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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What is an Element ? It's History, Classification and Various theories given by different Scientists to arrange the elements in the periodic table...!!

What is an Element ? It's History, Classification and Various theories given by different Scientists to arrange the elements in the Periodic Table.

The study of Elements is really a very vast and fascinating topic.

What is an Element ?

An element is a substance that is entirely made up of one type of atoms. Everything around us whether living or non-living is made up of elements.Elements are distinguished on the basis of their atomic number , which is the number of protons in the nucleus of their atoms . Elements are classified as Metals , Non-Metals and Metalloids. Atoms of the same element have different numbers of neutrons in their nuclei. That means that atoms of the same element can also vary in mass.

Short History of Elements :-

In the Ancient times People were aware of only a few basic elements like gold, silver and copper, as these elements were easily discovered in nature and were relatively simple to mine with their primitive tools. 

Aristotle Theory for defining a Element :-

Nearly around at the end of 329 B.C Aristotle a Greek Philosopher proposed that everything around us is made up of a mixture of one or more of four "roots". According to him these four roots were earth, water, air and fire. These roots were later renamed by another very famous Greek philosopher Plato as Elements . Both of these Aristotle and Plato further did nothing to discover more about elements. Thus up to 1 A.D. Only the elements like Gold, Silver, Copper, Iron, Lead, Tin, Mercury, Sulfur, Carbon Were discovered. After this up to 16th century other scientists discovered a few more elements like Phosphorus, Arsenic and Zinc.

Boyle Theory for defining a Element :-

In 1661 an English scientist Boyle Objected that it was impossible to combine the four Greek elements to form any substance and it was equally impossible to extract these elements from a substance. He therefore proposed a new definition of an element which said that an element is a substance that cannot be broken down into a simpler substance by a chemical reaction. This defination was followed for more than 300 years and even today is taught in a chemistry lecture. After this one after the other different Scientists discovered many new elements like cobalt, platinum, nickel, bismuth, hydrogen, nitrogen, oxygen, chlorine, manganese, tungsten, uranium, chromium, potassium, sodium, lithium, boron, magnesium, iodine and many more. 

After the discovery of so many elements now the Scientists were very curious to become the first to classify the elements.

Johann Wolfgang Dobereiner Theory for classification of elements :-

Thus in 1829 Johann Wolfgang Dobereiner, a German Scientist found a some groups of 3 elements which showed similar properties. He called these groups as Triads In these triads Atomic mass of the middle element was approximately the mean of the atomic masses of other two elements. Also these elements showed similar properties .

Eg. :- Lithium  Li 69

          Sodium  Na 23

       Potassium K 39

In the above triad made by Dobereiner the atomic mass of sodium is the mean of the atomic masses of lithium and potassium. These triads were known as Dobereiner's Triads. But Dobereiner could identify only a few triads from the elements. Other triads do not obey Dobereiner's rule. Hence the system of triads was not useful to classify the elements. By 1870 nearly 63 elements were discovered,due to this many Scientists were attracted towards the ways of classifying the elements

John Newlands Theory for classification of elements :-

After Dobereiner John Newlands an English chemist in 1865 arranged all these elements in an increasing order of their Atomic masses. He found that " Every 8 th element had properties similar to that of the first " .

Eg. :-

H    Li    Be   B   C   N   O

F    Na   Mg  Al  Si   P    S

Cl   K    Ca   Cr  Ti  Mn Fe

Here H is the first element and F is the Eighth element for it. Hence the properties of H and F were similar. Similarly Li is the first element and Na is the Eighth element for it. Hence they both have similar properties. But in the above table after Calcium every eighth element did not posses properties similar to that of the first. Also this periodic table did not include Inert gases, Noble gases etc.

Hence this theory was also not usefull. 

Dmitri Mendeleev Theory for classification of elements :-

In 1866 after Newland, A Russian chemist Dmitri Mendeleev examined the relation between the atomic masses of the elements and their physical and chemical properties. By analysing all the elements Mendeleev believed that atomic mass of the elements was the most fundamental property in classifying the elements . He arranged the elements in the increasing order of their atomic masses and found that the physical and chemical properties of the elements showed repetition after certain intervals. Proceding in this manner, he was able to arrange all the known elements according to their properties and thus created the first periodic table containing 63 elements.

Main Features of Mendeleev's Periodic table :-

1. Mendeleev was the first to successfully classify all the known elements.

2. Mendeleev's Periodic Law :-

The physical and chemical properties of elements are the periodic functions of their atomic masses.

3. Mendeleev kept some blank spaces in his periodic table.This vacant spaces were for elements that were yet to be discovered.

Demerits of Mendeleev's Periodic table :-

1. Hydrogen resembles alkali metals as well as halogens. Therefore no fixed position could be given to hydrogen in the periodic table.

2.There was no place for the isotopes of the various elements, which were discovered later.

3.Mendeleev's table did not include any of the noble gases.

However 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 next post would give you more knowledge about Modern Periodic Table and Classification of elements in it..!!

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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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What is an Atom ? It's Structure, Components and Properties.

What is an Atom ? It's Structure, Components and Properties.

To understand What is Atom ? First we need to know what is "Matter". Matter is anything that can be touched physically. Everything in the universe (except energy) is made of matter. An Atom is the smallest unit of Matter. Hence Everything in the universe is made of atoms.

In relation to Physics & Chemistry An Atom is defined as the smallest unit of an element, having all the characteristics of that element. Thus Atoms can join together to form molecules, which in turn form most of the objects around us.

Till the end of the 19th century, atoms were considered to be the ultimate particles of Matter, But Modern Researches have shown that an atom is not the ultimate particle of an element and it is composed of some fundamental particles. These fundamental particles of an atom are also known as "subatomic particles". These are electrons, protons and neutrons. An atom consists of two parts :- Nucleus which is present in the centre consisting of protons and neutrons and the extra nuclear portion containing electrons. The protons and neutrons present in the nucleus of an atom are collectively called as "nucleons".

1. Nucleus :- It is the central part of an atom consisting of protons and neutrons.It is positively charged.It's size(diameter) is about 10^-15 m (10^-15 m is typical for the smaller nuclei; larger ones go up to about 10 times that.) Density of nucleus is 1017 kg/m³ . Stability of Nucleus depends upon 3 factors. 

They are :-

a) Mesons :- Mesons were first predicted by a Japanese scientist Yukawa in 1935 to explain strong forces which binds together nuclear particles i.e protons and neutrons in the nucleus. The mesons may be positively charged or negatively charged or neutral. There is interconversion of proton in to neutron and vice-versa through the exchange of mesons. For eg. A proton may change into neutron by loosing a positive meson or by accepting a negative meson. Similarly a neutron may change into proton by loosing a negative meson or by accepting a positive meson.

b) Mass defect :- It is defined as the difference between the total calculated mass of nucleons and the actual observed mass of nucleons.

c) Binding Energy :- Mass defect is converted into energy which binds the nuclear particles i.e protons and neutrons in the nucleus of an atom this energy is called Binding Energy.

2.Protons :- Protons are present in the nucleus of an atom and are positively charged. E . Goldstein in 1886 discovered proton. Protons are denoted by the symbol " p ". The mass of a proton is about 1.673 x 10^-27 kg or 1.00783 amu (atomic mass unit ). It has a unit positive charge on it which is approximately equal to 1.602 x 10^-19 C .

3.Neutrons :- Neutrons are present in the nucleus of an atom and are electrically neutral. James Chadwick in 1932 discovered neutron. Neutrons are denoted by the symbol " n ". The mass of a neutron is about 1.675 x 10^-27 kg or 1.000866 amu( atomic mass unit ) . It has no charge on it.

4.Electrons :- Electrons are present outside the nucleus of an atom in the extra nuclear region revolving round the nucleus and are negatively charged. J.J. Thomson in 1896 discovered electron. Electrons are denoted by the symbol " e- ". The mass of an electron is about 9.1 x 10^{- 31} kg or 0.000545 amu. It has a unit negative charge on it which is approximately equal to -1.602 x 10^-19 c .

Important stuff regarding subatomic particles :-

1. The number of protons and electrons in an atom are always equal.

2.The number of neutrons are not necessarily equal to the number of electrons.

3. Stability of nucleus also depends upon neutron to proton ( n/p ) ratio.

Characteristics of Subatomic Particles :-

1. Atomic Number :- Atomic Number is defined as the total number of protons or electrons present in an atom. It is denoted by the symbol "Z". Chemical properties of elements depend totally on Atomic Number. Atoms of the same element have same Atomic Number.

2.Mass Number :- It is defined as the total number of protons and neutrons present in the nucleus of an atom. It is denoted by the symbol " A ". It represents the total mass of the atom. Chemical properties of elements are not dependent on mass number.Atoms of the same element can have different mass number.Thus A = p + n . An Element is represented as
 ^Z Element _A .

3. Neutron Number :- It is defined as the total number of neutrons present in the nucleus of an atom. It is denoted by the symbol " N ". Hence we can say that A = Z + N .

4. Isotopes :- Atoms of the same element having same Atomic Number( number of protons ) but different Mass Number( number of nucleons ) are known as Isotopes. Soddy in 1913 discovered Isotopes. Isotopes have different physical nature but same chemical nature. Isotopes of an element have same electronic configuration and are placed at the same position in periodic table. Eg., 8O16 , 8O17 , 8O18 .

5. Isobars :- Atoms of the different elements having same Mass Number( number of nucleons ) but different Atomic Number( number of protons ) are known as Isobars. Isobars have different physical and chemical properties. Isobars of an element have different electronic configuration and are placed at different positions in the periodic table. Eg. ¹⁸Ar₄₀ , ¹⁹K₄₀ .

6.Isotones :- Atoms of the elements having same number of neutrons but different number of nucleons are known as Isotones. Isotones of an element have different physical and chemical properties and also different electronic configuration and are placed at different positions in the periodic table. Eg, ¹⁵P₃₁ and ¹⁶Si₃₂ .

7. Isoster's :- Molecules having same number of atoms and same number of electrons are known as Isoster's. Eg, SO₂ and N₂O .

8. Isodiaphers :- Atoms of elements having different Mass Number( number of nucleons ) and different Atomic Number( number of protons ) are known as Isodiaphers. Eg. ¹⁹K₃₉ and ⁹F₁₉ .

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