Science Answer the questions.[Che-2M]

Hydrogen has been placed at the top of group 1, above the alkali metals in the modern periodic table because the electronic configuration of hydrogen is similar to those of alkali metals. Both, have 1 valence electron each.

1. Increasing atomic masses.
2. Grouping together of elements having similar properties.

1. Eka - boron.
2. Eka - aluminum.
3. Eka - silicon.

1. The vertical columns in a periodic table are called groups.
2. The horizontal rows of elements in a periodic table are called periods.

1. The greatest metallic character is found in the elements in the lowest part of the group.
2. The largest atomic size is found in the lowest part of the group.

Characteristics:

1. The elements in a period have consecutive atomic numbers.

2. The no. of elements in period is fixed by the maximum no. of electrons which can be accommodated in various shells. The vertical columns in a periodic table are called groups.

Characteristics:

1. The elements in a group do not have consecutive atomic numbers.

2. All the elements in a group have similar electronic configurations and show similar properties.​​​​​​​

1. B and C.
2. A and C.

1. C (2, 8).
2. B (2, 5).
3. A (2, 3).
4. 2^nd period (2 shells are filled).

1. X and Z. X and Z have zero valency hence they belong to same group: noble gases.
2. Y and Z. Y: 2,4 and Z: 2,8 so, both of them belong to second period with two shells filled.

The properties of elements are a periodic function of their atomic masses. It was the discovery of atomic number which led to a change in Mendeleev’s periodic law which was based on atomic mass.

The properties of elements are a periodic function of their atomic masses. It was the discovery of atomic number which led to a change in Mendeleev’s periodic law which was based on atomic mass.

1. In a group, all the elements have the same valency.
2. In a period, on moving from left to right, the valency of elements first increases from 1 to 4 and then decreases to zero.

1. All the chemically similar elements will have same valence electrons.
2. The 1st element in a period is determined by the no. of valence electrons in its atoms. The 1st element of every period has 1 valence electron.

1. Mg since atomic size decreases from left to right in a period.
2. K since atomic size increases on going down a group.

All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since, all the isotopes of an element have the same atomic number; they can be put at one place in the same group of the periodic table.

The atomic number of cobalt is 27 and that of nickel is 28. According to modern periodic law, the elements are arranged in order of increasing atomic numbers. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.

1. The atomic size increases gradually from lithium to francium.
2. The metallic character increases from lithium to francium.

1. All the elements of a group have similar chemical properties because they have same no. of valence electrons in their outermost shell.
2. All the elements of a period have different chemical properties because they have different no. of valence electrons in their atoms.

1. The position of isotopes could not be explained: If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table. But, the isotopes were not given separate places in Mendeleev’s periodic table. They were placed at the same place in the table. This placing of the isotopes at same place could not be explained by Mendeleev’s periodic law.
   
2. Wrong order of atomic masses of some elements could not be explained: In Mendeleev’s periodic table, when certain elements were put in their correct group on the basis of their chemical properties, it was found that the element with higher atomic mass comes first and the element with lower atomic mass comes later. Mendeleev’s periodic law could not explain this abnormal situation of wrong order of atomic masses.
   

1. Sodium (Na), magnesium (Mg) and aluminium (Al) belong to the same (third) period of the periodic table.
2. Lithium (Li), sodium (Na) and potassium (K) belong to the same group (group 1) of the periodic table.

1. Non - metal.
2. 6.
3. 2.
4. Oxygen.
5. Na 2 Y.

1. XCl 2.
2. XO.

1. XY.
2. Ionic bond.

1. 2.
2. Group 2.
3. XCl 2.

1. 2.
2. 2.
3. Metal.
4. Magnesium.

1. A in group 14; B in group 17.
2. Covalent bond.
3. AB 4.

1. Covalent bond is formed between two non-metals (A and B).
2. AB 2.

1. 9.
2. ₁₇Cl.
3. Both have the same number of valence electrons (7 electrons each) in their atoms.