What NEET Asks
- Direct questions on the number of periods/groups and their significance are common.
- Identification of period and group for elements based on atomic number.
- Understanding the general characteristics associated with elements in specific periods or groups.
Key Points
- The Modern Periodic Table is arranged in increasing order of atomic number.
- Periods are the 7 horizontal rows, representing the principal energy shells (n). The period number corresponds to the highest principal quantum number (n) of the valence shell.
- Groups are the 18 vertical columns, containing elements with similar valence shell electronic configurations and thus similar chemical properties.
- Number of elements per period:
- 1st period (n=1): 2 elements (H, He)
- 2nd period (n=2): 8 elements
- 3rd period (n=3): 8 elements
- 4th period (n=4): 18 elements
- 5th period (n=5): 18 elements
- 6th period (n=6): 32 elements (including Lanthanoids)
- 7th period (n=7): 32 elements (incomplete, including Actinoids)
- Group numbering: Groups 1, 2, and 13-18 are main group (representative) elements. Groups 3-12 are transition elements.
Must-Know Formula / Reaction
Relation between Period Number and Electronic Shell:
Period Number = Principal Quantum Number (n) of the valence shell
Explanation: This directly tells you the highest energy shell being filled in an element from that period.
Common Mistakes
- Students often confuse periods (horizontal rows) with groups (vertical columns).
- Don't confuse the total number of elements in a period with the number of d-block or f-block elements within it.
- Students often forget that the modern periodic table is based on atomic number, not atomic mass.
Rapid Revision
The Modern Periodic Table organizes elements by increasing atomic number into 7 periods (energy shells) and 18 groups (similar valence electrons/properties). Periods define the principal quantum number 'n', while groups define similar chemical behavior. Remember the element counts per period and their significance.