What is Modern Periodic Table ?? It's Features, Characteristics and Classification of Elements in it.

Modern Periodic Table

The Modern Periodic Table is a tabular display of all the chemical elements , arranged on the basis of their properties. Mendeleev's Periodic Table had only 63 elements whereas the Modern Periodic table have 109 elements.

In 1913 Sir Henry Moseley,an English Physicist discovered that atomic number is the most fundamental property of an element and not it's atomic mass. This discovery changed the whole perspective about elements and their properties. Also the Mendeleev 's periodic law was modified to Modern Periodic Law.

Modern Periodic Law :- The chemical and physical properties of elements are the periodic function of their Atomic numbers. The Modern Periodic Table was constructed based on the above law.

Features of the Modern Periodic Table :-

1)The elements in the periodic table are arranged in the increasing order of their atomic number or electronic configurations .

2) The horizontal rows are called as "periods".

3) The vertical columns are called as " groups" .

4)The Modern periodic table consists of 7 periods and 18 groups.

PERIODS :-

a) Each period starts with an alkali metal and ends with an inert gas element.

b) Elements present in the same period have same number of shells which is equal to the periodic number.

c) The 1st period is the shortest period containing only 2 elements i.e. Hydrogen (H) & Helium (He). In this period, only the 1s orbital is filled.

d) The 2nd period contains 8 elements starting with Lithium (Li) and ending with Neon (Ne). In this period, the 2s & 2p orbitals are filled.

e) The 3rd period also contain 8 elements starting with Sodium (Na) and ending with Argon (Ar). In this period the 3s & 3p orbitals are filled.

f) The 4th period is the long period with 18 elements, starting with Potassium (K) and ending with Krypton (Kr). In this period, 4s & 4p and also the 3d orbitals are filled.

g) The 5th period is also the long period with 18 elements, starting with Rubidium (Rb) and ending with Xenon (Xe). The 5s & 5p along with 4d orbitals are filled.

h) The 6th period is the longest period with 32 elements. It not only includes 10 elements belonging to 5d series i.e. from Lanthanum (La) to Mercury (Hg) but also contains 14 elements belonging the 4f series called lanthanides i.e from Cerium (Ce) to Lutetium (Lu). In this period, the 6s & 6p along with the 4f & 5d orbitals are filled.

i) The 7th period is an incomplete period. It includes Fr along with the 14 elements belonging to 5f series called actinides i.e from Thorium (Th) to Lawrencium (Lr). In this period, the 7s & 5f orbitals are filled.

GROUPS :-

a) The Modern Periodic Table consists of 18 groups or vertical columns.

b) Elements present in the same group show same physical and chemical properties.

c) Also the elements present in the same group have same number of electrons in the outermost shell.

d) According to American convention, the groups are denoted by roman numerals followed by either an capital alphabet "A" if the group is in the s-block or p-block, or by "B" if the group is in the d-block .

e) Thus the groups from 1 to 18 are denoted as IA , IIA, IIIB, IVB, VB, VIB, VIIB, VIII, IB ,IIB, IIIA, IVA, VA, VIA, VIIA and 0 (zero).

f) The elements from IA to VII A group i.e the elements in the groups 1, 2, 13, 14, 15, 16 & 17 are called as representative elements.

g) The 18th group or zero group elements are called as inert gases or noble gases . This group includes the elements He, Ne, Ar, Kr, Xe and Rn.

h) The elements in the groups from IIIB to IIB i.e., from group 3 to 12 are called as transition elements .

i) The two rows placed at the bottom of the periodic table i.e the Lanthanides and Actinides are also considered to be the part of IIIB group (i.e. group 3). These are usually called as inner transition elements .

The 14 elements with atomic number 58 to 71 are called as Lanthanides, whereas the 14 elements with atomic number 90 to 103 are called as Actinides.

Classification of Elements :-

At present elements upto atomic number 118 are known of which the recently discovered elements are man-made. With such a large number of elements,it is very difficult to study the chemistry of the individual elements and the innumerable compounds they form.Hence it is necessary to classify the elements for their systematic and organised studies. Therefore the Elements in the Periodic Table are classified on the basis of their electronic configuration into 4 blocks as s , p , d and f . 

1. S-block :- The Group 1 and 2 elements contain 1 or 2 electrons in their outermost shell and are called as s-block elements.

2. P-block :-The elements in the Groups from 13 to 17 and the 0 (zero ) group elements contain 3 to 8 electrons in their outermost shell and are called as p-block elements.

3. D-block :- The elements in the Groups from 3 to 12 contain 1 to 2 electrons in their outermost shell and are called as Transition elements. They are all metals.

4. F-block :- The 14 elements with atomic number 58 to 71 are called as Lanthanides whereas the 14 elements with atomic number 90 to 103 are called as Actinides.These 28 elements are collectively called as f-block elements.

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Electrons Behaviour in an Atom. What are orbits, Orbitals, Shells, Sub-shells, Quantum state & Quantum Numbers ??

Electrons behaviour in an Atom. What are Orbits, Orbitals, Shells, Subshells, Quantum states & Quantum Numbers?

Chemistry is mostly the study of electron interactions between atoms and molecules. Understanding the behavior of the electrons in an atom is an important part for understanding chemical reactions. We all know that Electrons are the negatively charged subatomic particles which revolve round the nucleus of an atom in specific orbits.

What are Orbits ????

Orbits are the paths in which an electron travels around the nucleus of an atom. But according to Heisenberg's Uncertainity Principle it is impossible to predict where an electron is and where it's going next.Thus it is impossible to locate the exact path in which the electron revolve round the nucleus and hence orbits cannot exist.

What are Orbitals ????

An Orbital is basically the region where the probability of finding the electron is maximum. Electrons are constantly spinning in these atomic orbitals and those shells or orbitals, are at specific distances from the nucleus.

Below is the very clear and straight forward explaination about shells, subshells and orbitals.

Electrons revolve around the nucleus in the Electronic shells :- K, L, M, N, O, P & Q

Each Electronic shell consists of some sub-shells :-

1. K contains only one sub-shell : s

2. L contains two subshells : s & p

3. M contains three subshells : s, p & d

4. N, O, P & Q contain 4 subshells : s, p, d & f .

Each sub-shell consists of some orbitals where the electrons are

present. Each orbital can hold up to 2 electrons.

1) s contain 1 orbitals and can hold upto 2 electrons.

2) p contain 3 orbitals and can hold upto 6 electrons.

3) d contain 5 orbitals and can hold upto 10 electrons.

4) f contain 7 orbitals and can hold upto 14 electrons.

Thus the Electrons have a fixed amount of energy depending upon the orbitals in which they are present. Hence they do not fall in to nucleus. Electrons in the first orbit closest to the nucleus has the lowest energy whereas electrons in the subsequent orbits posses higher energy. Electrons in an atom occupy shells in the increasing order of their energies.

Quantum Numbers and Quantum States :- 

Quantum numbers are often used to describe specifically the energies and shapes of orbitals in atoms.

Each electron in an atom is described by four different quantum numbers, they are : n , l , ml and ms .

1) The Principal Quantum Number " n " :- 

n is called the Principle Quantum Number. It is also known as the radial quantum number as it also defines the distance of the electron from the nucleus. The principal quantum number has integral values of n = 1, 2, 3... And as the value of n increases, the farther is the electron from the nucleus, the larger is the size of the orbital,and the larger is the Atom. n cannot be 0 or any negative integer, because there exists no atoms with zero or a negative amount of energy.

The shell "K" has been given the value n = 1, the"L" shell has been given the value n = 2 & so on...

Thus,      n 1  2  3 4

          shell K L M N ...

2) The Orbital Angular Momentum Quantum Number " l " :-

l is the orbital angular momentum quantum number as it determines the shape of an orbital, and therefore the angular distribution.It is also known as Azimuthal Quantum number.Each value of l indicates a specific s, p, d or f subshell.

The Azimuthal Quantum number has integral values of l = 0 to l = n - 1 for each value of n. This means that for n = 1, the first shell, there is only l =1-1 = 0 subshells i.e the shell and subshell are identical.

Thus to designate a particular subshell we write the number of the shell itself followed by the subshell designator as nl. This illustrates the relationship between "n" and "l". Thus 1s is the first shell having one orbital type associated with it.

3) The Magnetic Quantum Number "' ml " :-

ml is the magnetic quantum number as it determines the number of orbitals and their orientation within a subshell. The magnetic quantum number has integral values of ml = - l to + l including 0. It splits the subshells into individual orbitals.

4) The Electron Spin Quantum Number " ms " :- 

ms is the fourth and the final Quantum Number. Unlike the above three Quantum Numbers ms does not depend on another quantum numbers.It designates the direction of the electron spin.It has only two possible values of +1/2 or -1/2 .

Thus by specifying the four Quantum numbers the address of any electron in a given atom can be completely defined.

Electronic Configuration :- The arrangement of electrons of each element in their orbitals or shells is known as it's electronic configuration.

Pauli's Exclusion Principle :- In 1925 an Autrian physicist, Wolfgang Pauli, expressed the importance of electron spin in determining electronic configurations. According to Pauli's Exclusion Principle No two electrons in an atom can exist in the same quantum state. Pauli's Exclusion Principle implies that no two electrons can have the same set of four quantum numbers n, l, ml and ms . At least one quantum number should be different. This principle assigns quantum number to electrons.

For eg. 

Let us consider the case of an Helium atom which has 2electrons .

These electrons occupy n=1 orbital, so clearly for n=1 ,l=0 , ml =0 but ms can have 2 values +1/2 or -1/2 .

Hence 1 electron will occupy n=1, l=0 , ml =0 and ms = +1/2 where as the 

 another electron will occupy n=1, l=0 , ml =0 and ms = -1/2 orbital.

Also according to Pauli Exclusion Principle, the maximum number of electrons in an orbital with principal quantum number n is 2n²  . Thus, For 1st shell, n=1,maximum electrons that can be accomodated are 2  . Similarly the maximum no. of electrons that can be accomodated in 2nd , 3rd and 4th shell are 8 ,18 and 32 . Pauli Exclusion Principle is used in arranging the elements in the periodic table .

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