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Elements of d-block are called transition elements. Elements of f block are called inner transition elements.

(a) Atomic radii decrease when we move from left to right across a period. Atomic radii increase when we move down a group. 

(b) Enthalpy of ionisation increases across a period and decreases when we move down a group. 

(c) Electronegativity increases when we move across a period. It decreases when we move down a group.

(b) Actinides

Correct answer is (c) N(7)

Correct answer is (b) 16, 2

Correct answer is (a) 2

Fluorine atomic number 9 = 2,7 

Chlorine atomic number 17 = 2, 8,7 

Both of them show 7 valence electrons.

‘A’ – atomic number = 11 

Electronic configuration = K L M 2, 8, 1 

∴ Period number = Shell No. = 3 

Group number = Valence electron= 1

K L ∴ Atomic Number = 4 

2,2 Element = Beryllium

All noble gases show same valency i.e. ‘O’, all of them are inert gases, the chemical properties are same and hence they are placed in same group.

Group 2 = Metals 

Nature of oxide = Basic oxide

Correct answer is (c) 2, 8

(a) Two elements in same group – W and Z

(b) Elements in second period – W and X

Both of them will form ionic bond with the formula XY.

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

The electronic configuration of an atom conveys the valence electrons and number of shells. Valence electrons helps in detecting the group number. Number of shells in an atom tells the period to which it belongs.

Electronic configuration = 2, 8, 7 

∴ Atomic number = 17 

Nature of oxide = Acidic oxide

The elements which show some properties of metal and some properties of nonmetal are called semi-metals or metalloids.

Example – Boron, Silicon, Germanium, Arsenic.

All the elements in a group have the following characteristics: 

1. All elements in a group show same number of valence electrons, hence show similar properties. 

2. As we move top to bottom in a group the atomic radius goes on increasing and there is a slight gradation in properties.

(a) The atomic size goes on increasing as new shell is added in each element as we move from top to bottom. So it is Li <Na<K.

(b) The reactivity increases as it is easy to lose electrons if the size of atom increases, the nuclear force decreases.
The reactivity order is Li < Na < K.

In a period all elements have same number of shells. As we move from left to right in a period the number of electrons goes on increasing at the same time the number of protons also goes on increasing therefore attraction force of nucleus increases and pulls the valence electrons i.e. the outermost shell towards the nucleus and hence the size of atom goes on decreasing.

(a) The atomic size goes on increasing as new shell is added in each element as we move from top to bottom. So it is Li <Na<K.

(b) The reactivity increases as it is easy to lose electrons if the size of atom increases, the nuclear force decreases.

The reactivity order is Li < Na < K.

The element is aluminium. 

Its valency = 3.

As we move from left to right in a periodic table the valency first increases fill 4 and then again decreases to zero.

Dalton’s Atomic Theory:

• Each substance is made up of small particles which are called atoms.
• Atoms are indivisible.
• All atoms of an element have same properties, e.g. mass, size, chemical property, etc.
• Atoms of different elements have different properties, e.g. mass, size, chemical property, etc.
• Atoms of different elements combine in whole number ratios to form molecules.
• Atoms get rearranged during chemical reactions. A chemical reaction can neither create an atom nor destroy an atom.

Valency increases from 1 to 4 and then reduces to zero when we move from left to right across a period. Number of electrons valence increases from 1 to 8 when we move from left to right across a period.

Electron gain enthalpy: On moving from left to right atomic size decreases and effective nuclear charge increases thus, electron gain enthalpy becomes more negative. On moving down the group, atomic size increases thus, electron gain enthalpy becomes less negative.

When two atoms of the same element are joined by covalent bond, half of the distance between nuclei of two atoms is called covalent radius. In solid state, when two molecules of same substance are close to each other then half the distance between two atoms is called van der Waal radius.

The tendency to lose electrons will decrease across a period as the effective nuclear charge acting on the valence shell electrons increases.

Scandium, gallium and germanium.

Exit of electrons from outermost orbit results in formation of cation. In this process, removal of the outermost orbit increases the effective nuclear charge on remaining electrons. Hence, the size of a cation is smaller than that of a neutral atom. Acceptance of electron by an atom results in formation of anion. In this process, addition of extra electrons reduces the effective nuclear charge on electrons. Hence, size of an anion is larger than that of a neutral atom.

(a) Hydrogen resembles alkali metals, i. e. like alkali metals it combines with halogen, oxygen and sulphur to form compounds with similar formula as alkalies. 

(b) Like halogens, hydrogens also exists as diatomic molecule and combine with metals and non-metals to form covalent compounds.

Mendeleev arranged the elements on the basis of their increasing atomic mass and similarity of chemical properties.

He failed to arrange all the elements in triads having same chemical properties.

Two limitations of Newlands’ law of octaves are: 

1. The law was applicable only upto calcium. 

2. In order to fit elements into his table, Newlands adjusted two elements in wrong slot, and grouped unlike elements in same groups.

Newlands arranged the elements in the order of increasing atomic masses.

Reactivity of metals depends on the tendency to lose electrons. If the atomic size increases, the valence electrons are easily removed thereby forming positive ion. In case of metals the atomic size increases as we move down the table. Reactivity of non metals depends on the tendency to gain electrons. As we move down the group, the tendency to gain electrons decreases because the atomic size increases, the effective nuclear charge decreases.

Correct answer is (a) F

Correct answer is (b) B, C, N

(a) S < Si < P < Al

Metals are placed on the left side and nonmetals are placed on the right side of the periodic table.

(c) Atomic number

Chemical properties of elements are periodic function of their atomic number.

(a) (i) Sodium (ii) Fluorine

(b) (i) N₂ O₅ (ii) H₂O

(c) Atomic number of CO (27) is lower than Ni (28).
Also CO resembles with Rh and Ir whereas ‘Ni’ resembles with Pd and Pt.

(d) Germanium and scandium

(e)

Average atomic mass of Na 

= \(\frac{6.939 +39.102}{2}\) = 23.0205

The atomic mass of Na is the average atomic mass of Li and K and these elements resemble with each other.