Periodic Classification Of Elements

Since the atomic number of an element is 12 then its electronic configuration is 2, 8, 2. It has 2 electrons in valence shell so it is a metal. And we know that oxide of metal are basic in nature. Hence its oxide is basic.

Note-Non-metals always gain electrons and metals loose their electrons.

Note- Oxides of metals are basic in nature and oxides of non-metals are acidic in nature.

Elements in modern periodic table are classified into three parts metals, non-metals and metalloids.

Metals- Metals are those elements which donate electrons.

Ex- Na, K, Rb, Mg, Ca

Non-metals- Nono-metals are those elements which accept electrons.

Ex- N, O, F, Cl

Metalloids- Metalloids are those elements which show property of both metals and non-metals.

Ex- B, Si, Ge

Electronic configurations of X and Y are

X₁₀ = 2, 8

Y₁₄ = 2, 8, 4

Since element X has 8 electrons in outermost shell so it belongs to 18 group and element Y has 4 electrons in outermost shell so it belongs to 14 group.

The properties of elements of first group are

(i) These are highly reactive in nature.

(ii) They have a very low melting point.

The isotopes of same element have same atomic number. So they are placed at the same position in the periodic table.

(i) Group 1 elements are called alkali metals so alkali metals are Lithium (Li) and Sodium (Na).

(ii) Group 18 elements are called noble gases so noble gases are Helium (He) and Argon (Ar).

(a) Cations are smaller than corresponding atom because cations have less number of electrons as compared to corresponding atom but both have same number of protons. So the attraction of nucleus on valence electrons become strong in cations than corresponding atom.

(b) Because most elements discovered through their reactivity with other elements like oxygen but it was difficult for scientists to work with a substance that has no chemical properties due to lack of reactivity.

(a) We know that elements with same number of shells are placed in same period and period is equal to the number of shells contain. So elements B, C and D belong to same period and the period is 3.

(b) We know that those elements which have same electrons in outermost shell belong to same group. So elements A and D belong to same group. And there are 8 electrons in outermost shell so the group is 18.

Because the properties of cobalt were similar to those of rhodium (Rh) and iridium (Ir) (same group) and the properties of nickel were similar to those of palladium (Pd) and platinum (Pt) (same group). So cobalt with atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu.

The differences between Mendeleev’s Periodic Table and The Modern Periodic Table are

(i) The elements in Mendeleev’s Periodic Table were arranged by mass and the modern periodic table arranges elements by atomic number.

(ii) Mendeleev’s periodic table contains about 66 elements but modern periodic table contains presently 118 elements.

Hydrogen occupies a unique position in Modern Periodic Table due to two reasons

(i) Both hydrogen and alkali metals have one electron in valence (outermost) shell so some of the properties of hydrogen are similar to those of alkali metals and hence it can be placed in 1^st group.

(ii) Both hydrogen and halogens have one electron less than the nearest inert gas configuration in outermost shell so some properties of hydrogen are similar to halogens and hence it can be placed in group 17.

There are 7 periods and 18 groups in modern periodic table.

(i) Metallic character increases down a group because the size of elements increases down a group due to increment in the shells. So the effective nuclear charge decreases down a group. Due to this effective nuclear charge, the electronegativity decreases and the attraction of nucleus on valence shell electrons also decreases. So the tendency of loosing electrons increases. Hence metallic character increases down a group.

(ii) From left to right in a period, the effective nuclear charge increases because charge increases from left to right due to increment of proton. But the size remains same because number of shells remains same from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of nucleus on valence shell electrons also increases. So the tendency of the elements to lose electrons decreases. Hence metallic character decreases from left to right in a period.

Note- Effective nuclear charge is defined as the charge per unit volume.

(a) Electronic configuration of element D is

D₁₅ = 2, 8, 5

Element D has 5 valence electrons and its valency is 3 because it requires 3 electrons to acquire inert gas configuration.

(b) A₁₂ = 2, 8, 2

B₁₃ = 2, 8, 3

C₁₄ = 2, 8, 4

D₁₅ = 2, 8, 5

Since these elements are belong to same period because all elements have same shells and we know that radius of elements decreases from left to right in a period so element A will have largest radius.

(c) We know that metallic oxides are basic in nature and metallic nature decreases from left to right in a period so element A will form the most basic oxide.

(a) O₈ = 2, 6

Since valency of oxygen is 2 and element A produces oxide A₂O (i.e two atoms of element A and one atom O are present in A₂O molecule) so 1 electron must be present in the outermost shell of element A.

(b) Since element A contain 1 electron in outermost shell so it belongs to 1 group.

(c)Since it has 1 electron in outermost shell so it will donate this electron to acquire noble gas electronic configuration. Hence it is metal.

(a) Since element X belongs to 3^rd period and group 16 that means it has 3 shells and 6 valence electrons. So its valency is 2 because it requires 2 electrons to acquire stable noble gas configuration.

(b) Since hydrogen has 1 valency and element X has 2 valency. So the molecular formula will be H₂X.

(c) Since element X belongs to 3rd period and group 16 that means it has 3 shells and 6 valence electrons so its electronic configuration will be 2, 8, 6. Hence element X is sulphur (S) and it is non-metallic.

(a) Since element X burns in the presence of oxygen to form a basic oxide this means that it is metal and it’s atomic number is 20 so it is calcium (Ca). It’s electronic configuration is

Ca₂₀ = 2, 8, 8, 2

(b) We know that group number is equal to valence shell electrons and period is equal to number of shells contain by the element. Element X has 2 valence shell electrons and 4 shells so its group number is 2 and period is 4^th.

(c) CaO + H₂O → Ca(OH)₂

Note- Balancing of chemical equation-

(i) Here one Ca atom is in left side and one Ca atom is in right side. So Ca atoms are equal.

(ii) And we shall check that other atoms are equal or not. If the other atoms are equal to both side then the equation is balanced. If the other atoms are not equal then we shall equal them.

Now Ca atoms are equal, H atoms are equal and O atoms are also equal on both sides. So the chemical equation is balanced.

(a) Since the size of elements increases down a group due to increment in the shells. So the effective nuclear charge decreases down a group. Due to this effective nuclear charge, the electronegativity decreases and the attraction of nucleus on valence shell electrons also decreases. So the tendency of the elements to lose electrons increase down a group.

(b) From left to right in a period, the effective nuclear charge increases because charge increases from left to right due to increment of proton. But the size remains same because number of shells remains same from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of nucleus on valence shell electrons also increases. So the tendency of the elements to lose electrons decreases.

(a) Na₁₁ = 2, 8, 1

Since all elements have 3 shells so these elements belong 3^rd period.

(b) The missing element is phosphorus (P) which has 15 atomic number. So it should be placed between Si and S.

(c) Si shows the property of catenation because it has 4 valence electrons so it forms long chain.

(d) We know that noble gases are less reactive due to complete octet. So Ar is less reactive because Ar is a noble gas.

(a) Inert elements are placed in group 18.

(b) Elements B and E are placed in group 16 that means these have 6 electrons in outermost shell and these elements require 2 electrons to acquire noble gas configuration. So elements B and E will form B^2- and E^2- type ions.

And elements C and F are placed in group 17 that means these have 7 electrons in outermost shell and these elements require only 1 electron to acquire noble gas configuration. So elements C and F will form C^- and F^- type ions.

(c) We know that elements with similar properties are placed in same group. So element F would have chemical properties similar to C.

(d) Since A is placed in 1^st period so it has only one shell.

(e) A and D are placed in group 18 so these both elements have 8 electrons in outermost shell.

(f) Most abundant element in the earth’s crust is B because B is oxygen and oxygen is most abundant element in earth’s crust.

(a) (i) We know that noble gases have complete octet so element D is noble gas.

(ii) Element E is soft metal with a low density. Element E is Rubidium because its atomic number is 37.

(iii) Element B has giant covalent structure similar to diamond because element B has 4 valence shell electrons like carbon.

(iv) Element A can form a negative ion of the type X^3- because element A has 5 electrons in outermost shell and it requires 3 electrons to acquire noble gas configuration.

(b) On increasing the atomic number, the number of electrons in valence (outermost) shell increases but number of shells will be same in all atoms of a period.

Ex- The elements of 2^nd period are

Li₃ = 2, 1

Be₄ = 2, 2

B₅ = 2, 3

C₆ = 2, 4

N₇ = 2, 5

O₈ = 2,6

F₉ = 2, 7

Ne₁₀ = 2, 8