Mind Map on Ch 5 Periodic Classification of Elements Class 10​

Dobereiner’s Triads:

Dobereiner was taken group of three elements which is called triads. And he found that, when these three elements are arranged in the increasing order of their atomic masses, then the atomic mass of the middle element was found equal to the average of the atomic masses of the remaining two elements.

For example:

If lithium Li , sodium Na and potassium K are taken having atomic masses 6.9, 23, and 39 respectively. Then atomic mass of sodium is found to be equal to average of atomic masses of Li and K.

Newland’s law of Octaves:

  • Newland arranged the elements in the increasing order of their atomic masses and he found that the every eighth element had properties similar to the first element. And he named this as law of Octaves.
  • He had taken the elements from lithium to sodium and found that the eighth element Na has similar properties like first element Li.
  • There are some limitations of law of Octaves which are discussed as below.
  • The law of Octaves is only applicable up to the element calcium.
  • At that time Newland assumed that there were only 56 elements existing but later on new elements were discovered but they can’t obey law of Octaves.

Mendeleev’s periodic table:

  • Mendeleev arranged the elements in the order of their increasing atomic mass and observed their physical and chemical properties.
  • And he gave law called as Mendeleev’s periodic law. It states that the properties of the elements are the periodic functions of their atomic masses only.
  • Mendeleev’s periodic table has vertical columns which ha named as groups and horizontal rows named as periods.

Limitations of Mendeleev’s classification:

  • Like alkali metals hydrogen also reacts with halogens, oxygen and sulphur and forms compounds. But on the other hand like halogens hydrogen also exist in diatomic molecule form. Also it reacts with metals and non metals to form covalent compounds.
  • And hence no certain position was given to hydrogen in periodic table.
  • Isotopes were discovered which are having same chemical properties but different atomic masses which is challenged to Mendeleev’s periodic law.

The modern periodic table:

  • The modern periodic table is based on the modern periodic law which can be stated as all the properties of the elements are the periodic functions of their atomic numbers only.
  • As we know that, atomic number indicates the number of protons in the nucleus and it increases from element to next element when elements are arranged in their increasing atomic number.

Position of the elements in the modern periodic table:

  • In modern periodic table there are 18 vertical columns which are called as groups and 7 horizontal rows which are called as periods.
  • The elements present in the same group has same number of Valence electrons.

For example:

  • Cl and F belongs to 17th group and they are having same number of electrons in their outermost shell.
  • Also, the elements of the second period don’t have the same number of Valence electrons in their outermost shell but they possess same number of shells.
  • Also, as the number of Valence shell electrons increases by one then atomic number also increases by one when we move from left to right in period.
  • The formula 2n2 gives the maximum number of electrons that are occupied by the shell, where n is the number of given shell from the nucleus.

For example:      

  • For K shell: 2*(1)2= 2 and hence first period has only two elements.
  • For L shell: 2*(2)2=8 and hence second period has 8 elements.
  • Similarly, fourth, fifth, sixth and seventh period have 8, 18, 18, 32 and 32 elements respectively.

Trends in the modern periodic table:

Valency:

  • The number Valence electrons present in the outermost shell of an element is called as it’s valency.

Atomic size:

  • The atomic size of an atom is nothing but the distance between the centre of nucleus of an atom and its outermost shell.
  • For example: hydrogen has atomic radius of about 37pm.
  • And we see that the atomic radius decrease as we move from left to right in a period. This is because from left to right in a period the nuclear charge increases due to which outer electrons get attracted towards the nucleus and the radius of the atom get decreased.
  • But in group as we move from top to bottom the atomic radius increases. This is because as we move from top to bottom in group the number of new shells get added and which increases the atomic radius.

Metallic and non metallic properties:

  • The metals like Na and Mg are found on the left side of periodic table and non metals like sulphur and chlorine found on the right side of the periodic table.
  • And in the middle semi metal or metalloids like silicon exist.
  • We observe that, metallic character decreases across a period and increases down the group.

Updated: April 16, 2022 — 2:11 pm

Leave a Reply

Your email address will not be published.