Periodic Table And Periodicity Of Properties Chemistry Bise Rawalpindi Class 9

biise Rawalpindi board Grade 9th Physics Notes chapter 3 short long question answer Periodic Table and Periodicity of Properties.

Mcqs

1. Find the successive elements of the periodic table with ionization energies, 2372, 520, and 890 kJ per mol, respectively

(a) Li, Be, B

(b) H, He, Li

(d) He, Li, Be

Answer: (d)

2. In the modern periodic table, the number of periods of the element is the same as

(a) principal quantum number

(b) atomic number

(d) atomic mass

Answer: (a)

3. The correct order for the size of I, I+, I– is

(a) I > I– > I+

(b) I > I+ > I–

(d) I+ > I– > I

Answer: (c)

4. For the same value of n, the penetration power of the orbital follows the order

(a) s = p = d = f

(b) p > s > d > f

(d) s < p < d < f

Answer: (c)

5. Which of the reactions will need the maximum amount of energy?

(a) Na → Na+ + e–

(b) Ca+ → Ca++ + e–

(d) C2+ → C3+ + e–

Answer: (c)

6. Which of the following statements is incorrect?

(a) I.E.1 of O is lower than that of N, but I.E.2 O is higher than that of N

(b) The enthalpy of N to gain an electron is almost zero, but of P is 74.3 kJ mol-1

(d) The covalent radius of iodine is less than its Van der Waal’s radius

Answer: (c)

7. The correct order of electronegativity is

(a) Cl > F > O > Br

(b) F > O > Cl > Br

(d) O > F > Cl > Br

Answer: (b)

8. Two different beakers contain M1-O-H, and M2-O-H solutions separately. Find the nature of the two solutions if the electronegativity of M1 = 3.4, M2 = 1.2, O = 3.5, H = 2.1

(a) acidic, acidic

(b) basic, acidic

(d) acidic, basic

Answer: (d)

9. Which one is the most acidic among these?

(a) MgO

(b) CaO

(d) Na2O

Answer: (c)

10. Which one will have the highest 2nd ionisation energy?

(a) 1s2 2s2 2p6 3s1

(b) 1s2 2s2 2p4

(d) 1s2 2s2 2p6 3s2

Answer: (a)

Short Questions

Why are noble gasses not reactive?

An element is reactive when its outermost shell is not complete. They lose, gain or share electron(s) to gain a more stable electron configuration.
Noble gasses have their outermost shells filled and are in their most stable state. So, they do not react with other elements.

Q.2) Why does Cesium (at. 55) require little energy to release its one electron present in the outermost shell?

Answer:

Cesium has a large atomic radius. Electrons present in its outermost shell are far away from the nucleus. The attraction between the nucleus and outermost electron is less because of the high shielding effect. As a result, less energy is required to release its electron present in the outermost shell.

Q.3) How is the periodicity of properties dependent upon the number of protons in an atom?

Answer:

The number of protons in a nucleus of an atom represents the atomic number of that atom. In the periodic table, elements are arranged from left to right in increasing order of proton number. With this arrangement, properties of elements repeat at regular intervals, such that elements with similar properties come in the same group.

The physical and chemical properties of the elements change periodically with the atomic number.

Q.4) Why does the shielding effect of electrons make cation formation easy?

Answer:

The shielding effect weakens the attractive force exerted by the nucleus on the outermost electrons due to the presence of electrons between the nucleus and the outermost shell.

These electrons are thus less tightly held by the nucleus and are easy to remove. So, the greater the shielding effect is, the easier it is to lose electrons and become cation.

Q.5) What is the difference between Mendeleev’s and modern periodic laws?

Answer:

Mendeleev’s periodic law

Modern periodic law

Mendeleev’s periodic law states that the physical and chemical properties of the elements are the periodic functions of their atomic masses.

Modern periodic law states that the physical and chemical properties of the elements are the periodic functions of their atomic number.

Q.6) What do you mean by groups and periods in the Periodic Table?

Answer:

The vertical columns in a periodic table are called groups. The horizontal rows in a periodic table are called periods.

Q.7) Why and how are elements arranged in the 4th period?

Answer:

The 4th period is long, and eighteen elements are placed in it. These elements have the same number of shells, i.e., four. The first element of this period is potassium, an alkali metal with atomic number 19. The second element is calcium, an alkaline earth metal. The next ten elements are transition elements and belong to the d-block. And the last six elements are of p-block.

Q.8) Why does the size of the atom not regularly decrease in a period?

Answer:

As we move from left to right in a period, electrons are added within the same shell with a corresponding increase in the number of protons. This increases the force of attraction between the nucleus and valence shell, and it is pulled inward, decreasing the size of the atom. However, as we go rightward, the addition of more electrons in the same shell also increases the force of repulsion between the valence electrons and inner electrons, which tends to increase the size of the valence shell. The conflicting forces may introduce irregularity in the overall shrinkage of atomic radius.

Q.9) Give the trend of ionization energy in a period.

Answer:

The amount of energy required to remove the valence electron of an isolated gaseous atom is called ionization energy. As we move across a period from left to right, the ionization energy of elements increases because the atomic size decreases. This results in a strong attraction between the nucleus and valence electrons.

Long Questions

Q.1) Explain the contributions of Mendeleev to the arrangement of elements in his Periodic Table.

Answer:

Mendeleev was the most important contributor to the development of the periodic table. He did not create it from scratch. He improved the existing periodic table by turning an idea into a tool for categorizing elements.

He systematically arranged the known 63 elements in order of increasing atomic masses in a grid-like diagram; following this system, he could even predict the qualities of still-unknown elements. The elements with similar properties were in the same vertical columns. This arrangement of elements was called the periodic table.

In 1869, Mendeleev formally presented his discovery of the periodic law to the Russian Chemical Society. This law stated, “Properties of the elements are periodic functions of their atomic masses.”

At first, very few accepted his law in the international scientific community. But it gradually gained support over the following two decades. In London in 1889, Mendeleev presented a summary of their research in a lecture titled “The Periodic Law of the Chemical Elements.” His diagram, known as the periodic table of elements, is still used today.

Q.2) Show why in a ‘period’ the size of an atom decreases if one moves from left to right.

Answer:

As we move from left to right in a period, the atomic number increases. But the electrons are being added to the same energy level, and protons are being added to the nucleus. The effective nuclear charge increases due to the addition of protons. So the nucleus of protons attracts the valence shell electrons strongly with the gradual increase in atomic number. Therefore, atomic size decreases across the period.

For example, atomic size in period 2 decreases from lithium (Li) to fluorine (F). In lithium, only three protons attract the valence shell electrons, but there are nine protons attracting valence shell electrons in fluorine. So, fluorine will have a small atomic size as the valence shell would contract due to strong nuclear attraction. Thus the new size of the fluorine would be less than expected.

Q.3) Describe the electronegativity trends in a period and in a group.

Answer:

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

The trend of electronegativity in a period

From left to right across a period, electronegativity increases. As we move from left to right, the numbers of energy levels the electrons are in stay the same, so the valence electrons are in the same shell. However, the proton number increases due to which atomic size decreases. These factors enable the atom to attract the shared pair of electrons towards itself strongly. Hence electronegativity increases over a period.

Trends of electronegativity in a group

From top to bottom down a group, electronegativity decreases. As we move from top to bottom atomic radius increases because of the increase in the number and size of the energy levels, so the valence electrons in each atom are getting farther away from the nucleus. These factors weaken nucleus attraction for the shared pair of electrons. Hence electronegativity decreases down a group.

Q.4) Discuss the important features of the modern Periodic Table.

Answer:

Features of modern periodic table

i) The elements in the periodic table are arranged in the increasing order of their atomic number.

ii) The periodic table has 18 vertical columns called groups and seven horizontal rows called periods.

iii) The first period contains only two elements and is the shortest period. The second and third periods contain eight elements and are called short periods. The fourth and fifth periods contain 18 elements and are called long periods. The sixth period contains 32 elements and is the longest period. The seventh period has 23 elements and is incomplete.

iv) Elements of the same group show similar properties, and elements of the same period show different properties.

v) The elements are classified based on their electronic configuration into four blocks s, p, d, and f.

vi) The periodic table elements belong to 4 groups: metals, nonmetals, metalloids, and noble gasses.

Q.5) What do you mean by blocks in a periodic table, and why were elements placed in blocks?

Answer:

A block of the periodic table of elements is a set of adjacent groups. Elements with similar sub-shell electronic configurations are referred to as a block of elements.

The respective highest-energy electrons in each element in a block belong to the same atomic orbital type. There are four blocks in the periodic table named after their characteristic orbital, which is completed by the electrons:

s-block

Group 1 and group 2 elements have their valence electrons in s subshell. So, they are called s-block elements.

p-block

Group 13 to group 18 elements have their valence electrons in the p subshell. So, they are called p-block elements

d-block

The d-block elements have their valence electrons in the d subshell. The d-block lies between the s and p blocks.

f-block

The f-block lies separately at the bottom.

Q.6) Discuss in detail the periods in the Periodic Table?

Answer:

i) Short period

The first period consists of two elements, hydrogen, and helium, called a short period.

ii) Normal periods

The second and third periods are called normal periods. They have eight elements in each of them.

The second period contains the elements in the following order: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon.

The third period consists of sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, and argon. Neon and argon are noble gasses.

iii) Long periods

The fourth and fifth periods are called long periods, and each of them consists of eighteen elements.

iv) Very long periods

The sixth and seventh periods are called very long periods. In these periods, fourteen elements for each were accommodated and placed separately below the normal periodic table.

This series of elements start from atomic number 57 for the sixth period and atomic number 89 for the seventh period. These series are called Lanthanides and Actinides as they start after Lanthanum (Z=57) and Actinium (Z=89).

Q.7) Why and how elements are arranged in a Periodic Table?

Answer:

Elements are arranged in the periodic table in order of increasing atomic numbers from left to right in rows. These elements are arranged so that similar properties repeat at regular intervals, and elements with similar properties are placed in the same group.

This arrangement helps understand the properties of elements in groups rather than individually. Thus it makes it easy to understand the properties of elements. It establishes a trend of properties of periods and groups.

Q.8) What is ionization energy? Describe its trend in the Periodic Table?

Answer:

Ionization energy

Ionization energy is the energy required to remove valence electrons from an isolated gaseous atom.

The more the electrons are present in the valence shell, the more energy is required to remove them. For example, the sodium of group 1 has one electron in its valence shell; the energy required to remove that electron is called first ionization energy. Magnesium of group 2 has two electrons in its valence shell so that it will have more than one ionization energy value.

Trends of ionization energy in a period

As we move from left to right in a period, the atomic radius reduces, and thus the valence electrons are tightly bound to the atom’s nucleus. Therefore, the value of ionization energy increases from left to right as more energy is required to remove the tightly held electrons.

Trends of ionization energy in a group

As we move from top to bottom in a group, the atomic radius increases because more and more shells are added between the valence shell and the nucleus. The electrostatic force of the nucleus loosely holds the valence electrons. Therefore, the value of ionization energy decreases from top to bottom, as less energy is required to remove the loosely held electrons.

Q.9) Define electron affinity and why it increases in a period and decreases in a group in the Periodic Table.

Answer:

Electron affinity

is the change in energy when an electron is added to the outermost shell of an isolated gaseous atom to form a negative ion. An atom tends to accept electron(s). For example, the electron affinity for chlorine is -349 kJ mol-1. One mole of chlorine releases 349 kJ of energy to form one mole of chloride ions.

Cl(g) + e- → Cl-(g) E.A. = -349 kJ/mol

The minus sign indicates that the energy is released.

Trends of electron affinity in a period

From left to right in a period, electron affinity increases. As we move from left to right across a period, the size of the atom decreases. There is a strong attraction of the nucleus for the valence electrons. Thus, it will strongly attract the incoming electron, and more energy will be released.

Trends of electron affinity in a group

From top to bottom in a group, electron affinity decreases as we move from top to bottom, and the size of atoms increases. The nucleus loosely binds the valence electrons as they are farther away. The attraction forces between nucleus and valence electrons become weak. This results in poor attraction for the incoming electron, and less energy is released.

Q.10) Justify the statement bigger size atoms have more shielding effect, thus low ionization energy.

Answer:

Ionization energy is the energy required to remove a valence electron from an isolated gaseous atom. It depends upon the atomic size, effective nuclear charge, and shielding effect. Effective nuclear charge and shielding effect are correlated to atomic size.

Bigger size atoms have more shielding effects. This is because of more electrons between the nucleus and valence electrons. There is a reduction in the effective nuclear charge on the electron cloud. This results in a weak electrostatic force of attraction between the nucleus and the outermost electrons. The outermost electrons are then loosely bound by the nucleus and require less energy to remove. Thus, the ionization energy of bigger atoms is low.