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PERIODIC PROPERTIES
LAWS OF PERIODIC TABLE
MODERN PERIODIC LAW
PERIODIC LAW
The physical and
The physical and
chemical properties
chemical properties
of elements are
of elements are
periodic function of periodic function
their atomic
of their atomic
numbers.
weights.
STRUCTURAL FEATURES
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Periods: A horizontal row in the periodic table
is called period. There are 7 periods and each
period starts with different principal quantum
number.
Groups: A vertical column in the periodic table
is known as group. There are 18 groups in long
form of periodic table.
EFFECTIVE NUCLEAR CHARGE
In multielectron atoms, the electrons in the outermost shell are
screened from the nucleus by the inner electrons. As a result,
the electron in the outermost shell does not experience the full
charge of the nucleus.
 The actual nuclear charge felt by electron is called the
effective nuclear charge.
 The effective nuclear charge can be determined by subtracting
a screening constant for the inner electrons from the actual
nuclear charge( atomic number).
Z* =Z- S
where Z* is effective nuclear charge
Z is nuclear charge(atomic number)
S is screening constant.
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SLATER RULES TO CALCULATE SCREENING
CONSTANT
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Write the electronic configuration of the element in the
following order and grouping: (1s), (2s,2p), (3s,3p), (3d), (4s,4p),
(4d), (4f), (5s,5p), (5d), (5f),…………
Electrons in any group higher in the sequence than the electron
under consideration contribute zero to screening constant.
Electrons in the same group contribute S= 0.35 each. ( if
electron is present in 1s it will contribute S= 0.30).
For an electron in ns or np orbital, all electrons in (n-1) shell
contribute S= 0.85 each and all electrons in (n-2) shell
contribute S= 1.0 each.
For an electron in nd or nf orbital, all electrons in lower groups
contribute S= 1.0 each.
ATOMIC RADII
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The term atomic radius means
the distance from centre of the
nucleus to the outermost shell
of electrons.
An atom gets larger as the
number of electronic shells
increase; therefore the radius
of atoms increases as you go
down a certain group in the
periodic table of elements.
In general, the size of an atom
will decrease as you move from
left to the right of a certain
period.
IONIZATION ENERGY
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Ionization energy is the quantity
of energy that an isolated,
gaseous atom in the ground
electronic state must absorb to
discharge an electron, resulting
in a cation.
H(g)→H+(g)+e−
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Ionization energies are
dependent upon the atomic
radius. Since going from right
to left on the periodic
table, the atomic radius
increases, and the ionization
energy increases from left to
right in the periods and up the
groups.
ELECTRON AFFINITY
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Electron affinity is defined as the
change in energy (in kJ/mole) of a
neutral atom (in the gaseous phase)
when an electron is added to the
atom to form a negative ion.
On moving down the group, electron
affinity decreases.
On moving across a period, electron
affinity increases.
ELECTRONEGATIVITY
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Electronegativity can be understood
as a chemical property describing an
atom's ability to attract and bind
with electrons.
From left to right across a period
of elements, electronegativity
increases. If the valence shell of an
atom is less than half full, it requires
less energy to lose an electron than to
gain one. Conversely, if the valence
shell is more than half full, it is easier
to pull an electron into the valence
shell than to donate one.
From top to bottom down a group,
electronegativity decreases. This is
because atomic number increases
down a group, and thus there is an
increased distance between the
valence electrons and nucleus, or a
greater atomic radius.
PERIODIC PROPERTIES IN PREDICTING CHEMICAL
BEHAVIOR
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Ionic character: ionic character depends upon
electronegativity difference between bonded atoms. The
greater the difference in the electronegativities of the bonded
atoms, the higher is the ionic character of the bond. As we
move down the group the degree of ionic character in halides
increases because of increase in electronegativity difference.
HF > HCl > HBr > HI
As a result of ionic and covalent character , their properties
also change. For example, AlF3 is essentially ionic. AlCl3 has an
intermediate character and AlBr3 and AlI3 exist as covalent
dimers. Therefore, the melting point of these halides decrease
from AlF3 AlI3. AlF3 and AlCl3are conductors in fused sate
(ionic character) while AlBr3 and AlI3 are non-conductors
(covalent character).
• Oxidising and reducing character: Oxidation is a process in
which electrons are lost and reduction is a process in which
electrons are gained. Thus, an oxidising agent is a substance
which gains electrons while reducing agent is a substance which
loses electrons. Since the elements on the left of the periodic
table have low ionization energy values and therefore, have good
tendency to lose electrons. Consequently, they act as strong
reducing agents. On the other hand the elements on the right
hand side of the periodic table have great tendency to accept
electrons and therefore they act as good oxidising agents.
•Now , since ionization energy values decrease on moving down a
group therefore, reducing character of elements in a group
increases down a group. Conversely, the oxidising character of
elements decreases on moving down a group.
•As we move along a period from left to right, the ionization
energy increases and consequently, reducing character
decreases while oxidising character increases.
• LITHIUM IS THE STRONGEST REDUCING AGENT WHILE
FLOURINE IS THE STRONGEST OXIDISING AGENT.
• OXIDATION STATES: Like periodic properties, the
oxidation states of elements are closely related to the number
of electrons in the valence shell of their atoms. Oxidation
states of different elements correspond to the number of
electrons gained or lost by their atoms to acquire a complete
shell of eight electrons, ns2 np6.
Variation of oxidation state in a period: as we move along
a period , the positive oxidation state increases and negative
oxidation state decreases. For example, in second period, the
oxidation state increases from Li to C from +1 to +4 and then
decreases from nitrogen to fluorine from -3 to -1.
Variation in a group : In a group , the maximum oxidation
state shown by a p-block element is equal to the sum of its s
and p electrons, which is the same as its group number.
• acidic and basic character of hydroxides: The acidic or basic
character of hydroxides and oxides depend upon the position of the
element in the periodic table.
E------ O------H
There are two possibilities:
1. If the electronegativity of E is low, as in case of metals like Na, K, Mg
and Ca etc. the electrons of E-O bonds are drawn more closely to the
oxygen atom. This is because oxygen is highly electronegative element.
As a result bond between E-O breaks giving OH- ions. Therefore such
substance behaves as bases and readily react with acids like HCl to
get neutralized.
2. If the electronegativity of E is high as in case of non-metals like F, Cl,
N etc., the electrons of E-O bond are shared more equally between
non-metal and the oxygen atom. As a result, the oxygen –hydrogen
bond becomes weaker and cleaves forming a proton . Therefore, such
substances behaves as acids and are neutralized by bases.
•Reactivity of alkali and alkaline earth metals: As
the value of ionization energy decreases down the
group from Li to Cs, therefore, the reactivity of
alkali metal increases from Li to Cs. All the elements
are highly electropositive giving +1 ions. Because of
the very high second ionisation energies of these
elements, their oxidation state in compounds never
exceeds +1.
On the other hand , alkaline earth metals are in
general less reactive than alkali metals. This is
because of their relatively high ionization energies
and high heat of atomization in comparison to alkali
metals. The chemistry of this group is mainly
dominated by +2 oxidation state.
Some of the general important chemical trends are discussed below:
• The anomalous properties of elements in first short period (from Li to F)
are explained due to their peculiar atomic properties such as small size, low
ionisation energy and high electronegativity values.
•Diagonal relationship between elements of different groups (such as Li –
Mg, Be-Al, B-Si) are due to their similar atomic properties.
•Trends in bond type with the position of the element in the periodic table
and with oxidation state for a given element.
•The variable oxidation states of transition elements.
•The stability of compounds and their trends.
•Trends in stability of coordination compounds and the electron donor
power of various types of ligands.
ASSIGNMENT
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Electron affinity of F is less than that of Cl. Why?
What is shielding effect? Calculate the shielding constant and
effective nuclear charge for 3d electrons of zinc(z=30).
Electronegativity of H and F are 2.1 and 4.0 respectively. What is
the percentage of ionic character of H-F bond?
What is ionisation energy ? Write various factors on which it
depends.
Explain why successive electron affinities have negative values.
Calculate effective nuclear charge for 3p electron in phosphorus
atom
( z=15).
Define electronegativity and discuss paulings scale of
electronegativity.
Define electron affinity . What are the factors that govern it?