Periodic Trends in properties of elements
Periodic Trends in Physical Properties
The distance from the centre of the nucleus to the outermost shell of the electrons in the atom of any element is called its atomic radius.
Since it is very small, it is very difficult to measure. We measure using the below terms for different elements
- Half the inter-nuclear distance between two similar atoms of any element which are covalently bonded to each other by a single covalent bond is called covalent radius. This is used in case on non-metals
: Half the distance between the nuclei of the two adjacent metal atoms in a close packed lattice of the metal is called its metallic radius. This is used for metals
VAN DER WAALS’ RADIUS
: Half the inter-nuclear separation between two similar adjacent atoms belonging to the two neighboring molecules of the same substance in the solid state is called the van der Waals ‘radius of that atom.
Van der Waals ’radius > Metallic radius > Covalent radius
Trends of Atomic Radius on Periodic Table
It increases from top to bottom and decreases left to right.
Because on increasing down the group, no. of shells increases and on moving left to right effective nuclear charge increases.
The effective distance from the centre of the nucleus of an ion up to which it has an influence on its electron cloud is called its ionic radius.
A cation is smaller but the anion is larger than the parent atom.
: When we find atoms or ions which have same number of electrons, we call them iso-electronic species
In case of iso-electronic species, the cation with greater positive charge has smaller radius but anion with greater negative charge has the larger radii.
Which of the following species will have the largest and the smallest size?
, Al, Al3+
Atomic radii decrease across a period so Mg has larger atomic radius than aluminium. Now Cations are smaller than their parent atoms.
Now Mg has 12 electrons, Mg2+
has 10 electrons, AL has 13 electrons and Al3+
has 10 electrons.Among isoelectronic species, the one with the larger positive nuclear charge will have a smaller radius.
Hence the largest species is Mg and the smallest one is Al3+.
Ionization Energy/ Enthalpy (I. E.)
It is the energy to remove the valance electron from isolated gaseous neutral atom(X) or ion is ground state
Enthalpy change for the below reaction is called the Ionization Energy
(g) + e
We can likewise define successive ionization energy
(g) -> M++
(g) + e
This enthalpy is called second Ionization Energy. Similarly, other can be defined.
The term ionization enthalpy is taken for the first ionization enthalpy unless stated otherwise.
It is expressed in KJ/Mole
Trends on Periodic Table
I. E. is inversely proportional to atomic size. So, it decreases from top to bottom and increases left to right
Which of the following will have higher I. E.?
a) Beryllium and Boron
b) Nitrogen and Oxygen
a) In beryllium, the electron removed during the ionization is an s-electron whereas the electron removed during ionization of boron is a p-electron. The
penetration of a 2s-electron to the nucleus is more than that of a 2p-electron; hence the 2p electron of boron is more shielded from the nucleus by the inner core of electrons than the 2s electrons of beryllium. Thus I.E is higher for beryllium
b) The Electronic Configuration of Oxygen and Nitrogen
As Nitrogen has half filled stable configuration ‘N’ has higher I. E.
Electron Gain Enthalpy
When isolated neutral atom gain electron then the amount of heat energy gained or evolved is Electron Gain Enthalpy.
X(g) + e → X-
a) First Electron Gain Enthalpy is always –ve.
and onwards Electron gain enthalpy are +ve.
Because the heat energy is absorbed by the atom to overcome the inter electronic repulsion.
Trends in Periodic table
1)The electron gain enthalpy increases from left to right in a period.
2) The electron gain enthalpy decreases from top to bottom in a group.
It is the tendency or ability of an atom to attract the shared electron towards itself. It is measured by Pauling scale. Flourine is assigned the value of 4
Trends across the Periods and Groups
a) The electro-negativity increases from left to right in a period as the number of protons in the nucleus increases and attraction forces are increased
b) The electro-negativity decreases from top to bottom in a group as the distance from nucleus is increased
We know that Non-metallic elements have strong tendency to gain electrons. Therefore, electronegativity is directly related to that non-metallic properties of elements.So non metallic character increase across the period and decrease from top to bottom in a group
Iso – Electronic Species
The species having same no. of electrons are called as Iso – Electronic species.
8 + 2
9 + 1
11 – 1
12 – 2
Size for Iso – Electronic Species
A++ < A+ < A < A- < A
Cations < Parent atom < Anions
The above can be summarized in below figure
<img alt="Periodic trends in Physical properties of the elements" src="periodic-trends-elements.png"><br>
Periodic Trends in Chemical Properties
Periodicity of Valence or Oxidation state
Valence is a characteristics property of matter. It is the defined as the number of electron in the outermost shell of the elements or equal eight minus number of the electron in the outermost shell
It tells us about the chemical reactivity of the elements
Trend in Periodic table
a) The valency first increases then decreases from left to right in a period.
b) The valency remains constant from top to bottom in a group
Now a day we used oxidation state in place of valence
The tendency of an element to lose electrons and forms positive ions (cations) is called electropositive or metallic character. The elements having lower ionization energies have higher tendency to lose electrons, thus they are electropositive or metallic in their behavior.
Metallic character increase down the group due to increase in atomic size and decrease along the period. So, reactivity of the metals decreased from left to right in a right and increase from top to bottom in the group
Basic character of oxides increases down the group due to increase in electro positive character and decrease along the period.
Acidic oxide and Basic oxide:
Oxide + Water – Base
O + H2
O – 2Na OH
(Basic oxide) (Basic oxide)
Oxide + Water – Acid
O – HCl O4
(Acidic oxide) Perchloric acid
The tendency of an element to accept electrons to form an anion is called its non-metallic or electronegative character. The elements having high electro-negativity have higher tendency to gain electrons and forms anion. So, the elements in the upper right-hand portion of the periodic table are electro-negative or non-metallic in nature.
Anomalous Properties of Second Period Elements/Diagonal Relationship
The elements of 2nd
period differ in much respect from the other members of their respective group. This is known as diagonal relationship as these elements show similar properties to the elements placed diagonally.
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