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GSEB Solutions for Class 9 Science and Technology – Periodic Classification of Elements

GSEB Solutions for Class 9 Science and Technology –  Periodic Classification of Elements (English Medium)

GSEB SolutionsMathsScience
Exercise 31:

Solution 1.1:

C. 114
114 elements have been discovered in the universe. Out of this, 98 are naturally occurring elements and the remaining 16 are synthesised in the laboratory.

Solution 1.2:

D. 4
Modern periodic table is classified into 4 blocks viz. s, p, d and f block.

Solution 1.3:
C. Dobereiner
Dobereiner’s law of triads states that ‘if three elements are arranged in increasing order of their atomic masses, the atomic mass of the intermediate (middle) element is similar to the average of the atomic masses of the first and the third elements.

Solution 1.4:
B. B
Element B with atomic mass 18 is an inert gas element placed in the 18th group and the remaining elements are halogens placed in the 17th group.

Solution 1.5:
B. Newland
Newland’s law of octave states that ‘when the elements are arranged in the increasing order of their atomic weights, the properties of every eighth element are similar’; like the notes of music.

Solution 1.6:

D. f
The inner transition elements i.e. lanthanides and actinides are f-block elements placed at the bottom of the table.

Solution 1.7:

D. Silicon
Silicon has atomic number 14, it has 4 electrons in its valence orbit. Thus, it is a semimetal.

Solution 1.8:

B. 35
The electronic configuration of element having atomic number 35 is 2, 8, 18, 7. The element has a tendency to accept an electron to achieve stable configuration. Thus, it exhibits non-metallic property.

Solution 1.9:

C. K
Atomic number of K is 19. Thus, it has 19 electrons.

Solution 1.10:

D. Its atom forms negative ion.
The element having electronic configuration as 2,8,8,2 has valency of 2. It has tendency of losing two electrons and thus, it forms a positive ion.

Solution 1.11:

B. Moseley
Henry Moseley discovered the atomic number in 1913 from the study of X-ray spectra. This led to the construction of the modern periodic table.

 

Solution 2.1:

Lord Rayleigh and Sir Ramsay discovered inert gases in 1894.

Solution 2.2:

Mendeleef was the first scientist to prepare a periodic table.

Solution 2.3:

Mendeleef’s periodic table was constructed on the law which states that ‘the properties of the elements are the periodic function of their atomic masses’.

Exercise 32:

Solution 2.4:

By the discovery of gallium (Ga), the position of the element below Al was filled in Mendeleef’s periodic table.
Solution 2.5:

Silicon (Si) is placed above titanium (Ti) in Mendeleef’s periodic table.
Solution 2.6:

The vertical columns of the periodic table are called groups and the horizontal rows of the periodic table are called periods.

Solution 2.7:

There are 18 groups and 7 periods in the modern periodic table.

Solution 2.8:

There are 7 electrons in the outermost orbits in the elements of the 17th group.
Solution 2.9:

The elements of the 7th group and the 17th group have 7 electrons in the outermost orbit.

Solution 2.10:

53I is placed in the 17th group and the 5th period of the periodic table.
Solution 2.11:

The value of ionisation enthalpy is mentioned in kcal mol-1 or kJ mol-1.
Solution 3.1:

Dobereiner’s law of triads states that ‘if three elements are arranged in the increasing order of their atomic masses, the atomic mass of intermediate (middle) element is similar to the average of the atomic masses of the first and the third element.
Example – In a triad of Lithium (Li), sodium (Na) and potassium (K), atomic masses of Li is 7 and K is 39. The atomic mass of Na is 23
GSEB Solutions for Class 9 Science and Technology - Periodic Classification of Elements - Ex-32-3.1
; thus proving Dobereiner’s law of triads.
Mendeleef’s periodic law states that ‘the properties of the elements are the periodic function of their atomic masses’. When Mendeleef arranged the elements in increasing order of their atomic masses in seven groups, he found that the elements falling in the same group have similar properties.
Example – Li, Be, B, C, N, O and F placed in the second group of Mendeleef’s periodic table showed similar properties.
Solution 3.2:

Newland’s law of octave states that ‘when the elements are arranged in the increasing order of their atomic weights, the properties of every eighth element are similar’; such as the notes of music.
The modern periodic law for classification of elements states that ‘the properties of the elements are the periodic function of their atomic numbers’.

Solution 3.3:

Atomic number was discovered by Henry Moseley in 1913 from the study of X-ray spectra.

Solution 3.4:

Atomic radius is defined as the distance from the centre of the nucleus of the atom to its outermost shell.
Atomic radii of elements gradually increase in a group from top to bottom.
As we go down a group, the atomic number increases, thus, the number of shells increases. Also, as we move down the group, the valence electrons are present in a higher shell and thus, the distance of valence electrons from the nucleus increases. As a result of this, the force of attraction between the nucleus and the valence electron decreases. Therefore, atomic radius increases on moving down a group.
As we move across a period from left to right, the atomic radii decrease gradually.
In a period, there is a gradual increase in the nuclear charge from left to right. As the atomic number increases in a period, the electrons are added in the same shell. Thus, they are more and more strongly attracted towards the nucleus. As a result of this force of attraction of the nucleus, the atomic radii gradually decrease.

Solution 3.5:

The minimum amount of energy required to remove an electron from the outermost orbit of a gaseous atom in its ground state is called ionization energy.
Ionization energy of elements depends on various factors like nuclear charge, shielding effect, stability of electronic configuration and atomic radius.
Ionization energy decreases as we go down a group.
As we go down a group, the atomic radii of the elements increase, the number of electron shells also increases, thus, the nuclear attraction over a valence electron decreases and ionization energy also decreases.
Ionization energy generally increases across a period from left to right.
As we move across a period from left to right, the atomic radii decreases, and the nuclear attraction over a valence electron increases. Thus, ionization energy increases.
Solution 3.6:

The elements placed in the left side group of the periodic table are metals and the elements in the right side group are non-metals. The elements in the middle of the periodic table are called as semimetals. These are the elements in the group having 4 electrons in the outermost orbit.
Solution 3.7:

  • Ionization enthalpy: The minimum amount of energy required to remove an electron from the outermost orbit of a gaseous atom in its ground state is called ionization energy.
    Ionization energy of elements depends on various factors like nuclear charge, shielding effect, stability of electronic configuration and atomic radius.
    Ionization energy decreases as we go down a group and increases across a period from left to right.
  • Valence electrons: The number of electrons present in the outermost orbit of an atom of an element is called as valence electrons. The properties of elements in the modern periodic table depend on the number of valence electron in an atom of an element.
    In a group, the number of valence electrons remains the same whereas across a period, the number of valence electrons of atoms gradually increases.
  • Electron Affinity:  Elements have a tendency to gain electrons from outside the atom when attracted by the nucleus. As a result of this attraction, energy is liberated. This energy liberated when an electron is added to a neutral gaseous atom of an element is the electron affinity of that element.
    The greater the tendency of the atom to gain electrons, the greater is the value of electron affinity and more energy is released.
    In a group, as we move from top to bottom, the electron affinity decreases and in a period, the electron affinity increases as we move from left to right.

Solution 3.8:

The modern periodic law for classification of elements states that ‘the properties of elements are the periodic function of their atomic numbers’.
The modern periodic law is based on the atomic number and the electronic configuration of the elements.
In the modern periodic table, elements are arranged in the increasing order of their atomic number. The horizontal rows are called as “periods” and the vertical columns are called as “groups”. The Modern periodic table consists of 7 periods and 18 groups.
The elements falling in the same group or periods show regular gradation in properties when arranged as per the modern periodic law.

Solution 3.9:

Three metals of the periodic table:

  1. Sodium (Na)
  2. Potassium (K)
  3. Calcium (Ca)

Three non-metals of the periodic table:

  1. Phosphorus (P)
  2. Sulphur (S)
  3. Chlorine (Cl)

Three semi metals of the periodic table:

  1. Boron (B)
  2. Silicon (Si)
  3. Germanium (Ge)

Solution 3.10:

Elements having 1, 2 or 3 electrons i.e. those in the 1st, 2nd and 3rd group can easily lose electrons. And elements having 5, 6 or 7 electrons i.e. those in the 15th, 16th and 17th group can easily gain electrons.

Solution 3.11:

Number of electrons in the s-, p-, d- and f- blocks are as follows:

  1. s- block – 2 electrons
  2. p-block – 6 electrons
  3. d-block – 10 electrons
  4. f-block – 14 electrons

Solution 3.12:

In the modern periodic table, elements are arranged in the increasing order of their atomic number. The horizontal rows are called as “periods” and the vertical columns are called as “groups”. The Modern periodic table consists of 7 periods and 18 groups. PERIODS :-

  1. There are 7 periods in the modern periodic table.
  2. Each period starts with an alkali metal and ends with an inert gas element.
  3. Elements present in the same period have same number of shells which is equal to the periodic number.
  4. The 1st period is the shortest period containing only 2 elements i.e. Hydrogen (H) and Helium (He). In this period, only the 1s orbital is filled.
  5. The 2nd period contains 8 elements from Lithium (Li) to Neon (Ne). In this period, the 2s and 2p orbitals are filled.
  6. The 3rd period also contains 8 elements starting from Sodium (Na) to Argon (Ar). In this period, the 3s and 3p orbitals are filled.
  7. The 4th period is the long period with 18 elements, starting from Potassium (K) to Krypton (Kr). In this period, 4s and 4p and also the 3d orbitals are filled.
  8. The 5th period is also the long period with 18 elements, starting from Rubidium (Rb) to Xenon (Xe). The 5s and 5p along with the 4d orbitalsare filled.
  9. The 6th period is the longest period with 32 elements. It not only includes 10 elements belonging to 5d series i.e. from Lanthanum (La) to Mercury (Hg) but also contains 14 elements belonging to the 4f series called lanthanides. In this period, the 6s and 6p along with the 4f and 5d orbitals are filled.
  10. The 7th period is an incomplete period. It includes Fr along with the 14 elements belonging to the 5f series called actinides. In this period, the 7s and 5f orbitals are filled.

GROUPS :-

  1. The Modern Periodic Table consists of 18 groups or vertical columns.
  2. Elements present in the same group show similar physical and chemical properties.
  3. Also, the elements present in the same group have same number of electrons in the outermost shell.
  4. According to American convention, the groups are denoted by roman numerals followed by either a capital alphabet “A” if the group is in the s-block or p-block or by “B” if the group is in the d-block. Thus, the groups from 1 to 18 are denoted as IA , IIA, IIIB, IVB, VB, VIB, VIIB, VIII, IB ,IIB, IIIA, IVA, VA, VIA, VIIA and 0 (zero).
  5. The elements from IA to VII A group i.e. the elements in the groups 1, 2, 13, 14, 15, 16 and 17 are called as representative elements.
  6. The 18th group or zero group elements are called as inert gases or noble gases.
  7. The elements in the groups from IIIB to IIB i.e. from group 3 to 12 are called as transition elements.
  8. The two rows placed at the bottom of the periodic table i.e. the Lanthanides and the Actinides are also considered to be a part of IIIB group (i.e. group 3). These are usually called as inner transition elements.

Solution 3.13:

Periodicity is the tendency of a property to recur at regular intervals in the modern periodic table.
In the modern periodic table, the elements are classified on the basis of the electronic configuration of the outermost orbits of the elements.
In a given group, the electronic configuration of the valence orbit of all elements is same. Thus, the elements in same group show similar properties.