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CLASSIFICATION OF ELEMENTS AND
PERIODICITY IN PROPERTIES
Introduction:
Classification of elements was proposed in order to study all the elements in a systematic
manner
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Mendeleev’s Periodic Table was based on atomic masses
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Why Do We Need Classification?
Elements are the basic units of all types of matter
...

With such a large number of elements, it is very difficult to study individually the
chemistry of all these elements and their number of compounds
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Main aim behind this classification was to keep the elements
of same properties together, so that by studying one element out of that group, we can
have general idea about the properties of all the elements in that group
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Dobereiner’s Triads Law
1st attempt towards the classification of elements was made by Johann W
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He arranged elements in the groups of three and in such a way that the atomic
weight of middle element was equal or nearly equal to the average of atomic weights of
other two elements
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Now, a days, 1st and
9th elements have same properties in that series because noble gases were discovered
late
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Mendeleev arranged elements in horizontal rows and vertical columns of a table in order
of their increasing atomic weights in such a way that the elements with similar
properties occupied the same vertical column or group
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Horizontal rows are called periods
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Position of hydrogen: Position of hydrogen was not justified
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Position of isotope: Isotopes should be placed separately according to periodic law
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iii
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iv
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As
Tellurium (128 u) comes in VI group before Iodine (127 u)
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The long form of periodic table, also called Modem Periodic Table, is based on Modern
periodic law
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A modern version, the so-called “long form” of the Periodic Table of the elements, is the
most convenient and widely used
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Elements having similar outer electronic configurations in their
atoms are arranged in vertical columns, referred to as groups or families
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There are altogether seven periods
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The first period contains 2
elements
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The seventh period is incomplete and like the sixth period would have a
theoretical maximum of 32 elements
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The IUPAC names are derived by using roots for three digit atomic number of the
elements
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The roots are put together in order of digits which
make up the atomic number and “ium” is added at the end
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Characteristics of s-Block elements,
i
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ii
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iii
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iv
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v
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vi
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p-block: The p-Block Elements comprise those belonging to Group 13 to 18 and these
together with the s-Block Elements are called the Representative Elements or Main
Group Elements
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Characteristics of p-Block elements,
1
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2
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3
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4
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5
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d-block: These are the elements of Group 3 to 12 in the centre of the Periodic Table
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These elements have the general outer electronic configuration (n-1)d1-10ns0-2
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They are all metals with high melting and boiling points
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The compounds of the elements are generally paramagnetic in nature
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They mostly form
coloured ions, exhibit variable valence (oxidation states)
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They are of tenly used as
catalysts
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These elements have high melting point
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The last electron added to each element is filled
in f-orbital
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Characteristics of f-Block elements:
1
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Elements after uranium are known as transuranium element
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They form coloured compounds
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These two series of elements are called Inner Transition Elements (f-Block Elements)
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They are all metals
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5
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Periodic Properties
The properties which generally have a regular trend along a group or period are called periodic
properties
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Atomic Size: Atomic Radius is the distance from the centre of the nucleus to the
outermostshell containing electron
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In general, the ionic radii of elements exhibit the same trend as the
atomic radii
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The
radius of cation is always smaller than that of the atom
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The radius of the anion is always larger than
that of the atom
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Ionisation energy: It is the amount of energy required to remove the outer most electron
from an isolated atom in its gaseous state
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It is

denoted by (∆iH)
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Electron gain enthalpy: It is the enthalpy change when an electron is added to the gaseous
neutral atom
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It is the measured in the unit of kJ/mole
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X(g) + e- ⟶ X-(g)
iv
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Periodic Trends in Chemical Properties along a Period
1
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2
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3
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4
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5
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6
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7
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8
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Variation in Chemical Properties along a Group
1
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2
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As electronegativity of
elements decreases from top to bottom in a group
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Atomic Size: It increases along a
group
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Ionisation energy: It decreases across a period
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Electron gain enthalpy: It decreases across a period
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Electronegativity: It decreases across a period
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Basic nature of oxides: Since metallic character or electropositivity of elements increases in

going from top to bottom in a group basic nature of oxidise naturally increases
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Acidic character of oxides: It generally decreases as non-metallic character of elements
decreases in going from top to bottom in a group
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Reactivity of metals: It generally increases down a group
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Summary
1
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2
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3
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4
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Sixth period is the longest and first
period is the smallest
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s-Block elements: Elements of groups 1 and 2
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6
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Their general valence shell
electronic configuration is ns2np1–6
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d-Block elements: Elements of groups 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12
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Their general valence shell electronic configuration is (n–1)d1–10 ns1–2
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46Pd

8
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Also known
as inner transition elements
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9
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10
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11
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12
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Example: (i) N3–, O2–, F–, Na+, Mg2+, Al3+; (ii) P3–, S2–, Cl–, K+, Ca2+, Sc3+
13
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14
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15
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16
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17
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18
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19
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20
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21
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22
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23
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24
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25
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The
elements having low I
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are more electropositive or more metallic in character
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Valence of an element belonging to s or p-block is either equal to the number of valence
electrons or eight minus the number of valence electron
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The chemical reactivity is maximum at the two extreme ends of the periodic table and is

least in the centre
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Among alkali metals reactivity increases on descending the group while among halogens
the reactivity decreases on descending the group
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The basic character of oxides decreases while the acidic character increases on going from
left to right in a period
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Oxides of metals are generally basic while that of non-metals are acidic in nature
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The similarity between a pair of elements in period second and third located diagonally in
the periodic table is called the diagonal relationship

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