad) and .

WHEN COMPLETED, WORKSHEETS

BECOME SECTION SUMMARIES

Atomic Radius ae) across a period

because each successive element h as af ) .

(more/less) positive charge in the ag) to

attract the electron shell and pull it inwards. As yo u g o

down a group the radius ah) as each new

electron shell is added.

First ai) Energies aj) .

across a period, as the increasing amount of nuclear chargemakes it more and more difficult to ak) .

an electron. The values al) down a

group because each extra shell of electrons is am) .

(more/less ) s trongly held than the previous.

Successive Ionization Ener gies measure the energy

required to an)............................ another, subsequent

electron from an atom. T he energy required to remove the

next electron is always ao) .

(higher/lower). When the next electron happens to b e in

the next lower shell, the value ap) .

by a huge amount.

ag) is a value which describesthe power of an atom to ar) electrons.

The element with the highest value is as) ,

and values decrease as you move to the at) .

and asyou move au) the Periodic Table.

1. a) Write equations to represent the 1st, 2nd, 3rd & 4th

ionisations for a calcium atom.

b) Between which two o f these successive ionisations

would you expect a h uge increase in the required energy?

2. On each of the following Periodic Table diagrams label

the arrows with the word "increasing" or "decreasing" tocorrectly describe the trend in the direction shown.

a) Atomic

Radius

r;

I;o""P), (right)I}-t-~ - - - - - - - - ,

;;)(d?~J~:HffillE!b) Electro-

negativity

Also indicate

("H"&"L") the

position of

elements with

highest &lowest values.


5/9

207.2grams of

Lead

, contains.; ,. 6.022 x 1023 39.95 grams of

,. Lead atoms Argon

1~'(~1 39.95 207.2 l...- ..J 6.~~~t:i~;23 12.01grams ofI

_~~". C b1 1 " ~ . Argon atoms ar on

mo e, 1 mole.... 1 mole,. contains

= 12.01grams' = 39.95 grams"" = 207.2 grams .,...,. "'" 6.022 x 1023', _--_... ,., .; Carbon atoms

E AC H OF T HE SE HAS H~ SAME NUMBER OF ATOMS ". ".

... ~'" e

. 16 .,. .."Copyright 2005-2006

"......... ~----........_-_ ..... - ....

Quantities in Chemical Calculations

Atoms, molecules and ions always react with each other in

fixed, whole-number ratios. That's why balancing an

equation is so important... it actually brings the equation

into line with what is happening at the particle level.

For example, when hydrogen and oxygen react to form

water, the balanced equation is

This is a t r ue description of what is happening to the

molecules:

enCD

m

2 Moleculesof H20

However, when we carry out chemical reactions in the

laboratory or in Chemical Industry, we cannot see or count

the molecules. Instead, we measure the mass or volume of

substances.

To measure out the correct numbers of particles for a

reaction we need a simple w ay to convert masses and

volumes to numbers of molecules, and vice-versa. That's

the purpose of

The Mole

1 mole of a ny element or compound contains exactly the

same number of particles.

1 mole of each substance has a different mass, because the

atoms and molecules allweigh differently.

The really clever and convenient thing about the mole is its

link to the Periodic Table and the "Atomic Weights" shown.

6

CCarbon

18

AtArgon

82

PhLead

Defining the Mole

For technical reasons, the "atomic standard" used to

compare the masses of all atoms is the carbon atom,

which contains

6protons

6 neutrons

6 electrons

Since this is the standard of comparison, the formal

definition of the mole is:

"the number of atoms contained inexactly 12 grams of carbon-12"

Note: In Topic 1 it was pointed out that the Mass

Number for any atom is a whole number. It has still not

been explained why the ' 'Atomic Weights" in the

Periodic Table are mostly not whole numbers.

This WILL be explained in a later topic.

If you cannot wait, go find out about "Isotopes".

Avogadro's Number

Just how many atoms are in 1 mole?

Obviously, it is a very large number. We now know that it

is about 6,000 billion trillion.

Avogadro's Number

6.022 X 1023

particles in 1 mole of anything

This number is named in honour of an Italian scientist

who you will learn about s oon.


6/9


Calculating Mole Quantities

You need to be able to calculate mole quantities in terms of

both mass and number of particles.

Molar Mass

The "Molar Mass" of any chemical species is the mass (in

grams) of 1 mole of the substance.

You need to add up all the Atomic Weights

of all the atoms given in the formula.

Examples:

Name

Argon

Sodium

Formula

Ar

Na

Molar Mass (g)

39.95

22.99

Oxygen

CWorine

(16.00 x 2)

(35.4 5 x 2)

32.00

70.90

Water

Carbon Dioxide

Sodium chloride

H20 (1.008x2 + 16.00)

CO2 (12.01 + (16.00x2)NaCl (22.99 + 35.45)

18.016

44.01

58.44

Number of Moles in a Given Mass

When you weigh a chemical sample you then need to be

able to calculate how many moles this contains.

No. of moles = mass of substance vou havemolar mass

n= m

MM

Example Calculations

1. How many moles in a) 5.23g of magnesium?

b) 96.7g of water?

a) n = --ill..-

MM

5.23 = 0.215 mol24.31

b) n = -- ill..- _96_.7 _

MM (2x1.008 + 16.00)= 96.7/18.016= 5.37 mol

2. What mass is needed if you want to have 1.50 molesof s alt (sodium cWoride)?

n = --ill..-

M:M

so m = n x M M = -1.50 x (22.99 + 35.45)= 1.50 x 58.44

= 87.7 g


Moles and Numbers of Particles

Since one mole of any substance contains Avogadro's

Number of particles:

No. of moles = No. of particles you haveAvogadro's Number

Example Calculations

1. How many moles are present in a sample of lead

containing 7.88 x 1024

atoms?

7.88xl024

---23

6.022xlO

= 13.1 mol

2. a) How many atoms of lead are needed to make

0.0250 mole?

b) What would be the mass of this quantity?

Solutiona)n=~

NA

b) m = n x M1vl= 0.0250 x 207.2 (molar mass of Pb)= 5.18 g

23so N = n x NA = 0.0250 x 6.022x10

= 1.51 x 1022 atoms

Mole Quantities in Chemical Equations

When you consider an equation like

CD

C()

m2 Molecules

of H20

However, the number of molecules reacting is really just a

ratio. The actual numbers migh t b e

or, (let's use Avagadro's number)

(2 x NA) H2 + NA 02 --.- (2 x NA) H20

The Balancing Coefficients in a Chemical Equation

May be Interpreted as Mole Ratios


7/9


8/9


Comparing Mass Changes

When Metals Burn

Atoms always react in simple whole-number mole ratios,

but atoms have different masses, and compounds have

various formulas, so the result is that chemicals do NOT

react in simple ratios by mass.

That's why we need the mole... we measure quantities by

their mass, b ut this mak es no s en se u ntil moles are

calculated.

The syllabus requires that you should consider the mass

changes involved when various' metals combine with

oxygen to form their oxide compound.

The following table shows the mass changes for ;lOOg of

the metal in each case:

100g of Formula Mass 0z Mass of

Metal of oxide n eeded( g) O xide fo rmed

Lithium Liz 115 215

Iron FeZ

03

43 143

Zinc ZnO 49 149

Lead PbOz 15 115

Empirical Formulas vMolecular Formulas

Y ou are r emin ded that a molecular formula really does

describe the atoms present in a molecule.

The molecular compound methane,

has formula CH4

, because that's

exactly what each molecule contains ...

1 carbon atom and 4 hydrogen atoms.

Lattice structures, either ionic or covalent

are NOT molecular.

Example: sodium chloride, NaCl

The formula does NOT

describe a molecule, but only

gives the simplest ratio between

the bonded atoms ... this is an empirical formula.

On the previous page was an example of how formulas are

determined by analysing the mass composition of a

compound.

You should note that this metho d can o nly produce an

empirical formula. (In fact, the w or d "empirical" means

something determined by experiment, not by theory.)

If a molecular compound, with molecular formula XzY6

was analysed by mass measurements, its empirical formula

would be calculated t o be :>"''Y3

... simplest ratio of atoms.


A Little History ...

How the Mole was Invented

The "mole" a s a m easure of chemical quantities, is a

mathematically convenient device (a "trick") to help

chemical calculations.

Gay-Lussac's Law

Joseph Gay-Lussac wa s a French sc ie nt ist wi th an

unfortunat e name, bymodern standards. He lived 200 years

ago, and was very interested in flight using balloons, so he

investigated the way gases react chemically.

After a series of clever experiments, in which the volumes

of reacting gases were measured, in 1808 he propo sed the

"Law of Combining Volumes":

When measured

at constant temperature and pressure,

the volumes of gases in a chemical reaction

show simple, whole-number ratios

to each other.

The volume of a gas is easily changed by temperatur e and

pressure, so it is very important that the volumes are all

measured at the same conditions.

Hydrogen(g) + Chlorine(g)~ Hydrogen chloride(g)1 litre 1 litre 2 litres

Hydrogen(g) + Oxygen(g)2 litres 1 litre

. W ater (g) (vapour)

2 litres

Hydrogen(g) + Nitrogen(g) --.. Ammonia(g)3 litres 1 litre 2 litr es

Notice that in every case, t hat the volumes are alwaysi n a

simple, whole number ratio to each other.

Now con sider the balanced equ ation s fo r these thr ee

example reactions:

The mol e ra ti os a re t he same as the vo lume r atios

discovered by Gay-Lussac!

enter .Avogadro!


9/9


Avogadro's HypothesisThe Italian, Amadeo Avogadro (1776-1856) was trained in

Law, but became very interested in Science.

In 1811, he noticed the similarity between Gay-Lussac's

Law (an empirical "law" based on experiment) and the

concept that atoms must combine in simple, whole number

ratios to form compounds.

Equal Volumes of all Gases

Contain Equal Numbers of Molecules

(when measured at the same conditions

of temperature and pressure)

Prior to Avogadro, it was assumed that the the reaction

involved single atoms, like this:

Hydrogen{g) + Chlorine(g) ~ Hydroge n chloride (g)1 volume : 1 volume 2 volumes

Now, reasoned Avogadro, gases react in simple, whole-

number volume ratios because each litre of gas has the

same number o f molecules in it. Therefore, to get the

volume ratios shown above, each hydrogen molecule, and

each chlorine molecule, must have 2 atoms!

i.e. Hydrogen is H2(g) and Chlorine is Cl2(g)' and the correct

equation is

Then, for the same reaction, scientists could measure the

masses of these gases as well as volumes. This showed that

chlorine atoms must be about 35 times heavier than

hydrogen ... chemists were on the way to figuring out the

relative atomic weights of elements, and being able to

calculate chemical quantities.

Although he did not invent the concept of the mole, we

name the number of particles in 1 mole i n Avogadro's

honour ...


Molar Volume of a GasIf 1 mole of any chemical species contains the same

number of particles (Avogadro' s Number) AND if equal

volumes of gases contain equal number of particles

(Avogadro'S Hypothesis), then it follows that

1 mole of any gas must occupy the same volume,

if measured at the same temperature and pressure.

This volume is the "Molar Volume" and is the same for

every gas. It is measured at the standard set of conditions

known as Standard Laboratory Conditions (SLC); 25C

and 1 standard atmosphere of pressure.

Mole Calculations Involving GasesThis additional knowledge opens up the opportunity to

carry out quantity calculations which involve mass and

volumes of gases.

Example Problems1.

If 15.65g of calcium carbonate (CaC03) was completely

decomposed by heat, what volume of carbon dioxide

gas would be produced (if measured at SLC)?

Solution

Always begin with the balanced equation for the reaction.

CaC03(s) . CO2(g) + CaO(s)mole ratio = 1 : 1 : 1

Moles of CaC03: n =..J.!L = 15.65 = 0.1564 molj\1},,f 100,09

Mole ratio is 1 :1, so moles of CO2

formed = 0.1564

~

:. Volume of CO2 = 0.1564 x 24.8...... Molar Vol.

= 3.88 L (at SLC) of all gasesat S LC

2.

What volume of hydrogen g as ( at S LC) wo uld be

produced if 10.00g of lithium metal was reacted with

sulfuric acid?

Solution

2 Li(s) +2 :

. H2(g) + Li2SO4(aq)1 1

Moles of lithium: n = ..J.!L = 10.00 = 1.441 mol

MM 6.941

Mole ratio is 2:1, so moles of H2

= 1/2 X 1.441=0.7204

:. Volume of Hz = 0.7204 x 24.8

= 17.9 L (at SLC)

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Keep It Simple Science 2 - Metals