NEET Study Notes for Classification of Elements and Periodicity in Properties: Periodic table is a table that consists of all elements placed as per their similar characteristics. The arrangement of elements into groups and periods, helps us identify the properties of elements. Periodic Properties of an elements can be determined by valency and the number of shells in an atom.

• Classficiation of elements and periodicity in properties is one of the most important chapters in the Inorganic Chemistry Syllabus of NEET 2022. Around 2-3 questions can be expected from this unit. Check NEET 2022 Chemistry Syllabus
• The important topics in the unit classification of elements and periodicity in properties include modern periodic law and the long form of the periodic table, the periodic trend in properties of elements atomic radii amongst others.

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NEET Study Notes on Classification of Elements and Periodicity in Properties: Important Topics

What is a Periodic Table?

The table consists of all elements in which elements are placed according to similar characteristics

Modern Periodic Law and Long-form of Periodic Table

The basis of Modern periodic law was provided by Mosley. The square root of the frequency of rays emitted from metal is proportional to the atomic number. In a neutral atom, atom, the atomic number is equal to the number of electrons and protons. Thus the basis of the classification of elements should be their atomic number.

Modern Periodic law states that the physical and chemical properties of the elements are a periodic function of their atomic number.

Modern Periodic Table

On the basis of this modern periodic law, Bohr proposed a long form of the periodic table the horizontal lines in a periodic table are called periods, and the vertical lines in a periodic table are called groups.

Groups

• There are 18 such groups in a periodic table.
• The elements in a group 1, 2,13-17 are called representative or normal elements.
• Elements in groups 3-12 are called transition elements.
• Elements that is in a group is known as family. It is usually named after the first element.

Examples:

Alkali Metals- Elements of group 1

Alkaline earth metals- Elements of group 2

Pnicogens- Elements of group 3

Chalcogens- Elements of group 16

Halogens- Elements of group 17

Noble gases- Elements of 18

Periods

• Horizontal columns or rows are known as periods. Seven Periods of the periodic table is the shortest period, short period, long period, longest period, and Incompleted period.
• Shortest Period- It is the first period. (1H 2He) contain 2 elements.
• Short Periods- - 2nd period (3Li 10Ne) contain 8 element, 3rd period (11Na 18Ar) also contains 8 elements
• Long Periods- 4th period (19K 36Kr) contain 18 element. The 5th period from (37Rb 54Xe) also contain 18 elements. These are known as long periods.
• Longest Periods- The 6th period (55Cs – 86Rn) contains 32 elements.
• Incomplete period or the 7th period is (87Fr –) incomplete and contains only 26 elements.

Also, Read

• NEET Study Notes for Chemical Equilibrium

• NEET Study Notes for Dual Nature of Matter and Radiation

Blocks

Periodic table is divided into four main blocks (s,p,d and f) and depend upon the subshell to which valence electrons enter into:

• s-block- elements of group 1 and 2 constitute 2 compose the s-block.
• p-block- elements of group 13 to 18 compose of the p-block.
• d-block elements- elements of a periodic table in which the last electron occupies the d orbital, are called the d- block elements. These are also known as transition are elements. All these melements are metal except mercury which is a liquid element.
• f-block elements- f- block elements comprise of two horizontal rows in the bottom of the periodic table. There are a total of 28 f block elements in the periodic table. Lanthanides are elements from atomic number 58 to 71. These are also called rare earth elements Actinides are elements from 90 to 103, These are radioactive in nature.

Read NEET Study Notes for Reaction Mechanism

Periodic Properties

Properties that show gradation on moving from left to right in a period or from top to bottom in a group are called periodic properties. Such properties are atomic size, ionization energy, electron affinity etc.

What is Atomic Size?

The distance between center of nucleus of an atom to the outermost shell is known a atomic size. It is not possible to determine the absolute value of atomic radii as:

• Since the orbital does not have defined boundaries, the accurate positions electrons in an atom cannot be determined.
• The distribution of electrons in a group of atoms in influenced by other atoms. Isolation of an individual atom is not possible.

Ionic Radius

• The distance between nucleus of ion and electrons in outermost shell, which can affect the ionic bond is known as ionic bond.

Atomic Radii and Ionic Radii

Atomic and Ionic Radii decrease on moving from left to right in a period. On moving from top to bottom in a group, atomic and ionic radii increase. This increase occurs with increase in the number of shells and an increase in the atomic number.

Must Read

• NEET Study Notes for Redox Reactions

• NEET Study Notes for Hydrogen

Ionization Enthalpy

• Atomic radius decrerases when we move from left to right in a period, When atomic radius or the size of the atom decreases, attractive force between the nucleus and outermost shell increases. Due to this along the period from left to right. Ionization energy increases while the ionization energy from top to bottom decreases.
• As the number of shell increases, increases in a group, the outmermost shell will be distant from the nucleus leading to less neutral effective charge.
• Some of the factors that affect ioionization energy are- Atomic Size, number of electrons in inner shell, nuclear charge, stable configuration, penetration effect.

Electron gain Enthalpy

• When we move left to right, across in a period, electron gain enthalpy starts becoming negative.
• When energy releases on gaining an electron, electron gain enthalpy is negative.
• When electron is gained and energy is provided to atom, it is electron gain enthalpy is positive.
• Elements are highly reactive on the two extreme sides in a periodic table, while noble gases and the elements that occupy the center position are least reactive.the elements that gain electrons to form an anion are located on the extreme right. These are known as Halogens. The elements that lose electrons to form cations are called cations and these are located to the extreme left. These are known as alkalis. While moving across left to right in a period, the metallic property decreases while the non- metallic chcracteristics increases.
• Moving down in a group, the metallic and the non-metallic characteristics decreases.

Electronegativity

The tendency of an atom to to strract electrons bonded pair towards itself is known as electronegativity.

• Electronegativity can be measured on different scales like Mulliken scale and Pauling scale. However, the most commonly used scale is Pauling Scale. According to this scale, the most electronegativ element is fluorine while least electronegative element is cesium.

Periodic Trend in Electronegative elements

• In a period, Electronegativity increases across left to right. This is due to the decrease in atomic size and and increase in nuclear charge.
• In a group, Electronegativity decreases from top to bottom. This is due to increase in atomic size and increase in nuclear charge. This increase in nuclear charge, is however prevailed over by the addition of a shell.

Read NEET Study Notes on Basic Concepts of Chemistry

Most and Least Electronegative Element

• The most electronegative element is fluorine, its electronegative value is 3.98.
• The leaset electronegative element is cesium, its electronegative value is 0.79.

Factors that affect Electronegativity

Electronegativity of an element depend on various factors like atomic radii, nuclear charge, effect of substituent, oxidation state, hybridization.

• Atomic Radii- The greater the atomic radii, the less will be the electronegativity of an element. As the electrons are at a distance from the nucleus, the force of attraction will be less.
• Nuclear Charge- The greater the nuclear charge, the more will be the electronegativity of an element. An increase in nuclear charge will lead to a greater force of attraction between electrons.
• Oxidation State- When the positive oxidation state increases, electronegativity also increases.
• Hybridization- The more s- character is exhibited in hybrid orbital, the more will be the electronegativity.
• Substituent Effect- Electronegativity is also affected by the nature of substituent connected to the atom. The chemical behaviour of an atom is determined by the difference in electronegativity of an atom.

Check NEET 2022 Preparation Books

Valency

The number of electrons that are gained or lost or shared with atoms in order to form compounds is known as the valency of elements.

Valency for Group 1 and 2 elements- number of electrons in outermost shell

Valency for Group 13 to 14 elements- group number - 10

Valency for Group 15-18- 8 deducted by number of electrons in outermost shell.

NEET Sample MCQs for Classification of Elements and Periodicity in Properties

Questions: In the modern periodic table, halogens occupy which group?

1. 18th
2. 17th
3. 1st
4. 16th

Answer: 17th

Question: What are the elements of IInd period known as ?

1. Bridge Element
2. Typical Element
3. Transitional Element
4. Normal Element

Answer: Bridge Element

Question: What is the most electropositive halogen?

1. CI
2. Br
3. F
4. I

Answer: I

Question: Who gave the modern periodic table of elements?

1. Mosley
2. Mendeleev
3. Dalton
4. Dobereiner

Answer: Mosley

Question: When moving up in a group, what happens to the metallic characteristics?

1. Remains the same
2. Increases
3. First increases then decreases
4. Decreases

Answer: decreases

Question: The number of period of elements in modern periodic table is same as of—?

1. Atomic mass
2. Atomic number
3. Azimuthal quantum number
4. Principal quantum number

Answer: principal quantum number

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