Class 9 Chemistry Cha 2 Periodic Table and Periodicity of Properties
Class 9 Chemistry Cha 2 Periodic Table and Periodicity of Properties short question, long question, and numerical problems for all kpk boards.
Short Questions Class 9 Chemistry Cha 2 Notes
Table of Contents
Q.1) Which element of group IA is not an alkali metal and why?
Answer: Hydrogen present in group IA is not an alkali metal. This is because it forms cations (H+) more reluctantly than the other alkali metals. Alkali metals form alkaline oxides and hydroxides but hydrogen forms a neutral compound.
Q.2) Place the following elements in order of increasing ionization energy: Na, S, Mg and Ar.
Answer: Elements in order of increasing ionization energy: Na < Mg < S < Ar
Q.3) Name the group and state the group number of each of the following elements:
a) K b) Ne c) Be d) Cl e) C
Answer: a) K Alkali metal Group I b) Ne Noble family Group VIII c) Be Alkaline earth metal Group II d) Cl Halogen Group VII e) C Carbon family Group IV
Q.4) Which element is most electronegative among C, N, O, Br and S? Which group does it belongs to?
Answer: Oxygen is the most electronegative having the value of 3.5. Electronegativities of carbon, nitrogen, bromine and sulphur are 2.5, 3.0, 2.8 and 2.5 respectively. Oxygen belongs to group VI.
Q.5) How do the first ionization energies of representative elements vary across a period and down a group?
Answer: From left to right in a period, the atomic radius reduces and thus the valence electrons are tightly bound to the nucleus of the atom. Therefore, value of ionization energy increases, as more energy is required to remove the tightly held electrons. From top to bottom in a group, the atomic radius increases because of addition of more and more shells. The valence electrons are loosely held by the electrostatic force of nucleus. Therefore, value of ionization energy decreases, as less energy is required to remove the loosely held electrons.
Q.6) Which element is found in,
a) Period 2, group VII b) Period 4, group III c) Period 5, group VI d) Period 1, group VIII
Answer: a) Period 2, group VII Fluorine b) Period 4, group III Gallium c) Period 5, group VI Tellurium d) Period 1, group VIII Helium
Q.7) How will you differentiate between representative and transition elements?
Representative elements All s and p-block elements are called representative elements. Representative elements are placed at left and right side of the periodic table.
Transition elements The d-block elements are called the transition elements. Transition elements are placed between s and p-blocks.
Q.8) Make a general sketch of the periodic table showing s, p, d and f block elements (without showing the symbols of elements)
Q.9) Why the s-block elements have two groups only?
Answer: s-block elements contain their valence electrons in an s orbital. s orbitals can accommodate only 2 electrons which makes it have a general configuration ns2. Therefore, only two groups are present in s-block i.e. group 1 and group 2. Groups 1 and 2 have only one and two electrons in their outermost shell respectively.
Q.10) What type of elements is Sulphur (S), a representative element, a transition element or lanthanide element?
Answer: The Representative Elements are those elements within the first two families (Groups I and II on the far left) and the last six families or groups (on the right) of the Periodic Table. Sulphur is a representative element as it is present in the p-block of the periodic table. It is a member of group VI
Long Questions Chemistry Class 9th Notes Chapter 2
Q.1) How modern periodic table is different from the Mendeleev’s periodic table.
Mendeleev’s periodic table i. In Mendeleev’s periodic table, elements are arranged in order of increasing atomic mass. ii. There were gaps for elements not discovered at that time. iii. Mendeleev’s periodic table contains about 65 elements. iv. Transition elements were included with the other elements. v. Noble gases were not placed.
Modern periodic table i. In modern periodic table, elements are arranged in order of increasing atomic number. ii. There are no gaps.
iii. Modern periodic table contains about 118 elements. iv. Transition elements are placed in a separate block. v. Noble gases are placed in a separate group.
Q.2) Differentiate between atomic radii and covalent radii. Explain the trends of atomic radius in the periodic table.
Atomic radius It is the distance between the nucleus and the valence shell of the atom.
Covalent radius It is the one half of the distance between the nuclei of two similar atoms of the same molecule containing a single covalent bond.
Trends of atomic radius in the periodic table i)The atomic radii decrease while going from left to right in a period as effective nuclear charge increases because of the addition of more proton. On the other hand, the number of electrons increases in the same shell (shell number remains the same). Due to increased effective nuclear charge, strong attraction of nucleus on the outermost electrons toward itself increases thus causing the decrease in size. ii) As we move from top to bottom in a group a new shell of electrons is added up in the successive period. Since there are more electron shells, the distance between nucleus and the outermost shell increases. The nuclear charge does not hold its influence on the outer most electrons and thus increased shielding effect results in increased size of an atom.
Q.3) What is electronegativity? Identify the most and least electronegative groups of elements in the periodic table. Why is fluorine special in terms of electronegativity?
Answer: Electronegativity The ability of an atom to attract the shared pair of electrons towards itself in a covalent bond is called electronegativity. In other words, we can say that the power of attraction for the shared pair of electrons is called electronegativity. If two atoms have the same ability to attract the shared pair of electrons, they have the same electronegativity. Group VIII is the most electronegative group while group I is the least electronegative group. From left to right across a period, electronegativity increases. This is because as we move from left to right, the valence electrons are in the same shell. However, the proton number increases due to which nuclear pull on valence electrons increases thus decreasing atomic size. These factors enable the atom to attract the shared pair of electrons strongly towards itself. Hence electronegativity increases along a period. Fluorine has the highest electronegativity among all other elements because of its small radius. It has 7 electrons in the valence shell and needs only 1 electron to complete its octet. Because of its smaller atomic radius, the positive protons have a very strong attraction towards the negative electrons. Therefore, it has the highest electronegativity of 4.0.
Q.4) Define shielding effect and draw its affects the ionization energy, electron affinity, and electronegativity.
Answer: Shielding effect The electrons present between the nucleus and valence electrons reduce the attraction of valence electrons towards the nucleus. They repel each other making the atom larger. Therefore, the valence electrons experience less nuclear charge. Shielding effect is the reduction in force of attraction between nucleus and the valence electrons due to the presence of electrons in the inner sub-shells. It has a direct impact on ionization energies, electron affinities and electronegativities of the elements. As we move from left to right across a period, the atomic number increases. The number of electrons in the outermost shell increases but the number of shielding electrons does not change. The increase in nuclear charge results in the contraction of atomic radii from left to right across a period. This results in increase in ionization energies, electron affinities and electronegativities of the elements across the period. As we move from top to bottom in a group, a new shell is added with each period. This results in increase of inner shells or shielding shells, which screen the outermost electrons more effectively. The increasing shielding effect wins over the increasing nuclear charge. This causes decrease in ionization potentials, electron affinities and electronegativites of the elements down the group.
Q.5) Explain the following terms. a) Periodicity of properties b) Electron affinity c) Modern Periodic law
Answer: a) Periodicity of properties Periodicity of properties means the repetition of a property after a certain interval. For example, the elements in the periodic table are arranged in order of increasing atomic number such that the elements with similar properties fall within the same group. It means after some time, properties of a certain element repeat. b) Electron affinity Electron affinity (E.A) is the amount of energy released when an electron is added to the outermost shell of an isolated gaseous atom to form a negative ion. Affinity means attraction. Elements of the periodic table possess varying tendencies to accept the electrons in their outermost shell. Since energy is released, the electron affinity is given the negative sign as shown below: F + e– → F– E.A = -328 kJmol-1 It means one mole of fluorine release 328 kJ of energy to form one mole of fluoride ions. Based on this sign convention, it means that a higher electron affinity indicates that an atom accepts electrons easily. Whereas a lower electron affinity indicates that an atom does not accept electrons easily. c) Modern Periodic Law The modern periodic law states that the physical and chemical properties of the elements are the periodic function of their atomic number. Periodic table is formed on the basis of this law. In the simplified form, it is divided into eight vertical columns know as groups and seven horizontal rows known as periods.