Ch. No. 1. SCHOOL OF ELEMENTS
Q1. A. Fill in the blanks.
1. The formula of chloride of metal M is MCl2. The metal M belongs to group 2.
2. 18th group in the periodic table contains elements that are all gases at room temperature.
3. The arrangement of elements in a group of three is known as traids.
4. The law used by Newlands to arrange elements is known as Newlands’ Octaves.
5. The element eka – aluminum in Mendeleev’s periodic table is known as Gallium in modern periodic table.
6. Elements showing properties of both metals and non – metals are known as metalloids.
Q1. B. Match the columns.
| Column I | Column II |
| 1. Sodium 2. Cerium 3. Sulphur 4. Manganese | a. Lanthanide b. Metalloid c. Transition metal d. Noble metal e. Metal f. Non – metal. |
Ans.
| Column I | Column II |
| 1. Sodium 2. Cerium 3. Sulphur 4. Manganese | e. Metal a. Lanthanide f. Non – metal c. Transition metal b. Metalloid |
Q2. Give scientific reasons.
1. Atomic size increases down the group.
Ans.
i. The atomic numbers of the elements increase as we go down the group. Thus, the elements placed lower have more electrons.
ii. To accommodate these electrons new shells are added to the atom.
iii. These new shells take the outermost electrons farther from the nucleus causing atomic size (radius) to increase as we go down the group.
2. Metallic character decreases from left to right in a period.
Ans.
i. Atoms of metals tend to lose electrons, whereas those of non – metals tend to gain them.
ii. In a period, as we go from left to right, atomic number increases, increasing the number of electrons and protons.
iii. These additional electrons are placed in the same outer shell and are strongly attracted towards the positively – charged nucleus.
iv. This strong attraction does not allow loss of electrons, causing metallic character to decrease from left to right in a period.
3. Elements in the same group show the same valency.
Ans.
i. Valency is the number of electrons given, taken or shared by an atom to complete its outermost shell.
ii. All the elements in the same group have the same number of electrons in their outermost shells.
iii. Thus, all these elements take, give or share the same number of electrons.
iv. Hence, elements in the same group show the same valency.
Q3. Answer the following
1. How could the modern periodic table remove various anomalies of Mendeleev’s periodic table?
Ans.
i. Hydrogen was placed in group 1 and in group 17 as it resembles alkali metals as well as halogens. Therefore, no fixed position could be given to hydrogen in Mendeleev’s periodic table. But in Modern periodic table it is placed in group 1.
ii. Mendeleev arranged the elements in their increasing order of atomic masses. But some elements with higher atomic masses are placed before those having lower atomic masses. E.g. Cobalt (Co) with atomic mass 58.93 is placed before Nickel (Ni) having atomic mass 58.71. But in Modern periodic table the elements were arranged on the basis of the atomic number of elements. Thus atomic number of Co is 27 and that of Ni is 28. So Nickel is placed after Cobalt.
iii. Though isotopes of the same elements have different atomic masses, they cannot be placed separately in the periodic tale. Since isotopes are chemically similar, they had to be given the same position in the periodic table. Isotopes occupy the same position in the Modern periodic table.
iv. In Modern periodic table elements are classified according to their electronic configuration into different blocks. Thus, various anomalies (irregularity) which were seen in Mendeleev’s table due to the arrangement according to atomic masses were overcome by the Modern Periodic table.
2. In modern periodic table, which are the metals, non metals and metalloids among the first 20elements?
Ans. In the modern periodic table, among the first 20 elements, following are the metals, non – metals and metalloids.
| Metals | Non metals | Metalloids |
| Lithium (Li) Beryllium (Be) Sodium (Na) Magnesium (Mg) Aluminium (Al) Potassium (K) Calcium (Ca) | Hydrogen (H) Helium (He) Carbon (C) Nitrogen (N) Oxygen (O) Fluorine (F) Neon (Ne) Phosphorus (P) Sulphur (S) Chlorine (Cl) Argon (Ar) | Boron (B) Silicon (Si) |
3. What are the demerits of Mendeleev’s periodic table?
Ans.
i. Hydrogen was placed in group 1 and in group 17 as it resembles alkali metals as well as halogens. Therefore, no fixed position could be given to hydrogen in Mendeleev’s periodic table.
ii. Though isotopes of the same elements have different atomic masses, they cannot be placed separately in the periodic table. Since isotopes are chemically similar, they had to be given the same position in the periodic table.
iii. Some elements with higher atomic masses are placed before those having lower atomic masses, e.g. cobalt with atomic mass 58.93 is placed before nickel having atomic mass 58.71.
iv. Dissimilar elements were placed in the same group. E.g. Mn (Manganese) was placed in the group of halogens which totally differ in the properties.
4. Define atomic size. How does it vary in a period and in a group?
Ans.
i. Atomic size is determined by using atomic radius.
ii. For an isolated atom, its atomic radius is the distance between the centre of the atom (i.e. centre of its nucleus) and its outermost shell.
iii. In a period, atomic radius generally decreases from left to right. This is because the electrons are added to same shell and experience greater pull from the nucleus.
iv. Atomic radius increases in a group from top to bottom as new shells are added bringing outermost electrons farther from the nucleus.
Q. 4. Name
1. Three elements having a single electrons in their outermost shell.
Ans. Hydrogen, Lithium, Sodium, Potassium.
2. Three elements with filled outermost shell.
Ans. Helium, Neon, Argon, Krypton, Xenon.
3. Three elements having 7 electrons in their outermost shell.
Ans. Fluorine, Chlorine, Bromine, Iodine.
