Hydrogen: The Unique Element in the Periodic Table, Lecture notes of Biochemistry

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PERIODIC TABLE OF THE ELEMENTS

Chapters 14 and 23. PROPERTIES OF THE ELEMENTS First, let’s look at Periodic Table and Group numbering. 14.1 Hydrogen. 14-

The simplest atom, ~90% of all atoms in the universe. The only element whose isotopes are each given a different symbol andname.^1 H (or H, protium) = one proton (p

+-), plus one e

surrounding it.

2 H (or D, deuterium) = one p

+^ and one neutron (n), plus one e

3 H (or T, tritium) = one p

+^ and two n, plus one e

(^2) Deuterium ( H) was produced in the ‘Big Bang’ — it is too fragile to survivefusion conditions in the stars (which produce the lighter elements) orsupernovas (which produce heavier elements).Hydrogen is the exception in the periodic table — it cannot besatisfactorily classified in any group: it has similarities both to (a) group 1metals such as Li, Na, etc, in forming H

+^ and (b) group 17 non-metals such

as F, Cl, etc, in being H

(H-H with a single covalent bond) in its stable 2 elemental form (compare F

, Cl, etc) and also forming H 22

-^ (hydride ion)

analogous to F

• -, Cl, etc.

Covalent Hydrogen Compounds 14-

.^ Very common and stable: CH

, NH^ , H^ O, 4 3 2

HF, etc, etc. These other elements have higher electronegativity than H (H= 2.2, C = 2.5, N = 3.1, O = 3.5 F = 4.1) ^ we think of these as containing H

+^ oxidation state and C

4-3-^ 2- , N^ , O^ , F

oxidation states.

e.g.^ F^ (g) + H^2

(g)^ →^ 2HF (g) 2 His a very important gas, for many reasons. For example:^2

N(g) + 3 H^2

(g)^ ^ 2 NH^3

(g)

G < 0 (spontaneous) but very slow under normal conditions due to verystrong NN^

^ reaction run in industry at high T (~400 ºC) and pressure (250 atm) with an Fe catalyst to speed it up. This is called the

Haber

process , and is the main source of NH

for the fertilizer industry. 3

Metallic (Interstitial) Hydrides

.^ H^ molecules and H atoms can occupy^2 space in-between the atoms of a metal.

In particular, palladium (Pd) has a

high affinity and can hold vast amounts (Pd:

^ 935 times its volume = PdH

).0.^

Best thought of as a solution of the gas in the metal!

Came to people’s

attention during Cold Fusion stories of late 1980’s.Formation of Pd/H

is used to purify H 2

from gas mixtures. 2

Many transition metals form 14-

metallic (interstitial) hydrides

in which H

molecules and H atoms occupy the holes in the 2

Figure 14.2 crystal structure of the metal.

A metallic (interstitial) hydride

Table 14.1 14-

Trends in Atomic, Physical, and ChemicalProperties of the Period 2 Elements. Trends in atomic radius, ionization energy, and electronegativityacross Period 2.

GROUP 1. 14-

ALKALI METALS

(^1) (ns ) +^ Found as M in minerals (loss of ns

1 electron). Too reactive (strong reducing agents) to be found as the free metal.

Prepared in industry

by electrolysis of melted salts.All well-studied except Francium — radioactive, longest-lived isotope^283 is^ Fr: half-life of only 21.8 min. Estimated only ~25 g on Earth atany one time Properties.^

Soft, silvery metals at 20 ºC.

All react vigorously with

water to give H

gas: the reactivity increases down group. 2 M (s) + HO (l)^2

^ H(g) + MOH (aq)^2

(video)

half-reactions:

M (s)^ 

+^ - M (aq) + e

-^ HO (l) + e 2 ½H(g) + OH^2 -^ (aq)

overall:^ M (s) + H

O (l)^ ^ ½H 22

+^ (g) + M (aq) + OH

-^ (aq)

Important Question – WHY is Li different? 14-

The difference between Li and the rest

of group 1^ (e.g. gives oxide with O

, reacts with N 2

, etc) is due to the small size of Li 2

• leads to high charge-to-size ratio. This also causes Li compounds to have significantcovalent character (i.e. LiCl is still fairly ionic but has noticeable covalent character,whereas NaCl is very ionic). Li

+^ has too high a charge-to-size ratio (“charge density”) to be happy as completely free Li

+^ ion, so it shares electrons a bit with anions (i.e. some covalency).*** For this and other reasons (see later),

the top member of every group is significantly different from the rest *** Note 1:^ The decreasing charge-to-size ratio down the group (i.e. Li

+^ +^ +^ > Na> K> Rb

+^ >

+^ Cs) explains the products on reaction with O

. The highest charge density (highest 2

charge-to-size

• ratio) is Li,^ and^ this^ forms

the^ highest^

charge^ density

2-^ Osalt,

whereas^ down

the^ group^ the

metals^ form^

2-^ the O(peroxide) 2 and^ then^ the

• O 2

(superoxide) salts, i.e. decreasing charge density ions.i.e., high charge density Li

+^ favors forming a salt with high charge density O

2-, but

down^ the^ group

progressively

lower^ charge

+^ density M favor^ formation

of

progressively lower charge density anions. Note^ 2:^ Group

1 metals^ have

nd very high 2 Ionization^ Potential

2+^ -means Mnot

possible to make (in stable form at 25 ºC). Of course, in places such as the surface ofthe sun, all sorts of other ions are possible.

GROUP 2. 14-

ALKALINE EARTH METALS (ns

Occur in nature as M

2+^ ―^ too reactive to exist as free metals. Radium (Ra) radioactive:

226 Ra: half-life of 1599 years. Again, Be slightly different from the rest due to high charge-to-size ratio (cf. Li in group 1): very hard metal and toxic.

Others

softer and not toxic.

Also, Be compounds are mostly covalent, since 2+^ Be has too high a charge density to be happy as the free ion inionic compounds – therefore it shares electrons with other atoms,decreasing its charge density (i.e. covalent bonding).Be is unreactive to H

O; Mg reacts slowly with cold H 2

O, but 2

fast with steam; others react vigorously with H

O. 2

M (s) + 2 HO (l)^2

^ M(OH)^ (aq) + H^2

(g) 2

All react with O

to give oxides MO except Ba, gives peroxide BaO 2

Mg reacts with N

gas to ionic nitride, like Li. Others only at high T. (^2) 3Mg( s ) + N^ ( g^2 )^ →^ MgN( s )^32

(Mg + CO

st^ nd^1 and 2^ ionizations are both very easy

2+^  M (no M

rd^3 ionization very difficult

3+ ― no M

Section 14.5. GROUP 13. 14-

(^21) (ns np)

Metallic character decreases moving right, and we find that boron (B) is not ametal = “metalloid” or “semi-metal”.

Al video **** Metalloid = element with props between metals and non-metals **** BOis acidic (cf., Li 2 3

O is basic, gives OH 2

-^ in water).^ Down group, metallic

character increases - remainder of group 13 are metals but oxides of Al and Gaare amphoteric (see below), while those of In and Tl are basic. Compare : B :^ BO^2

(s) + 6 NaOH (aq)

^ 2 Na^ BO^ (aq) + 3 H^3

O (l) 2

acid^ base

salt

water ( cf.^ HCl (g)^ +

NaOH (aq)^ 

NaCl (aq)^

+^ H^ O(l) )^2

in water, forms B(OH)

(or H^ BO^ ), known as boric acid. (^3 3 3) B(OH) + 2 H^ O^ ⇌^ B(OH) 3 2

-^ +^ + H^ OpK 43

= 9.25a

Al, Ga : oxides react with bases as above, but also with acids. They are“amphoteric” (= can behave as both acids or bases)AlO^2

(s) + 6 NaOH 3 ^ 2 Na^ AlO^3

(aq) + 3 H^ O (l) 2 acid^ base

salt^

water

AlO(s) + 3 H^2

SO^ (l)^ ^ Al 2 4

(SO^ )^ (s) + 3 H 2 4 3

O (l) 2

base^ acid

salt^

water

In, Tl : oxides are basic and react only with acids.

In^ O(s) + 3 H^2

SO^ (l)^ ^ In 2 4

(SO^ )^ (s) + 3 H 2 4 3

O (l) 2

Base^ acid

salt

water

PERIODIC TABLE OF THE ELEMENTS

Uses :^ Plentiful. 14-

BOused in production of borosilicate glass.^23

B(OH)^ (boric acid)^3

used as disinfectant, eyewash, insecticide.

Na^ [BO(OH)^2

]8HO used in washing 42

powders.Boron hydrides B

Hvery important class of compounds.xy^

Figure 14.

Aluminum sulfate used in water purification, dye industry, antiperspirants, etc.AlOused as a support for industrial catalysts, chromatography supports, etc.^2 3 BN compounds similar to analogous C compounds.e.g. borazene, like benzene.Borazon (BN)^

has a similar structure to diamond and thus also very hard. Diagonal Relationships

:^ Be with Al (and B with Si) Be and Al have similar properties e.g.,^ similar covalency in their compounds.

Figure 14.8 14-

Some boron hydride structures (“boranes”) Borane is BH^3 Diborane

Allotropes of Carbon

C^60

C^70

multi-wallednanotube

Diamond^

Graphite

graphene – single sheet of graphitesingle-wallednanotube

Oxidation states 14-

:^ **** multiple ones now more common! ** C** :^ non-metal:

compounds all covalent except C

4-^ (carbide) ion e.g.

CaC.^ Oxide (CO^2

) acidic:^ almost all oxid. states from -4 to +4 ( 2

e.g.,^ CH^4

to CO^ ) known.^2 Si, Ge :^ metalloids – essentially all compounds are +4 ox. state, but a feware +2. Sn, Pb :^ metals

+4 (covalent)

e.g.,^ MO^2 +2 (ionic usually)^ e.g.,^ MO^

Figure 14.

C usually four-coordinate (exception C

O).^ Other elements show more exceptions ( e.g.,

2-^ SiF , GeCl^66 2-^ 2-^ , Sn(OH)^ , Pb(OH)^6

2-^ ) but still usually 6

four-coordinate.Note difference:

(^2) C has 2s2p 2 outer configuration with no available d orbitals.^ Remainder of elements have d orbitals they can use in bonding ^ can form six bonds (sp

32 dhybrids).