Science [2 Mark Questions]

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

The no. of valence electrons in the atoms of elements decides which element will be the 1st element in a period and which will be the last in a period.

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. B and C.
2. A and C.

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

The noble gases are placed in a separate group because they are chemically very inert or unreactive (having completely filled outermost electron shells).

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

The similarity in the atoms of lithium, sodium and potassium is that all of them have 1 valence electron each.

The elements having similar chemical properties form oxides and hydrides having similar formulae. Mendeleev used these properties for creating his periodic table.

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

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

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.

Position of hydrogen: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.

On going down in a group of the periodic table, the atomic size increases. When we move from top to bottom in a group, a new shell of electrons is added to the atoms at every step due to which the size of atom increases.

The main characteristic of last elements in a period is that they all have 8 valence electrons in their atoms except helium. Such elements are called noble elements.

Mendeleev’s periodic law: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.

Position of Cobalt and Nickel: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.

The fundamental difference between the electronic configuration of group 1 and group 2 elements is that group 1 elements have 1 valence electron in their atoms whereas group 2 elements have 2 valence electrons in their atoms.

	Elements	Least atomic radius	Chemically least reactive
(i)	F, Cl, Br	F	Br
(ii)	Li, Na, K	Li	Li

The horizontal rows of elements in a periodic table are called periods.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.​​​​​​​

In order to make sure that the elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table.

The real significance of atomic number in the modern periodic classification is that it relates the periodicity in the properties of elements to the periodicity in their electronic configurations.
Example: The atomic number increases from 3 in lithium to 11 in sodium, there is a repetition of electronic configuration from 2, 1 to 2, 8, 1 (both having 1 valence electron).

The vertical columns in a periodic table are called groups.

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.

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

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

The elements are classified into groups so that the elements with similar properties fall in the same group and hence the study of a large number of elements is reduced to the study of a few group of elements.

Noble gases. Since they are chemically unreactive, so they got a place in the periodic table in the form of a separate group and hence did not disturb the original order of Mendeleev’s periodic table.

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.

Position of isotopes: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.

1. XY.
2. Ionic bond.

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

In the given set of elements, i.e., calcium, magnesium, sodium and beryllium, sodium does not belong to the set. This is because sodium is a group 1 element and the remaining three elements​belong to group 2. Elements of the same group show similar chemical properties.

Valency of X = 2.

1. XCl 2.
2. XO.

Helium and neon atoms have completely filled outermost electron shells (containing the maximum number of electrons which can be accommodated in them).

In the given set of elements, i.e., oxygen, nitrogen, carbon, chlorine and fluorine, chlorine does not belong to the set. The element chlorine belongs to the third period while the remaining elements belong to the second period.

Out of eight groups in the original periodic table of Mendeleev, first seven groups are of normal elements and eighth group is of transition elements. Noble gases were not known at that time. So, there was no group of noble gases in Mendeleev’s table.

N (2, 5 ); P (2, 8, 5); Nitrogen will be more electronegative because its atom has small size due to which the attraction of its nucleus for the incoming electron is more.

According to Mendeleev’s periodic law: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.

Modern periodic table arranges the elements according to increasing atomic numbers. So, the atomic number of cobalt (27) comes first whereas the atomic number of nickel (28) comes later.

Anomalies of Mendeleev’s classification of elements:

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

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.

Eka - aluminium and gallium are the two names of the same element as Eka - Aluminum has almost exactly the same properties as the actual properties of the gallium element. The properties: atomic mass, density, melting point, formula of chloride and formula of oxide are almost the same.

The properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table because the electronic configurations of the elements are repeated in this manner.

The isotopes of chlorine, Cl - 35 and Cl - 37 are placed in the same slot because they have similar chemical properties and same atomic number.

The 1st period has two elements because the 1st electron shell of an atom c a n take a maximum of two electrons only. The 2nd period of the periodic table has 8 electrons because the maximum no. of electrons which can be put in the 2nd shell of an atom is 8.

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