These are the free notes for class 9 chemistry. It is about periodic table and periodicity written by sir umair khan. Class 9, Chapter 8, Periodic table and periodicity.
Solved exercise
Q1: Multiple-choice questions. (MCQs)
1. In which period and group will you place the element which is an important part of the solar cell?
(A) Third period and 14th group (Correct)
(B) Second period and 14th group
(C) Third period and 15th group
(D) Third period and 16th group
2. Identify the electronic configuration of the outermost shell of a transition metal.
(A) ns2 np4
(B) ndxns2 (Correct)
(C) ns2 np6
(D) ns2np5
3. Which is the softest metal?
(A) Na (Correct)
(B) Ca
(C) Al
(D) Zn
4. A yellow solid element exists in allotropic forms which is also present in fossil fuel
Indicate the name.
(A) Carbon
(B) Iodine
(C) Aluminium
(D) Sulphur (Correct)
5. How many electrons can nitrogen accept in its outermost shell?
(A) 2
(B) 3(Correct)
(C) 4
(D) 5
6 Which element is the most reactive element?
(A) Oxygen
(B) Chlorine
(C) Fluorine (Correct)
(D) Nitrogen
7. Which element has the highest melting point?
(A) Na
(B) K
(C) Rb (Correct)
(D) Cs
8. In what order does the metallic character change in the second group?
(A) Mg > Ca > Ba > Sr
(B) Sr > Ba > Ca > Mg
(C) Mg > Sr > Ca > Ba
(D) Ba > Sr > Ca > Mg (Correct)
9. Which of the following best describe the correct order of oxygen, fluorine, and
Nitrogen’s atomic radii?
(A) O < F < N
(B) N < F < O
(C) F < O < N (Correct)
(D) O < N < F
10. The element having less value of ionization energy and less value of electron affinity is likely
to belong to:
(A) Group 1 (Correct)
(B) Group 13
(C) Group 16
(D) Group 17
Q2: Questions for short answers.
Q.1 Why was the atomic number chosen to arrange the elements in the periodic table?
Ans: ATOMIC NUMBER TO ARRANGE ELEMENTS
The atomic number was chosen to arrange the elements in the periodic table because the atomic number is a more fundamental property, and many of the discrepancies in Mendeleev’s periodic table were removed when they were arranged in increasing order of their atomic number.
Q.2 What is the significance of the word periodic?
Ans: SIGNIFICANCE OF THE WORD PERIODIC
The significance of the word periodic is that it represents the recurring pattern after regular intervals. Periodic” means something happening or repeating at regular intervals of time.
Q.3 Why does the size of a period increase as we move down the periodic table?
Ans: SIZE OF PERIOD DOWN THE P/T
Ans. As you go down, the number of electronic shells increases, Addition of d-sub
shell elements or transition metals, the presence of f-sub-shell lanthanide and actinide causes an increase in period size..
Q.4 In a group, the elements have the same number of electrons in the outermost shell.
Why is it so?
Ans: VALENCE ELECTRONS IN A GROUP
In a group, the elements have the same number of electrons in the outermost shell because the group number represents the number of valence electrons (electrons in the outermost shell). Due to same number of valence electrons, all elements in a particular group will have similar chemical properties.
Q.5 Do you expect calcium to be more reactive than sodium? Give the reason of your
answer.
Ans: REACTIVITY OF CALCIUM AND SODIUM
No, sodium is more reactive than calcium because sodium has 1 electron in the valence shell (calcium has 2 electrons in the valence shell). It becomes easier for the sodium to lose 1 electron from its valence shell as compared to calcium.
Q.6 Which element has the maximum atomic radius, and which element has the
minimum atomic radius in the third period?
Ans: MAXIMUM AND MINIMUM ATOMIC RADIUS IN 3RD PERIOD
Ans. In the 3rd period, the element with the maximum atomic radius is sodium (186 pm) and the element with the minimum atomic radius is chlorine (79 pm).
Q.7 Why are the most electronegative elements present in sixth and seventh groups?
Ans: ELECTRONEGATIVE ELEMENTS IN SIXTH AND SEVENTH GROUPS
The most electronegative elements are found in the sixth (16th) and seventh (17th) groups, specifically oxygen and fluorine, respectively, because they have a high effective nuclear charge due to the smallest atomic sizes among the rest of the elements in the respective groups.
Q.8 The first ionization energy value of magnesium is less than the second one. Give a reason.
The first ionization energy value of magnesium is less than the second one because after the removal of one electron, it becomes a positive ion, and nuclear attraction on the remaining electrons increases, thus it becomes difficult to remove the second electron from the ion.
Q.9 Is it possible for two metals or two non-metals to form an ionic bond?
Ans: IONIC BOND BY METALS OR NON-METALS
No, it is not possible for two metals or two non-metals to form an ionic bond. An ionic bond only forms between a metal and a non-metal because it requires one atom to readily lose electrons (metal) and another atom to readily gain electrons (non-metal). Similarly, two metals can only form a metallic bond.
Q.10 Which element has the lowest value of ionization energy, and which element has the highest value of electronegativity?
Ans: LEAST AND HIGHEST ELECTRONEGATIVITY
Ans. The element with the least value of ionization energy is caesium (377 KJ/mol), and the element with the highest value of electronegativity is fluorine (4.0).
Additional short questions and answers.
Q.11 Define the periodic table.
Ans. The elements are arranged according to ascending order of their atomic numbers. From left to right in a horizontal row, properties of elements are found repeating after regular intervals. This results in the form of a table in which elements of similar properties are placed in the same vertical columns.
Q.12 What do you mean by groups and periods in the Periodic Table?
Ans.
- Groups: Vertical columns in the periodic table are called groups. They are studied from top to bottom. There are 18 groups in the periodic table.
- Periods: Horizontal rows in the periodic table are called periods. They are studied from left to right. There are 7 periods in the periodic table.
Q.13 Why are the properties of elements different in period and the same in group?
Ans. In the modern periodic table, the electronic configuration of the elements continues changing when we move from left to right in a period. Due to this, the elements in a period show a gradual change in the properties while the elements in a group show similar properties due to having the same electronic configuration of last shell electrons.
Q.14 What do you mean by periodicity?
Ans. The periodic table shows repetition of the properties of elements after regular intervals. The study of properties in a sequence is called periodicity in the properties of elements.
Q.15 Why are lanthanides and actinides placed at the bottom of the periodic tables?
Ans. In the 6th and 7th periods, two series of fourteen elements each have been accommodated. Because of the space problem, these two series were placed at the bottom of the periodic table to keep it in a manageable and presentable form. The first series starts after lanthanum (La=57), and it is called lanthanides. The second series starts after actinium (Ac=89), and it is called the actinides.
Q.16 Define lanthanides. To which period do they belong?
Ans. The elements which follow the Lanthanum with atomic number 58 to 71 are called lanthanides. They belong to the 6th period. The name of the series is based on the element named lanthanum.
Q.17 Define actinides. To which period do they belong?
Ans. The elements that follow actinium, having atomic numbers 90 to 103, are called actinides. They belong to the 7th period. The name of the series is based on the element named actinium.
Q.18 Define normal or representative elements.
Ans. Elements which belong to ‘s’ and ‘p’ blocks are called normal or representative elements. In these elements, all the inner shells are completely filled with electrons. Only their outermost shells are incomplete.
Q.19 Define transition elements.
Ans. Those elements that belong to ‘d ‘ and ‘f’ blocks are called transition elements. Their d-subshells or f-subshells are in the process of completion with electrons.
Q.20 Define s-block elements.
Ans. Group 1 and Group 2 elements have the electronic configuration that ends at the s-subshell. Therefore, these elements are called s-block elements.
Q.21 Define p-block elements.
Ans. Elements in groups 13–18 (except He) are known as p-block elements because their electronic configuration ends at the p-subshell.
Q.22 Define f-block elements.
Ans. Lanthanides and actinides are known as f-block elements since their valence electrons lie in f sub-shell.
Q.23 What are halogens?
Ans. All elements in group 17 have an ns2 np5 configuration in their outer shells. They show a strong tendency to gain one electron to become an anion. They are called electronegative elements. These elements are very reactive non-metals and form salts with metals. Hence, they are called halogens (salt-forming).
Example: F, Cl, Br, I
Q.24 What do you know about the elements of Group-16?
Ans. Elements present in group 16 have an ns2, p4 configuration inThese are the free notes for class 9 chemistry. It is about periodic table and periodicity written by sir umair khan. Class 9, Chapter 8, Periodic table and periodicity. as a strong electronegative element.
O2(g) + 2e– → 2O2-(g)
Q.25 How many elements did Mendeleev arrange, and what mistake did he make in his periodic table?
Ans. Mendeleev arranged only 63 elements in his periodic table because only these elements were discovered at that time. He left many space vacant for those elements which were yet to be discovered.
Q.26 How many total versions of the periodic table and what are they?
Ans. EG. Mazurs collected 700 different published versions of the periodic table. Many forms retain the regular rectangular structure. Some forms had spirals circles and triangular shapes.
Similarities in the Chemical Properties of Elements in the Same Group
Q.27 Why are the elements with similar chemical properties placed in the same group?
Ans. The chemical properties of elements depend largely upon the number of electrons present in their outermost shells. Since in a group of the periodic table all the elements have the same number of electrons in the outermost shell, they are expected to show similar chemical properties.
Q.28 What do you mean by group number and period number?
Ans.
- Group Number: It tells about the number of electrons present in the outermost shell of an atom.
- Period Number: It tells about the number of electronic shells present in an atom.
Variation of Periodic Properties in Periods and Groups
Q.29 Is atomic size always increase with increasing atomic numbers?
Ans. Although you might expect atoms to become larger with the increase in their atomic numbers, this does not always occur because the size of atoms is determined by the diameter of their electron shells.
Q.30 Describe the trends of electronegativity in the period and group.
Ans. It increases from left to right in a period and decreases from top to bottom in a group. Thus, the most electronegative atoms are found at the top right-hand corner of the periodic table. The most electronegative atoms are F, O, N, and Cl.
Q.31 Which property explains why chemical reactions occur?
Ans. Electronegativity is one of the most well-known properties for explaining why chemical reactions take place.
Q.32 Define atomic radius. Give an example.
Ans. Atomic radius is defined as half the distance between the nuclei of two identical bonded atoms. It is expressed in pm (1 pm = 10-12m). For example, the distance between the nuclei of two bonded carbon atoms is 154 pm. Half of this distance, i.e., 77pm, is therefore the radius of a carbon atom. This is also called the covalent radius of a carbon atom.
Q.33 Why does the atomic radius of elements increase down the group?
Ans. The atomic radius increases from top to bottom in a group.
Reason: Because the increase in electronic shell in atoms of successive periods decreases the effective nuclear charge.
Q.34 What is the advantage of the metallic character of metal?
Ans. The metallic character of a metal generally determines its level of reactivity.
Q.35 Why does the size of atoms decrease in a period?
Ans. In a period, the atomic radii gradually decreases form left to right. Because of the increase in atomic number, the effective nuclear charge increases gradually from left to right due to the addition of one more proton in the nucleus of every next atom. This increased nuclear force pulls the outermost shell more strongly towards the nucleus.
Q.36 Define ionization energy.
Ans. The ionization energy is the amount of energy required to remove the most loosely bound electron from the valence shell of an isolated gaseous atom. Its unit is kJ mol-1. The ionization energy of sodium is 496 kJ mol-1.
e.g.: Na → Na+ + 1e– (ΔH = +496 KJ/mol)
Q.37 Why is the 2nd Ionization energy of an element higher than the first one?
Ans. When an electron is removed from an atom, a mono-positive ion is formed. A mono-positive ion has one extra proton than the number of electrons. Its nuclear charge attracts the remaining electrons more strongly. Hence, more energy is needed to remove the second electron. That is why the 2nd I.E. has greater value.
Q.38 Give the trend of ionization energy in the periodic table.
Ans.
- Along Group: Ionization energy decreases while moving from top to bottom in a group.
Reason: Because in a group, more and more shells lie between the valence shell and the nucleus, reducing the electrostatic force of attraction on valence electrons, so they are taken away easily. - Along Period: Ionization energy increases while moving from left to right in a period.
Reason: Because in a period, the size of atoms reduces and valence shell electrons are held strongly by the electrostatic force of attraction of the nucleus.
Q.39 Define electron affinity. Write down its unit.
Ans. The amount of energy released when an electron is added to the outermost shell of an isolated gaseous atom. Its unit is kJ/mol.
Example: Electron affinity of fluorine is -328 kJ/mol.
F + e– → F– (ΔH = -328 kJ/mol)
Q.40 Why does electron affinity decrease in a group?
Ans. Electron affinity decreases in a group because of increasing atomic size and the shielding effect in a group.
Q.41 Why does electron affinity increase in a period?
Ans. Electron affinity increases in a period because of a decrease in atomic size from left to right in a period.
Q.42 What is the difference b/w first and second electron affinities?
Ans. First electron affinity can be negative, positive, or zero, while the second electron affinity is always positive.
Q.43 Define electronegativity. Write the electronegativity of Nitrogen and oxygen.
Ans. The ability of an atom to attract the shared pair of electrons towards itself in a molecule is called electronegativity. The electronegativity of Nitrogen is 3.0, while that of Oxygen is 3.4.
Metallic Character and Reactivity
Q.44 What do you know about electropositive metals/ alkali metals and their reactivity?
Ans. All elements of group I have one electron in their outermost shell, so they show a strong tendency to lose their valence electron, forming cations. They are thus known as electropositive metals. These metals react vigorously with water, producing hydrogen and giving alkali in the solution.
Na(s) + H2O(ℓ) → NaOH(aq) + H2(g)
Alkali metals also react with halogens, giving halides.
Na(s) + Cl2(g) → 2NaCl(s)
The reactivity of alkali metals gradually increases down the group.
Q.45 What do you know about alkaline earth metals?
Ans. The second group elements also show a tendency to lose both of their outermost electrons, forming dipositive ions called alkaline earth metals.
Mg(s) → Mg2+(g) + 2e–
Q.46 Why does the reactivity of metals increase down the groups?
Ans. The reactivity of metals increases down the group because of increasing atomic size and decreasing ionization energy. As the atomic size increases down the group due to the addition of shells, it becomes easy to remove an electron from the outermost shell of a metal, which causes increased reactivity of metals.
Q.47 What is electropositivity? Explain with an example.
Ans. Electropositivity is the property of an element to readily lose its valence electrons and gain a positive charge. Metals are highly electropositive elements.
e.g., Sodium atom can lose 1 electron to form a positive ion. Na(s) → Na+(g) + 1e–
Q.48 Why does metallic character decrease along a period and increase in a group?
Ans.
- In a period: Atomic radius of elements decreases in a period so the removal of electrons becomes more difficult, resulting in a decrease of metallic character.
- In a group: Atomic radius and shielding increase down the group, so removal of electron becomes easier, which results in an increase of metallic character.
Q.49 What is the trend of variation in densities of alkali metals?
Ans. Densities of alkali metals increase down the group in the periodic table due to an increase in atomic masses.
Densities of First Group Elements
| First group Elements | Li | Na | K | Rb | Cs |
|---|---|---|---|---|---|
| density g/cm3 | 0.53 | 0.97 | 0.89 | 1.63 | 1.879 |
but varies less significantly from left to right in a period. It is expressed in g/cm3.
Q.50 Why are aluminum and silicon less reactive than sodium and magnesium?
Ans. Aluminum and silicon are less reactive than sodium and magnesium. This is because a comparatively larger number of valence electrons makes it difficult to lose electrons.
Q.51 What is density? Write its trend.
Ans. It is the mass of a substance in a unit volume. The density of elements generally increases from top to bottom in a group.
Q3: Constructed response questions.(CRQs)
Q.1. Suppose a new element is discovered. Where would you like to accommodate this element? in the periodic table:
Ans. It would be placed in the periodic table according to its atomic number, and if the new element has similar properties to existing elements, it would go in the same group.
For example, if it behaves like a metal, it might be placed in one of the groups on the left side.
If it has properties similar to non-metals, it could be placed on the right side.
Q.2. What is the first element of the periodic table? Will it lose an electron or gain it?
Ans. The first element of the periodic table is hydrogen. It can either lose an electron to achieve a stable configuration like a metal or gain an electron to achieve a stable configuration like a non-metal.
Q.3 Atomic radii of boron and aluminum are 88 pm and 125 pm, respectively. Which element is expected to lose an electron or electrons easily?
Ans. Aluminum is expected to lose electrons more easily than boron since it has a larger atomic radius and a higher shielding effect.
Q.4 How would you find the atomic radius of an atom?
Ans: We can determine the atomic radius of an atom by measuring the distance between the nuclei of two atoms and dividing that distance by two.
Example:
The distance between the nuclei of two carbon atoms in its elemental form is 154 pm, which means its half 77 pm is the radius of a carbon atom.
Units:
The units of atomic radius are as follows:
Nanometer (10–9m)
Picometer (10–12m)
Q.5 Why is it not possible for an oxygen atom to accept three electrons to form O3- ion like nitrogen, which can accept electrons to form N3-?
Ans: It is not possible for oxygen atom to accept three electrons to form O3- ion, like nitrogen, which can accept electrons to form N3- because the valence shell of an oxygen atom has the capacity to accept two electrons only.
Q5: Investigative Questions.
Q.1 Arrangement of the elements in the form of a periodic table is a remarkable achievement of chemists. Comment on this statement, citing the benefits of this table.
Ans: BENEFITS OF PERIODIC TABLE
The following are the benefits of the periodic table:
(i) It helps scientists to find information about an element, such as its atomic number, atomic mass, and chemical symbol, quickly.
(ii) It helps to understand the trends in physical properties of the elements, such as ionization energy, electronegativity, and atomic radius.
(iii) Similarly, it helps to understand the trends in the chemical properties of elements in the periodic table.
(iv) It helps scientists to predict the properties of elements and compounds easily.
Q.2 Both lithium and beryllium show behaviour different from rest of the alkali and alkaline
earth metals respectively. Can you think of the possible reasons for this difference?
Ans: DIFFERENT BEHAVIOUR OF LITHIUM AND BERYLLIUM
Both lithium and beryllium show behaviour different from rest of the alkali and alkaline
earth metals respectively because of the following reasons:
(i) Small size of atoms:
Both lithium and beryllium have considerably smaller atomic radii compared to other elements in their groups which results in an increased attraction between the nucleus and the valence electrons.
(ii) Higher Ionization energy of atoms:
Due to their small size, lithium and beryllium require more energy to remove their outermost electrons, making them less reactive than other alkali and alkaline earth metals.
(iii) High polarizing power of atoms:
The small size and high positive charge of lithium and beryllium cations allows them to strongly polarize the electron clouds of nearby anions, leading to a more covalent character in their bonds.
(iv) Absence of d-orbitals:
Lithium and beryllium lack d-orbital in their atoms.
Q.3 Modern periodic table is the amended form of the earlier table developed by
Mandeleev. Elaborate how these two tables are different from each other.
Ans: DIFFERENTIATION
The differences between mole and Avogadro’s number are as follows:
| Mendeleev’s Periodic Table | Modern Periodic Table |
| Definition | |
| · A table obtained by arrangement of elements into groups and periods in ascending order of their atomic masses is called Mendeleev’s periodic table. | · A table obtained by arrangement of elements into groups and periods in ascending order of their atomic number is called Modern periodic table. |
| Basis of Periodic Table | |
| · It is based on arrangement of elements in ascending order of their atomic masses. | · It is based on arrangement of elements in ascending order of their atomic masses. |
| Periodic Law | |
| · Properties of elements are periodic function of their atomic masses. | · Properties of elements are periodic function of their atomic number. |
