Periodic Classification of Elements
Periodic Classification of Elements
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| Periodic Table |
Dobereiner’s Triads
Dobereiner made
groups of three elements each, having similar chemical properties and called
them triads. He arranged the three elements in a triad in an increasing order
of atomic mass and showed that the atomic mass of the middle element was
approximately equal to the mean of the atomic masses of the other two elements.
Newlands’ Law of Octaves
In the year
1866 Newlands arranged the elements known at that time in an increasing order
of their atomic masses. He found that every eighth element had properties
similar to those of the first. Newlands compared this similarity with the
octaves in music. He called the similarity observed in the eighth and the first
element as the Law of octaves.
Merits of Mendeleev’s periodic table
1) Mendeleev considered the fundamental property of elements, namely, the atomic mass, as standard and arranged 63 elements known at that time in an
increasing order of their atomic masses.
2) Atomic masses of some elements were revised so as to give them proper place
in the periodic table in accordance with their properties.
3) Mendeleev kept vacant places in the periodic table for elements not
discovered till then. Three of these unknown elements were given the names
eka-boron, eka- aluminium and eka-silicon from the known neighbours and their
atomic masses were indicated as 44, 68 and 72, respectively.
4) Later on these elements were discovered and named as eka-boron [scandium
(Sc)], eka- aluminium [gallium (Ga)] and eka-silicon [germanium (Ge)]
5) There was no place reserved for noble gases in Mendeleev’s original
periodic table. However, when noble gases such as helium, neon and argon were discovered
towards the end of nineteenth century, Mendeleev created the ‘ zero’ group
without disturbing the original periodic table in which the noble gases were
fitted very well.
Demerits of Mendeleev’s periodic table
1)
Hydrogen was placed in group 1 and
in group 17 as it resembles alkali metals as well as halogens. Therefore, no
fixed position could be given to hydrogen in Mendeleev’s periodic table.
2)
Though isotopes of the same elements
have different atomic masses, they cannot be placed separately in the periodic
table. Since isotopes are chemically similar, they had to be given the same
position in the periodic table.
3)
Some elements with higher atomic
masses are placed before those having lower atomic masses, e.g. cobalt with
atomic mass 58.93 is placed before nickel having atomic mass 58.71.
4)
Dissimilar elements were placed in
the same group. E.g. Mn (Manganese) was placed in the group of halogens which
totally differ in the properties.
Modern periodic table
1. The modern periodic table contains seven horizontal rows called the periods
1 to 7.
2. The eighteen vertical columns in this table are called the groups 1 to 18.
3. The arrangement of the periods and groups results into formation of boxes. Atomic
numbers are serially indicated in the upper part of these boxes.
4. Apart from these seven rows, two rows are shown separately at the bottom of
the periodic table. These are called lanthanide series and actinide series,
respectively.
5. There are 118 boxes in the periodic table including the two series. It
means that there are 118 places for elements in the modern periodic table.
6. The entire periodic table is divided into four blocks,viz, s-block,
p-block, d-block and f-block.
7. The s-block contains the groups 1 and 2. The groups 13 to 18 constitute the
p-block. The groups 3 to 12 constitute the d-block, while the lanthanide and
actinide series at the bottom form the f-block.
8. The d-block elements are called transition elements. A zig-zag line can be
drawn in the p-block of the periodic table.
9. The three traditional types of elements can be clearly shown in the modern
periodic table with the help of this zig-zag line.
10.
The metalloid elements lie along the border of
this zig-zag line. All the metals lie on the left side of the zig-zag line
while all the non-metals lie on the right side.
Give scientific reasons.
1. Atomic radius goes on increasing down a group.
The atomic numbers of the
elements increase as we go down the group. Thus, the elements placed lower have
more electrons.
To accommodate these electrons
new shells are added to the atom.
These new shells take the
outermost electrons farther from the nucleus causing atomic size (radius) to
increase as we go down the group.
2 Metallic character goes on decreasing while going from left to right in a period.
Atoms of metals tend to lose
electrons, whereas those of non – metals tend to gain them.
In a period, as we go from
left to right, atomic number increases, increasing the number of electrons and
protons.
These additional electrons are
placed in the same outer shell and are strongly attracted towards the
positively – charged nucleus.
This strong attraction does
not allow loss of electrons, causing metallic character to decrease from left
to right in a period.
3 Elements belonging to the same group have the same valency.
Valency is the number of
electrons given, taken or shared by an atom to complete its outermost shell.
All the elements in the same
group have the same number of electrons in their outermost shells.
Thus, all these elements take,
give or share the same number of electrons.
Hence, elements in the same
group show the same valency.
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