Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
Group 3	Group 3	3	3	\({ LuF }_{ 3 }\)
Group 17	Group 17	7	8 - 7 = 1

Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
Group 13		3	3	\({ AII }_{ 3 }\)
Group 17		7	8 - 7 = 1

(a) The electronic configuration of an element having 5 electrons in its outermost subshell should be ns²np⁵. This is the electronic configuration of the halogen group. Thus, the element can be F, Cl, Br, I, or At.
(b) An element having two valence electrons will lose two electrons easily to attain the stable noble gas configuration. The general electronic configuration of such an element will be ns². This is the electronic configuration of group 2 elements. The elements present in group 2 are Be, Mg, Ca, Sr, Ba.
(c) An element is likely to gain two electrons if it needs only two electrons to attain the stable noble gas configuration. Thus, the general electronic configuration of such an element should be ns²np⁴. This is the electronic configuration of the oxygen family.
(d) Group 17 has metalloid, non–metal, liquid as well as gas at room temperature. F and Cl are the gaseous states, Br is liquid state and I is solid state. Here, the F, Cl, Br and I are non-metals whereas, At is metalloid.

The outer electronic configuration of 1s²,2s²,2p⁵ is 2s²,2p⁵. Therefore, this element is a p-block element and belongs to the period and group 17. Thus, the element is fluorine, F.

Main features of long form of periodic table:
(i) Groups. The vertical columns in the periodic table are known as groups. There are 18 groups in the long form of periodic table. Each group having the same electronic configuration in the outermost shell.
(ii) Periods. There are 7 periods in the long form of periodic table. It is denoted by n which means highest principal quantum number.
(iii) Lanthanoids. Group of 14 elements in the sixth period. They are placed after Lanthanum.
(iv) Actinides. Group of 14 elements in the seventh period after actinium. Both Lanthanoids and actinoids are placed in separate panel at the bottom of the periodic table.
Advantages of long form of periodic table:
(i) It gives a suitable link between the position of element and its electronic configuration.
(ii) On the basis of atomic numbers it easier to remember all the elements.
(iii) The elements in the same group have similar properties due to their outer-most (valence shell) configuration. Thus it gives is a logical classification.
(iv) Justified positions are provided to transition and inner transition elements.
(v) It makes the study of elements systematic and simple.

S.No	Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
(a)	Lithium	Group 1	1	1	\({ Li }_{ 2 }O\)
	Oxygen	Group 16	6	8 - 6 = 2

S.No	Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
(b)	Group 13	Group 2	2	2	\({ Mg }_{ 3 }{ N }_{ 2 }\)
	Group 17	Group 15	5	8 - 5 = 3

S.No	Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
(c)	Group 13		3	3	\({ AI I}_{ 3 }\)
	Group 17		7	8 - 7 = 1

S.No	Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
(d)	Group 14		4	4	\({ SiO }_{ 2 }\)
			6	8 - 6 = 2

S.No	Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
(e)	Group 15		5	3 or 5	PF3 or PF5.
	Group 17		7	8 - 7 = 1

S.No	Element	Group number	Electrons in valence shell	Valency	Formulae of binary compound
(f)	Group 3	Group 3	3	3	\({ LuF }_{ 3 }\)
	Group 17	Group 17	7	8 - 7 = 1

a. Lithium is an alkali metal (Group1). It has only one electron in the valence shell, therefore, its valence is 1.Oxygen is a group 16 element with a valence of 2. Therefore, formula of the compound formed would be Li₂O (lithium oxide).
b. Magnesium is an alkali earth metal (Group2). hence has a valence is 2.Nitrogen is a group 15 element with a valence of 8−5=3. Thus, the formula of the compound formed would be Mg₃N₂ (magnesium nitride).
c. Aluminium is group 13 elements with a valence of 3 while iodine is a halogen (group17) with a valence of 1. Therefore, the formula of the compound formed be AII₃(Aluminium iodide.).
d. Silicon is group 14 elements with a valence of 4 while oxygen is a (group16) with a valence of (8−6=2). Hence, the formula of the compound formed be SiO₂(silicon dioxide).
e. Phosphorus is a group 15 element with a valence of 3 or 5 while flurine is group 17 element with a valence of 1. Hence, the formula of the compound formed would be PF₃ or PF₅.
f. Element with atomic number 71 is a lanthanoid called lutetium (Lu). Its common valence is 3. Fluorine is a group 17 (halogen) element with a valence of 1. Therefore, the formula of the compound formed would be LuF₃(lutetium fluoride).

In p -block, when we move from left to right in a period, the acidic character of the oxides increases due to increase in electronegativity. e.g.
( i ) 2nd period
B₂O₃ < CO₂ < N₂O₃ acidic character increases.
( ii ) 3rd period
Al₂O₃ < SiO₂ < P₄O₁₀ < SO₃ < Cl₂O₇ acidic character increases.
on  moving down the group, acidic character decreases and basic character increaseas.e.g.
(a) Nature of oxides of 13 group elements 

B2O3	\(\underbrace { { Al }_{ 2 }{ O }_{ 3 } \ { Ga }_{ 2 }{ O }_{ 3 } } \)	In2 O3	Tl2O
Weakly acidic	Amphoteric	Basic	Strongly basic

Nature of oxides of 15 group elements
N₂O₅   P₄O₁₀    As₄O₁₀    Sb₄O₁₀   Bi₂O3
Strongly acidic   Moderately acidic  Amphoteric  Amphoteric Basic
Among the oxides of same element, higher the oxidation state of the element, stronger is the acid. e.g. SO₃ is a stronger is the acid than SO₂.
B₂O₃ is weakly acidic and on dissolution in water, ti forms orthoboric acid. Orthoboric acid does not act as a protonic acid ( it does not ionise ) but acts as a weak Lewis acid.
B₂O₃  + 3H₂O  \(\rightleftharpoons\)  2H₃BO₃
Boron trioxide                       Orthoboric acid
B ( OH )₃  +  H-----OH \(\longrightarrow\) [ B ( OH )₄ ]^- + H⁺
Al₂O₃ is amphoteric in nature. It is insoluble in water bur dissolves in alkalies and react with acids.
Al₂O₃  + 2NaOH \(\overset { \triangle }{ \longrightarrow } \) 2NaAlO₂       +              H₂O
Aluminiun trioxide                     Sodium meta ailuminate
Al₂O₃ + 6HCl \(\overset { \triangle }{ \longrightarrow }\) 2AlCl₃  + 3H₂O
Aluminium chloride
Tl₂O is as basic as NaOH due to its lower oxidation state ( +1 )
Tl₂O + 2HCl \(\longrightarrow \) 2TlCl + H₂O
P₄O₁₀ on reaction with water gives orthophosphoric acid.
P₄O10  +  6H₂O  \(\longrightarrow\) 4H₃PO₄
Phosphorus pentaxide                   Orthophosphoric acid
Cl₂O₇ is strongly acidic in nature and on dissolution in water, it gives perchloric acid.
Cl₂O₇  +  H₂O \(\longrightarrow\) 2HClO₄
Dichlorine heptoxide       Perchloric acid