Periodic Classification Of Elements

Noble gases were not known when Mendeleev formulated his periodic table.
This is because the noble gases such as He, Ar etc where not discovered at that time.

First group of the periodic table is mainly known as alkali metals and they have valency = 1 and oxide with the valency = 2 so the formula of oxide would be M₂O.

The noble gases are placed in a separate group because these are unreactive and present in a very low concentration in the atmosphere and their properties do not resemble with any other group so they are placed in a separate group.

The valency of an atom refers to the number of electrons present in the outermost shell.

The element in the 3^rd period and 2^nd group is Magnesium (Mg). Its configuration is 2, 8, 2 which shows that its outermost shell has 2 valence electrons and its valency is 2.

In periodic table elements of the same group have the same chemical properties so aluminium will have same chemical properties are Gallium (Ga) and Indium (In).

These all elements belong to the same group so they have similar chemical properties.

Both the elements with atomic number 11 and 14 lie in the same group so when we move from right to left in a period atomic size decreases. So, an element with atomic number 11 will be in bigger in size.

The element with atomic number 12 is Magnesium (Mg) and element with atomic number 17 is Chlorine (Cl).

When Mg reacts with hydrogen it forms MgH₂ and chlorine forms HCl. MgH₂ is solid and HCl is liquid so MgH₂ has a higher melting point.

‘X’ has 1 valence electron and it belongs to a 4^th period so it is Potassium (K) and the ‘Y’ is of the 3^rd period and it has 7 electrons in an outermost cell with atomic number 17 so it is Chlorine (Cl). We know that poopt5assium is a metal.

Fluorine is a most reactive element in group 17 because of its small size and high electronegativity by which it accepts electron easily and forms a bond with another element.

13^th group consists of the following elements:

Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Thallium (Ti).

‘Al’ lies in the 3^rd period so; the formula of oxide of Aluminium is ‘Al₂O₃’.

Elements belonging to the same period have the same number of shells.

Example: Every element in the first period (top row) has one orbital for its electrons. All the elements in the second period (the second row) have two orbitals for their electrons.

(ii) Elements belonging to the same group have same number of valence electrons.

The valency of an atom refers to the number of electrons present in the outermost shell. The number of valence electrons in the atom will determine the last number of the atom's electron configuration.

For example Sodium has an electron configuration of 2,8,1, so it has 2 valence electrons.

Atomic number represents the number of electrons in the element and also in electronic configuration and physical and chemical properties.

These all elements (Li, Be, B, C, N, O) belong to the same period i.e. Period 2
One similarity between them is they have the same number of shells.

As Chlorine and Bromine consists of the same number of valence electrons in the outermost shell so they are kept in the same group of the periodic table.

We will first write the electronic configuration of all the elements mentioned:

17:- 2,8,7

10:- 2,8

20:- 2,8,8,2

12:- 2,8,2

19:- 2,8,8,1

15:-2,8,5

So, the elements with atomic number

17, 12, 15 belong to the same period as they have the same number of the shell.

20, 19 belong to the same period as they have the same number of the shell.

The achievements by Mendeleev were:

•Mendeleev kept some blank spaces in the periodic table for the elements that were yet to be discovered.

Eg:- Eka-aluminium, Eka-boron, Eka-silicon.

•He also predicted properties of some elements which were yet to be discovered.

Eg:- The properties of Eka-aluminium predicted by Mendeleev and those of element gallium were almost the same.

•When noble gases were discovered, they could be placed in a new group without disturbing the order.

The formula of the hydrides and oxides formed by an element were treated as one of the basic the properties of an element for its classification.

The modern periodic table contains of groups and periods.

The atoms with identical outer shell electronic cell electronic configuration are in the same groups.

The atoms with same number of shells are in same period.

(a) The atomic radii decreases from Li to F because of increase in nuclear charge which tends to pull electrons closer to nucleus and reduces the size of an atom.

b.(i) As we go down the group metallic character increases and decreases along the period.

So, the most metallic element is Li (Lithium)

(ii) As the metallic character decreases along the period, the non-metallic character increases.

The most non-metallic element is F (fluorine).

(a) Along the group non-metallic character increases so; Cl (Chlorine) is more non-metallic.

(b) As we see the trend as the atomic no. increases atomic mass also increases (other than few exceptions). So, Cl has a higher atomic mass than Al.

“The physical and chemical properties of the elements are the periodic functions of their atomic weights”.

According to Mendeleev periodic law, the characteristic property of the elements is based on their atomic weights. In this periodic table, elements are arranged in increasing order of their atomic weight.

Two observations which posed a challenge to Mendeleev’s periodic Law are:

•Isotopes of all elements posed a challenge to Mendeleev’s periodic law.

•The atomic masses do not increase in a regular manner in going from element to another so, it was not possible to predict how many elements could be discovered between 2 elements.

(i) Na has similar properties as of the same group members as K, Rb, Cs. They contain 1 valence in the outermost shell so all of them are reactive.

(ii) Nitrogen:- 2,5

Phosphorus:- 2,8,5

Electronegativity is a measure of how strongly atoms attract bonding electrons to themselves.

Nitrogen is more electronegative because in nitrogen outermost shell is nearer to nucleus and nucleus will attract electrons easily.

(i) Li and Na have a single electron in the outermost shell. They can easily lose 1 electron and produce cation which causes chemical reactions which are explosive in nature.

(ii) Na and Mg both lie in the same period so along the period atomic size decreases. So, the size of Mg is smaller than Na.

(iii) Electronegativity of F is more than Cl i.e. F has more affinity to attract electrons because of its small size due to this F is more reactive than Cl.

(i) As the effective nuclear charge acting on the valence shell increases across the period, the tendency to lose electrons decreases due to this metallic character decreases.

(ii) Atomic size increases in a group because new shells are being added as we go down the group. This increases the distance between the outermost shell and nucleus.

(iii) The reactivity of element depend upon their tendency to lose or gain electrons to complete their outermost shell, greater is the tendency to lose/gain electron greater is the reactivity

So, in a period first tendency to lose electron decreases so reactivity decreases and the tendency to gain electron increases so the reactivity increases.

(i) Element A is closer to noble gas and is on the right side of the period and in the periodic table, non-metals are on the right side.

(ii) As we go down in periodic table atomic size increases, so the size of element C is greater than element A.

(iii) Element B is in the first group and it consists of elements having 1 electron in the outermost shell so it has valency 1.

(i) Ionisation energy:- It is the amount of energy required to remove an electron from an isolated gaseous atom.

(ii) Electron affinity:- Electron affinity is the energy which is released when an extra electron is added to an isolated gaseous atom in its outermost shell.

(iii) Atomic radii:- The distance from the center of the nucleus to the outermost shell is defined as atomic radii.

(iv) Modern periodic law:- Properties of the element are a periodic function of their atomic number.

If we check all options in option (a) it is true that elements become less metallic decreases because tendency to lose e^- decreases, in option (b) this statement is also true that valence electrons increases in option (c) it is not true because tendency to lose electrons is difficult across the period and (d) option is also correct as the oxides become more acidic.

So, the correct answer is an option (c).

If we see all the orders in option (b) is only the correct order in others the sequence is not correct and in option (a) the arrows are wrong.

We know that across the period atomic size decreases. Na has the greatest size in all these elements so it should come first this makes the option (c) and (d) invalid and in option (a) the arrows are wrong.

If we break this configuration the we see that it is 2,8,8 which completely fills the atom do it belongs to the Noble Gases.

Moseley only proposed atomic number of an element is a more fundamental property than it’s

atomic mass by doing this prediction of properties of elements could be done with more precision.

When Mendeleev started the work, many elements were not even discovered. Example – noble gases.

It was assumed that only 63 elements existed in nature and no new elements would be discovered in the future. But later on several new elements were discovered whose properties did not fit into the law of octaves.

He could only find 3 triads in elements due to this his concept was discarded.

After calcium, every 8^th do not possess properties similar to that of the first.

The element which is mentioned is Calcium(Ca) it is located in the fourth period and second group.

In the above options we know that among the elements that Na has highest metallic character Cl has lowest. So, only (b) option matches.

In the above mentioned elements we know that F has highest non- metallic character so it should come in the last and across the period non- metallic character increases so (b) option matches with this.

Eka-aluminium has the properties similar to the aluminium so it forms the same type of oxide as that of aluminium i.e. E₂O_3.

Silicon and Germanium are metalloids but boron is a non-metal but there is no such option mentioned so we will go with option (c).

if we see the options in (a) at. No 7 is nitrogen which forms 5 types of oxides in which some of them are neutral so we can’t say it forms acidic oxide and option (b) is at. no 3 which is lithium, it is a metal it forms basic oxide and same goes with option (c) it is magnesium which forms basic oxide too. In option (d) at. no 19 which is chlorine which forms all acidic oxides.

It is written that it forms acidic oxide so it is confirmed that the element is non- metal and atomic number 14 element is carbon (C).

In the modern periodic table it consists of 18 vertical columns known as Groups and 7 horizontal rows known as periods. According to this definition, only option (c) matches.

the elements B(Lithium), C(Nitrogen), D(Neon) all lie in the same 2^nd period.

The all the above mentioned atomic numbers belong to 2^nd and 3^rd groups which have a difference of 8 between them so the numbers which have a difference of 8 is ‘9’ and ‘17’. So, the correct option is (c).

2,8 is completely filled electric configuration so it must be noble gas which lies in group 18.

Carbon is the element which is essential constituent of all organic compounds and lies in group 14.

Highest atomic number in period 2 is 10 which has an electronic configuration of 2,8. So, L shell is the outermost shell in period 2.

Down the group metallic character increases and across the period metallic character decreases. Only option (a) follows the right order.

Germanium was the element which found the place later in periodic table but Mendeleev predicted germanium to as eka-silicon.

False

The assumption in the statement is wrong he assumed to be having 56 elements existed in nature.

True

True

True

False

It was Henry Moseley who proposed that the atomic number of an element is a more fundamental property than its atomic mass.

False

Na, Mg, Al, Si, P and Cl belong to 3^rd period but Li and F belong to the 2^nd period of the modern periodic table.

False

The shell just inner to the outermost shell is called pen ultimate shell. So, the above definition gets wrong for atomic size.

True

True

True