Here in this article, the students of Grade IX will find the Long question's answers in their Chemistry textbook. The answers are presented in an easy and understandable way. These answers can be used as Notes/Study material for students or teachers of the majority of boards such as BISE Federal, Punjab, KPK, Sindh, or any other board of intermediate and secondary education.
Define and explain the term Atomic size or Atomic radii. Also, describe its trend (variation) in the periodic table.
Atomic size/Atomic radii
The size of an atom is called atomic size.
Explanation
The size of an atom is different in different combinations. There are various factors that contribute to this variation. It must be remembered that an atom in isolation has a different size (atomic volume) than when it is in combination with other atoms. The atomic size of an atom is expressed in terms of Atomic radii, covalent radii, and Ionic radii.
Atomic radius
The distance between the nucleus and the valence shell (outer shell) of the atom is termed atomic radius.
The atomic radius is represented by “r”. the atomic radius
is directly proportional to the number of shells.
Unit of measurement of atomic size
Atomic radius is expressed in the nanometer (1.0 x 10-9 m) or the Pico meter (pm=1.0 x 10-12 m).
Atomic and covalent radii can find out by spectroscopy
and X-ray studies.
Variation (Trend) of atomic size in the periodic table
Atomic size varies in the periodic table in the following manner
a. Variation in Periods
In moving from left to right in periods of periodic viable the atomic sizes gradually decrease.
Reason
From left to right in periods the number of protons (positive charges) and electrons (negative charges) increases, so they will pull the valence shell due to increased force of attraction and so the size of the atom collapses and thus decreases.
b. Variation in Groups or Families
In moving from top to bottom in groups of the periodic table the atomic sizes increases.
Reason
From top to bottom in each group the number of shells increases due to which the size of the atom expands and hence increases.
Covalent radii
The one-half of the distance between the nuclei of two similar atoms of the same molecule containing a single covalent bond is called covalent radii.
Define and explain the term Ionization Energy. Also, describe its variation in the periodic table.
Ionization energy or Ionization Potential
The minimum amount of energy required to remove the outermost electron from a gaseous atom to form a positive ion is called Ionization energy(I.E) or Ionization Potential (I.P).
Explanation
Ionization is the process of removing electrons from an atom and requires energy which is called Ionization energy.
Units of measurement of I.E.
Ionization energy can be measured in the units of joules or kilo-joules per mole of the element. For example
Na ...............> Na + e- I.E
= +496kjmol-1
The above reaction indicates that the valence electron of Sodium (Na) can be removed by applying +496kjmol-1 of energy.
The plus (+) sign indicates that energy must be given in order to remove electrons.
Types of Ionization energy
Ionization energy may be
A. 1st Ionization energy
The minimum amount of energy required to remove the 1st electron from the valence shell of an atom is called 1st ionization energy.
For example
Mg ...............> Mg+ + e- I.E = +738 kJ mol-1
So +738 kJ mol-1, must be supplied in order to remove 1st valence electron from the Magnesium.
B. 2nd Ionization energy
The minimum amount of energy required to remove the 2nd
electron from the valence shell of an atom is called as 2nd
ionization energy.
For example
Mg .................> Mg+2 + 1e- I.E = +1450 kJ mol-1
So +1450 kJ mol-1, must be supplied in order to remove the 2nd valence electron from the Magnesium.
Here it should be noted that 2nd ionization energy is greater than 1st, as can be seen from the above values.
Factors affecting the ionization energy
The following factors affect the ionization energy
1. The Atomic radius of the atom
The greater the atomic radius lesser will be the ionization energy require and vice versa.
2. Nuclear charge of the atom
Greater is the nuclear charge greater will the ionization energy require and vice versa.
3. Shielding effect of low-lying electrons
Greater is the shielding effect lesser will be the ionization energy requires and vice versa.
4. Electronic configuration of the atom
An atom with more electrons in its shell will require less ionization energy and vice versa.
Variation or Trend of Ionization energy
a. Variation in periods
From left to right in the periods of the periodic table the ionization energy increases.
Reason
In periods the atomic size decreases, with the decreased atomic sized atom will require a high amount of ionization energy to remove its valence electron, and thus ionization energy increases.
b. Variation in groups
From top to bottom in groups of the periodic table, the ionization energy decreases.
Reason
As the group, atomic size increases, the atom with larger sizes will require less amount of energy to remove its valence electrons and thus ionization energy decreases.
Define and explain the term Electron Affinity. Also, describe its variation in the periodic table.
Electron Affinity (E.A)
The amount of energy evolved (released) whenever an electron enters into a gaseous atom to form an anion is called Electron affinity.
Explanation
Electron affinity means love for acceptance electron. In the periodic table, different elements have different electron affinities because each has its own electron-gaining ability. When an electron is absorbed by an atom it will be tightly bounded to a new atom through attractive forces of the nucleus.
The energy released by the addition of the first electron is called the first electron affinity, and the energy released by the addition of the second electron is called the second electron affinity.
Unit of measurement of E.A
Electron affinity is usually measured in units of energy i.e.
joules
or kilo joules per mol (kj/mol).
For example
Cl + e- >>>>>>> Cl- E.A = -348.8 kJ mol-1
The negative sign indicates that energy is released.
So -349 kJ mol-1 is released when an electron is accepted in the valence shell of Chlorine (Cl).
Factors affecting electron affinity
The following factors affect electron affinity
1. Atomic radii
Greater is the atomic radii lesser will be the electron affinity and vice versa.
2. Nuclear charge
The greater the nuclear charge greater will the electron affinity and vice versa.
3. Shielding effect
Greater is the shielding effect lesser will be the electron affinity and vice versa.
Variation of Electron Affinity in the periodic table
A. Variation in periods
From left to right in the periods of the periodic table electron affinity increases.
Reason
In periods the atomic size decreases from left to right, the atoms with decreased size will accept electrons easily because of the higher force of attraction.
B. Variation in groups
From top to bottom in groups of the periodic table, the electron affinity decreases.
Reason
As the group, atomic size increases, the atoms with larger sizes will have less force of attraction for incoming electrons and thus electron affinity will decrease.
Define and explain the term Shielding Effect. What is its effect on ionization energy and electron affinity? Also, describe its variation in the periodic table.
Shielding Effect
The effect in which the inner electrons shield the outer electrons from the nuclear force of attraction is called Shielding Effect.
Explanation
As we know that electrons are distributed in various shells of an atom, in such a case the inner electrons form a shield for outer electrons and thus nucleus has less force on the outer electrons. If one could see the outer electrons will not have a clear view of inner electrons easily.
Effect of shielding effect on ionization energy
When there will be shielding effect then less force will be required to remove the valence electrons, so we can say that in presence of a shielding effect the ionization energy decreases.
Effect of shielding effect on electron affinity
In the presence of a shielding effect the electron affinity decreases. It is because when there is a shielding effect there is decreased force of attraction so the ability to accept electrons (electron affinity) decreases.
Trend (variation) of shielding effect in the periodic table
A. Trend along periods
In moving from left to right in periods of the periodic table, shielding effect neither increases nor decreases but remains same.
Reason
As shielding effect is due to inner electrons which are the same in any particular period of the periodic table. So as the number of inner electrons remains the same so we can say that from left to right shielding effect remain constant.
B. Trend along groups
In moving from top to bottom in groups of periodic table shielding effect increases.
Reason
From top to bottom in groups, the number of inner electrons increases. As the shielding effect is due to inner electrons, which increase, so shielding effect also increases down the group.
Define and explain the term Electronegativity. Also, describe its variation in the periodic table.
Electronegativity
The ability of an atom to attract the shared pair of electrons in a Covalent bond is called electronegativity.
Explanation
The term electronegativity is associated with atoms that are only covalently bonded. A covalent bond may be
I. Polar covalent bond
That type of covalent bond in which the shared pair of electrons is attracted more towards more electronegative atom and so electronegativity will be different.
e.g. HCl >>>>>>> H+ + Cl-
II. Non-polar covalent bond
That type of covalent bond in which the shared pair of electrons is equally attracted by both the bonded atoms and so electronegativity will be same.
e.g. Cl2 >>>>>>> Cl Cl
Factors affecting Electronegativity
The following factors affect the electronegativity of an atom
1. Atomic size
Greater is the atomic size lesser will be the electronegativity and vice versa.
2. Nuclear charge
Greater is the nuclear charge greater will be the electronegativity and vice versa.
3. Shielding effect
As the number of shells increases between the nucleus and
the valence shell, so the removal of electrons from the outer shell is easy.
Therefore, the electronegativity will be lower.
4. Electronic configuration
The completely filled orbitals and half-filled orbitals are stable. The addition of electrons to these orbits is difficult. Therefore, the electronegativity is very low.
Pauling calculated the electronegativities of the elements and assigned Fluorine a maximum electronegativity value of 4.0 and Cesium has a minimum of it which is 0.70.
Variation of Electronegativity in the periodic table
A. Variation in periods
In periods from left to right the electronegativity of elements increases.
Reason
In periods the atomic size decreases from left to right, the atoms with decreased size will attract the shared pair of electrons easily.
B. Variation in groups
From top to bottom in groups of the periodic table, the electronegativity decreases.
Reason
As the group atomic size increases, the atoms with larger sizes will have less force of attraction on shared pair and thus electronegativity will decrease.
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