Course: B.Sc. Microbiology/Biotechnology/Biochemistry

Sem II Sub: Inorganic Chemistry

Unit 2

Periodic properties

Atomic RadiusAtomic Radius

Radius

Atomic radius is the distance from the center of an atom’s nucleus to its outermost electron.

Atomic Radii

• The atomic radius : size of the atom.• determine : finding half the length between

the nucleus’ of two atoms of the same elements.

Ato m

ic Ra

di us i nc re

a ses

Atomic Radius increases

HLiNa

K

Fr

F

At

Atomic RadiusAtomic Radius

Atomic Radii

0

0.05

0.1

0.15

0.2

0.25

1 2 3 4 5 6 7 8

Element (atomic number)

Rad

ius

(nm

)

Series1

Atomic RadiusAtomic Radius

Trend #1Trend #1Atoms get larger going down a group

Hydrogen (1s1)

Lithium (1s2 2s1)

Sodium (1s2 2s2 2p6 3s1)

Shielding EffectShielding Effect

Atomic RadiusAtomic RadiusTrend #2Trend #2

Shrinking EffectShrinking Effect

5p+ 6p+ 7p+

1s2 2s2 2p1

1s2 2s2 2p2

1s2 2s2 2p3

Trends within periods

• The size of atoms (and therefore the atomic radii) increase as we go down groups on the periodic table (since more energy levels are added), BUT as we go to the right, the size decreases because…

• Electrons are being added to the same shell, which does not change the size of the atom, but protons are also being added to the nucleus, which pulls the electrons inwards towards the center, decreasing the radius.

Look at the trend within the periods.

Ionic Radius

• The ionic radius is the radius of an atom’s ion.• When an atom gains or loses electrons, it

becomes an ion. • Because an electron is negatively charged,

atoms that lose or gain acquire a net charge– If an atom loses electrons it becomes _________– If an atom gains electrons it becomes _________

Ionic Radius Continued

• atom - loses electrons : become smaller.• nucleus : able to pull them closer.

• atoms - gain electrons : become larger.

• Silicon4+ : (the atoms lose more and more electrons, making the pull from the nucleus stronger each time).

• Silicon4- : the atomic radius increases by a lot, because it has 4 extra electrons that repel, forcing the ion out (and therefore getting bigger). Then the ionic radii continues on it’s decreasing pattern, because the ions after silicon each have one less extra electron then the previous element, and therefore push out (repel) less.

Look at the trends for ionic size as you move across a period.

Ionization EnergyIonization Energy

Energy needed to remove one of atom’s electrons from its outermost shell

A + E A+ + e-

Reflection of how strongly an atom holds onto its outermost electron.

Atoms with high ionization energies hold onto Atoms with high ionization energies hold onto their electrons very tightly.their electrons very tightly.

Atoms with low ionization energies are more Atoms with low ionization energies are more likely to lose one or more of their outermost likely to lose one or more of their outermost electron.electron.

Ion

iza

tion

en

erg

y in

crea

ses

Ionization energy increases

HLiNa

K

Fr

F

At

Ionization EnergyIonization Energy

• first ionization energy : from neutral atoms to cations with a 1+ charge.

• second ionization energy : form 2+ cations from 1+ cations:• M+(g) M2+(g) + e-

• third ionization energy : form 3+ cations:• M2+(g) M3+(g) + e-

- IE : positive numbers because energy must be supplied (an endothermic energy change) to separate electrons from

atoms. .

Periodic table:

IE : decreases from top to bottom in groups, and increases from left to right across a period.

He : largest first ionization energy francium : one of the lowest.• From top to bottom :.• orbitals - higher values of the principal quantum number (n),

which : further away from the nucleus. • Since the outermost electrons are further away, they are less

strongly attracted by the nucleus, and are easier to remove, corresponding to a lower value for the first ionization energy.

• From left to right : more protons are being added to the nucleus, but the number of electrons in the inner, lower-energy shells remains the same.

• The valence electrons feel a higher effective nuclear charge — the sum of the charges on the protons in the nucleus and the charges on the inner, core electrons.

• The valence electrons are therefore held more tightly, the atom decreases in size , and it becomes increasingly difficult to remove them, corresponding to a higher value for the first ionization energy.

•

Metallicity

Ability of an atom to lose an electron

TREND:Increases from top to bottomDecreases from left to right

Electron AffinityElectron Affinity

A + e- A- + E

Measure of an atom’s attraction, or affinity, for an extra electron.

Energy released when an atom gains an electron to form a negative ion/anion.

increases within a period from left to right.

- As one goes down a group, electron affinity decreases.

• Periodic Trends :• increases : left to right across the periodic table, from the alkali

metals to the halogens.• small changes in the electron affinity are observed as you move

down a group.• electron affinity : measure of how stable the products are with

respect to the reactants. • products - much more stable, a large amount of energy will be

released during the process and EA will be a large negative number.

• reactants - much more stable than the products, then it becomes very difficult to add an electron and the EA will be positive.

• Halogens (group 7A, F to At) : Most negative EA values, addition of an e- leads to noble gas configuration, very favorable.

• Group 5A (N to Bi): filled shell discourages addition of an electron, EA values less negative than neighbors (groups 4A & 6A).

• Alkaline Earths (group 2A, Be to Ba) : Filled s-subshell discourages addition of an electron, EA values nearly zero.

• Noble Gases (Group 8A, He to Rn) : Completely filled shell strongly discourages addition of an electron, EA values are positive.

ElectronegativityElectronegativity

Ability of an atom to attract electrons in a chemical bond.

Ele

ctro

ne

gativ

ity in

cre

ase

s

Electronegativity increases

HLiNa

K

Fr

F

At

Periodic table:

• Move to the right : electronegativity increases. • valence shell of an atom is less than half full -less energy to

lose an electron than gain one and thus, it is easier to lose an electron.

• Conversely, when the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.

• Move down a group: decreases. • because the atomic number increases down a group and thus

there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.

• Important exceptions :noble gases, lanthanides, and actinides.

• noble gases : complete valence shell .• lanthanides and actinides : more complicated chemistry that

does not generally follow any trends.

• Therefore, noble gases, lanthanides, and actinides : electronegativity values.

• for transition metals : while they have values, there is little variance among them as you move across the period and up and down a group. This is because of their metallic properties that affect their ability to attract electrons as easily as the other elements.

Ele

ctro

ne

gativ

ity in

cre

ase

s

Ele

ctro

n a

ffini

ty in

cre

ase

s

Electronegativity increases

HLiNa

K

Fr

F

At

Ion

iza

tion

en

erg

y in

crea

ses

Ionization energy increases

Electron Affinity increases

Summary of Periodic Summary of Periodic TrendsTrends

Ato m

ic Ra

di us i nc re

a ses / i o

nic

s iz e

Metallicity inreases

Atomic Radius increases/ Ionic size

Me

talli cit y in

cr eas es

• Summary of Periodic Table Trends• Moving Left → Right• Atomic Radius Decreases• Ionization Energy Increases• Electronegativity Increases• Moving Top → Bottom• Atomic Radius Increases• Ionization Energy Decreases• Electronegativity Decreases

Look at the trends in ionic size as you move down a group

References

• Essentials of Physical chemistry by Bahl Arun, S Chand, 2012• General chemistry by Ebbing Darrell D, 5th, A I T B S Publishers, 2002