So many attempts of classifying elements
 
had failed. Then in 1913 an
 
english physicist Henry Moseley found out
 
that the fundamental property of an
 
element is not atomic mass but it is
 
it's atomic number. Because atomic number
 
tells us the number of electrons present
 
in any atom of the element and number of
 
electrons are actually responsible for
 
the chemical properties of an element
 
Give and take of electron determines the
 
chemical property of any element. Thus, after this
 
the arrangement of elements was done
 
according to their atomic numbers and
 
using this the periodic table formed by
 
Mendeleev was corrected. The modern
 
periodic law states that "the properties
 
of elements are the periodic function of
 
their atomic numbers". So when you arrange
 
the elements according to their atomic
 
numbers will find that the properties of
 
elements are similar found after a
 
particular interval that the periodic
 
table of Mendeleev that was corrected is
 
now known as the modern periodic table.
 
The modern periodic table has seven rows.
 
But 18 columns, this periodic table is
 
called the long form of periodic table,
 
whereas Mendeleev's periodic table is called the short
 
form of periodic table. Modern periodic
 
table,
 
let us study about this long form of
 
periodic table in detail. The group 1, 2, 13, 14
 
15, 16, 17 elements are called normal
 
elements.
 
Normal elements because their last orbit
 
are incomplete, either they have not
 
completed duplet or octet state.
 
Then the group 18, these elements are called
 
inert gases. Because their last orbits are complete
 
 
 
either they have completed their duplet
 
state or their octet state, whereas
 
the elements in group 3 to 12, they are called
 
transition elements. Their last two shells
 
or the last two orbits in the atoms are
 
incomplete and the rows that lie below
 
the table, these two rows have elements,
 
which have last three shells incomplete
 
in their atoms. They are called inner
 
transition elements. The elements that we
 
will be studying about in this lesson
 
are the normal elements that are present
 
in the group 1, 2 and 13 to 18. Out of
 
these elements also we are going to
 
study the first 20 normal elements,
 
ranging from atomic number 1 to atomic
 
number 20. And they are Hydrogen
 
Helium, Lithium, Beryllium,
 
Boron, Carbon, Nitrogen, Oxygen, Fluorine,
 
Neon, Sodium, Magnesium, Aluminium,
 
Silicon, Phosphorus, Sulphur, Chlorine, Argon
 
Potassium(K) and Calcium.
 
For all these elements can
 
be remembered using a trick, but to remember
 
the first 10 elements the trick goes
 
like this "Hi He Lies Because Boys Can
 
Not Open Fish Net". Hi he lies
 
because boys cannot open fish net. This
 
line can make you remember the 10
 
elements, how, that is because the first
 
or the first two letters of each word in
 
the line represent the symbol of the
 
element. For example, 'H' in Hi
 
represents Hydrogen, 'He' in he
 
represents Helium, 'Li' in lies represents
 
Lithium and so on. The other 10 elements
 
in the list can be remembered using
 
another trick. And the trick goes like
 
this "Naughty Mowgli Always Sings
 
Pop Songs Claimed by Ar K Ca". So here
 
too the first two are the first letter of
 
each word in the line represents the
 
symbol of the element. Now, let us see
 
what are the merits of modern periodic
 
table? And, why is it better than the
 
Mendeleev's periodic table? OK! now,
 
according to Mendeleev's periodic table
 
position of isotopes wasn't clear. But,
 
when you come to modern periodic table,
 
it is clear that isotopes of an element
 
are placed in the same position. For
 
isotopes of an element are nothing but
 
atoms of the same element having
 
different atomic mass number but same
 
atomic number. So, because our modern
 
periodic table is arranged according to
 
atomic number both the isotopes of an
 
element can be placed at the same
 
position. For example, Chlorine; Chlorine
 
has two isotopes Chlorine 35 and Chlorine 37.
 
Both their masses are different but
 
both their atomic numbers are same
 
that is 17 so both the chlorine
 
isotopes are placed at the same position.
 
The next concept that Mendeleev's periodic
 
table couldn't explain is the position of
 
Cobalt and Nickel. Cobalt having higher
 
atomic mass was placed before Nickel
 
having lower atomic mass but, when we talk
 
about the modern periodic table, it tells
 
us that Cobalt's atomic number is 27.
 
Whereas Nickel's atomic numbers is 28. So, that
 
is the reason why Cobalt is placed
 
before Nickel. And the next thing that
 
Mendeleev's periodic table couldn't explain is
 
the position of Hydrogen, but modern
 
periodic table hydrogen is given as the
 
special position. It is placed in group
 
one but, it is not considered to be
 
metal nor is it considered to be a
 
halogen. So there's given a special
 
position in the modern periodic table
