Modern Periodic Classification of Elements
These notes are based on the chapter periodic classification of elements from class 10 science NCERT book and CBSE syllabus.
Law of Modern Periodic Table states that properties of elements are the periodic function of their atomic numbers. In the modern periodic table, elements are arranged in order of their increasing atomic numbers.
- Explanation of modern periodic table
- Groups in modern periodic table
- Periods in modern periodic table
- Trends in modern periodic table
Explanation of Modern Periodic Table
- Elements are arranged in order of their increasing atomic numbers.
- The vertical columns are known as groups and horizontal columns are known as periods, in the modern periodic table.
- There are 18 groups and 7 periods in the modern periodic table.
Elements having same number of valence electrons are placed in the same group. For example, elements having valence electrons equal to 1 are placed in the 1st group, elements having valence electrons equal to 3 are placed in the 13th group, elements having valence electrons equal to 2 are placed in 2nd group except helium which is placed in 18th group, since it is an inert gas.
Elements having same number of shells are placed in the same period.
Groups in Modern Periodic Table
1st group: Alkali metals are placed in the 1st group in the modern periodic table. Hydrogen is also placed in the 1st group although hydrogen is not an alkali metal.
2nd group: Alkaline earth metals are placed in the 2nd group in the modern periodic table.
Elements placed in 1st and 2nd groups in the modern periodic table are collectively known as light metals.
3rd to 12th group: Transition elements are placed from 3rd to 12th group in the modern periodic table.
13th group: Metals are placed in the 13th group; except boron which is a metalloid.
14th group: Carbon, silicon, germanium, tin and lead are placed in this group. Among them, carbon is a non-metal, silicon and germanium are metalloids and tin and lead are metals.
15th group: Nitrogen, phosphorous, arsenic, antimony and bismuth are place in the 15th group; among which nitrogen and phosphorous are non-metals, arsenic and antimony are metalloids and bismuth is a metal.
16th group: Oxygen, sulphur, selenium, tellurium and polonium are placed in this group, among which oxygen, sulphur and selenium are non-metals, tellurium is metalloid and polonium is a metal.
17th group: Non-metals are placed in the 17th group. Since, halogens are placed in this group hence this group is also known as group of halogen.
18th group: Noble gases are placed in the 18th group. This group is also known as zero group.
Periods in Modern Periodic Table
At present there are seven periods in the Modern Periodic Table.
1st period: This is known as very short period as there are only two elements, i.e. hydrogen and helium.
2nd and 3rd period: There are total 8 elements in each of the 2nd and 3rd periods. These periods are known as short periods.
4th and 5th period: There are total 18 elements in each of the 4th and 5th periods. These periods are known as long periods.
6th period: There are total 32 elements in 6th period. This period is known as very long period.
7th period This period is known as incomplete period. Blank spaces in this period are supposed to be filled by the elements discovered in future.
Position of elements in the Modern Periodic Table:
Elements are placed in groups according to their valence electrons and placed in periods according to the number of shells present in them.
| Position of Elements | |||
|---|---|---|---|
| Valence Electrons | Group | No. of Shell | Period |
| 1 | 1 | 1 | 1 |
| 2 | 2 | 2 | 2 |
| 3 | 13 | 3 | 3 |
| 4 | 14 | 4 | 4 |
| 5 | 15 | 5 | 5 |
| 6 | 16 | 6 | 6 |
| 7 | 17 | 7 | 7 |
| 8 | 18 | ||
Helium has valence electrons equal to 2, but it is placed in group number 18 because it is a noble gas and has completely filled outermost shell.
Example: Hydrogen: Atomic Number = 1
Electronic configuration of hydrogen: K1
Valence electron = 1
Number of shell = 1
Position in periodic table: 1st group 1st period
Example: Sodium: Atomic number = 11
Electronic configuration of sodium: K2L8M1
Valence electron = 1
Number of shell = 3
Position in periodic table: 1st group 3rd period.
Example: Carbon: Atomic number = 6
Electronic configuration of carbon: K2L4
Valence electron = 4
Number of shell = 2
Position in periodic table: 14th group 2nd period.
Example: Neon: Atomic number = 10
Electronic configuration of neon: K2L8
Valence electron = 8 i.e. equal to 0 (zero)
Number of shell = 2
Position in periodic table: 18th group 2nd period.
Trends in Modern Periodic Table
Valence electrons
- Number of valence electron remains the same while moving from top to bottom in a group.
- Number of valence electron increases while moving from left to right in a period.
Valency
- Valency remains the same on moving from top to bottom in a group.
- Valency first increases upto 4 on moving from left to right in a group and then decreases upto zero.
| Trend of valency of elements in 3rd Period | ||||||||
|---|---|---|---|---|---|---|---|---|
| Group Number | 1 | 2 | 13 | 14 | 15 | 16 | 17 | 18 |
| Elements | Na | Mg | Al | Si | P | S | Cl | Ar |
| Valency | 1 | 2 | 3 | 4 | 3 | 2 | 1 | 0 |
Atomic size
Atomic size increases on moving from top to bottom in a group and decreases on moving from left to right in a period.
| Atomic size in first group | ||||||
|---|---|---|---|---|---|---|
| Period Number | 1 | 2 | 3 | 4 | 5 | 6 |
| Elements | Ha | Li | Na | K | Rb | Ks |
| Atomic size | 37 | 152 | 186 | 227 | 248 | 265 |
Atomic radii
Atomic radius increases on moving from top to bottom in a group and decreases on moving from left to right in a period.
| Atomic size in third period | ||||||||
|---|---|---|---|---|---|---|---|---|
| Group Number | 1 | 2 | 13 | 14 | 15 | 16 | 17 | 18 |
| Elements | Na | Mg | Al | Si | P | S | Cl | Ar |
| Atomic Size | 186 | 160 | 143 | 118 | 110 | 103 | 99 | 98 |
Metallic character of elements
Metallic character decreases on moving from left to right in a period.
Tendency to lose electron: Tendency to lose electrons decreases on moving from left to right in a period and tendency to lose electrons increases on moving from top to bottom in a group.
Tendency to lose electron is also known as electropositive character, thus electropositive character decreases on moving from left to right in a period and increases on moving from top to bottom in a group.
Tendency to gain electron: Tendency to gain electrons increases on moving from left to right in a period. Tendency to gain electron is called electro-negativity, thus electro-negativity of elements increases on moving from left to right in a period.