Classification and Development of Elements:
Dobereiner's Law of Triads( in 1829):
"All the three elements are arranged"
It suggested a relationship between the properties of elements and their atomic weights.
According to Dobereiner, the atomic weight of the middle element is nearly the same as average of the atomic weights of other two elements.
Newlands Law of Octaves(1864) :
In 1864, Newlands made an attempt to classify elements. There are seven musical notes in music.
Every eighth note is similar to the first one and it is the first note of the next scale. Similarly Newland stated that the eighth element starting from a given one is a kind of repetition of the first like the eight notes of an octave of music. So he called this relation ship as the Law of octaves.
According to Newlands' law of octaves when the elements are arranged in order of increasing atomic weights then every eighth element has properties similar to that of the first element.
Mendeleev's law(1913):
stated the famous periodic law according to which "Properties of elements are a periodic function of their atomic weight".
Mendeleev arranged elements in the order of their increasing atomic weights in the form of a table, which is known as Mendeleev's Periodic Table.
Some gaps were left as the elements to fill up these gaps had not been discovered at that time.
Modern Periodic Table (1913):
Moseley modified Mendeleefs periodic law. He stated “Physical and chemical properties of elements are the periodic function of their atomic numbers.” It is known as modern periodic law and considered as the basis of Modern Periodic Table.
i) When the elements were arranged in increasing order of atomic numbers, it was observed that the properties of elements were repeated after certain regular intervals 01 2, 8, 8, 18, 18 and 32.
ii) These numbers are called magic numbers and cause of periodicity in properties due to repetition of similar electronic configuration.
Structural Features of Long Form of Periodic Table:
i) Long form of Periodic Table is called Bohr’s Periodic Table.
There arc 18 groups and seven periods in this Periodic Table.
ii) The horizontal rows are called periods.
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First period (1H – 2He) contains 2 elements. It is the shortest period
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Second period (3Li – 10Ne) and
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Third period (11 Na – 18Ar) contain is elements each.These are short periods.
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Fourth period (19K – 36Kr) and fifth period (37Rb – 54Xe) contain 18 elements each. These are long periods.
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Sixth period (55Cs – 86 Rn) consists of 32 elements and is the longest period.
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Seventh period starting with 87 Fr is incomplete and consists of 19 elements.
iii) The 18 vertical columns are known as groups.
The periodic table can be classified based on groups:
Elements of group 1 are called
alkali metals.
Elements of group 2 are called
alkaline earth metals.
Elements of group 16 are called chalcogens [ore forming elements].
Elements of group 17 are called
halogens. (sea salt forming).
Elements of group 18 are called noble gases.
Properties of the periodic table:
Ionization energy:
The quantity of energy required to remove a specific electron from an atom, ion, or molecule.
i) Ionization energy decreases with increasing atomic size.
ii) with increasing distance between the electron to be removed and the nucleus.
Electron Gain Enthalpy (EGE or ΔHe g):
It is the amount of energy released when an electron is added in an isolated gaseous atom.
Various factors with which electron gain enthalpy varies are :
(i) Atomic size: varies directly
(ii) Nuclear charge: varies directly
Along a period, electron gain enthalpy becomes more and more negative while on moving down the group, it becomes less negative.
Electro-negativity (EN):
It is defined as the tendency of an atom to attract the shared electron pair towards itself in a covalent bond.
Various factors with which electronegativity varies are :
(i) Atomic size: varies inversely
(ii) Charge on the ion: varies directly,
In periods as we move from left to right electronegativity increases,
while in the groups electronegativity decreases down the group.
Valency:
It is defined as the combining capacity of the element.
On moving along a period from left to right, valency increases from 1 to 4 and then decreases to zero (for noble gases).
while on moving down a group the valency remains the same.
Atomic radius:
It is defined as the distance between the centre of the nucleus and the outermost shell of electrons.
i)it's difficult to measure from isolated an atom from other atoms.
ii) Within a period, there is a regular decrease in atomic radius with increasing atomic number.
iii) Within a group, there is a regular increase in atomic radius with increasing atomic number.
iv) Atomic Radius can be measured by covalent radius, ionic radius,van der waal's radius.
Ionic radii:
It is defined as "The distance between the nucleus of an ion and a point upto which the nucleus has influence on its electron cloud".
(i)Ionic radius increases going from top to bottom and decreases across the periodic table.
Hydrogen bonding:
It is defined as "the force of attraction existing between hydrogen atom covalently bonded to highly electronegative atom (N, O or F) and the electronegative atom belonging to another molecule of the same or different substance" .
Types of hydrogen bonds are:
(i) Intermolecular H-bonding :
H-bonding involving two or more molecules.
(ii) Intramolecular H-bonding :
H-bonding within a molecule.
Anomalous properties of water:
1)Abnormal Boiling Point:
i) Due to hydrogen bonding water has an abnormal boiling point.
ii) water molecules are held by hydrogen bond which causes association of water molecules.
iii) More energy needed to break the bond, hence B.P of water is 100'c.
2)Density of ice less than the density of water:
i) The number of hydrogen bonds is larger in ice than the water.
ii)As ice is a solid and rigidly heldhin space.
iii) oxygen atoms are tetrahydrally sarrounded by 4 Hydrogen atoms. And 2 H-H atoms are bounded covalently.
iv) In ice these are arrenged cage like structure with vacant space.
v) hence ice has an expanded structure.
3)Water has a maximum density at 4°c:
As temperature rises to over 4° C, Water starts to flow and the molecules start to move away from each other due to which the Volume again increases and Density decreases.
Thus, Density is maximum at 4°C.
4)High Polar nature of water:
Water is highly polar due to the presence of highly electronegative oxygen atom present in the molecule.
5)Due to hydrogen bonding, water has abnormally high specific heat, high latent heat of fusion and vapourisation and high tension.
