1. Why Do We Need Classification of Elements?
As the number of known elements increased over time, it became increasingly difficult to study each element individually. Scientists realized the importance of organizing elements in a systematic manner so that elements with similar behaviors and properties could be studied together. This need for organization led to the development of periodic classification.
Key Benefits of Classification
- Simplifies the study of elements by grouping similar ones
- Helps predict properties of new or undiscovered elements
- Reveals patterns and trends in element behavior
- Makes chemical knowledge more organized and accessible
- Facilitates understanding of chemical reactions and bonding
2. Meaning of Periodicity
Periodicity refers to the recurring pattern of properties in elements when they are arranged in a specific order. When similar properties of elements repeat at regular intervals, this repetition is called periodicity. The arrangement that demonstrates this repetition is known as the periodic table.
Understanding Periodicity
Imagine the days of the week - they repeat every 7 days. Similarly, in the periodic table, certain properties of elements (like reactivity, metallic character, atomic size) repeat after specific intervals when elements are arranged in order of increasing atomic number.
Visualizing Periodicity in the Periodic Table
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Kr
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
3. Early Attempts at Classification
Before the modern periodic table, scientists made several attempts to classify elements based on observable characteristics:
Early Classification Methods
Scientists initially grouped elements based on:
- Similar physical properties: State (solid, liquid, gas), density, melting point, boiling point
- Similar chemical reactions: How elements react with oxygen, water, acids, etc.
Limitations of Early Methods
These early classification systems were not fully successful because:
- They could not explain all similarities and differences between elements
- Some elements showed similarities in one property but differences in others
- Newly discovered elements often didn't fit into existing groups
- There was no fundamental basis for the classification
4. Mendeleev's Contribution (Conceptual Overview)
Dmitri Mendeleev, a Russian chemist, made a groundbreaking contribution to the classification of elements in 1869. He arranged elements primarily based on their atomic masses and chemical properties, noticing that elements with similar properties appeared at regular intervals.
Achievements of Mendeleev's Table
- Successfully grouped elements with similar properties together
- Left gaps for undiscovered elements, predicting their existence
- Accurately predicted properties of new elements (Gallium, Germanium, Scandium)
- Arranged elements in order of increasing atomic mass
- Established the concept of periodicity in properties
Drawbacks of Mendeleev's Table
- Some elements did not fit properly according to atomic mass order
- Isotopes (atoms of same element with different masses) could not be explained
- Position of hydrogen was uncertain (shows properties of both metals and non-metals)
- Could not explain the cause of periodicity
- Some dissimilar elements were placed in same groups
5. Modern Periodic Law
To overcome the limitations of earlier classification systems, scientists introduced a new, more fundamental basis for organizing elements.
Modern Periodic Law
"The physical and chemical properties of elements are periodic functions of their atomic numbers."
This means the arrangement of elements in the periodic table depends on their atomic number (number of protons), not their atomic mass.
Atomic Number Basis
- Atomic number determines the electronic configuration
- Electronic configuration determines chemical properties
- Explains position of isotopes (same atomic number)
- Resolves anomalies in Mendeleev's table
Why Atomic Number?
- Atomic number is more fundamental than atomic mass
- It uniquely identifies each element
- Determines the number of electrons and their arrangement
- Directly relates to chemical behavior
6. Structure of the Modern Periodic Table
The modern periodic table arranges elements in a well-defined structure based on increasing atomic number.
(a) Periods
- Definition: Horizontal rows in the periodic table
- Total: 7 periods
- Key Feature: Elements in the same period have the same number of electron shells
- Period Number: Equal to the number of electron shells
Example: All elements in Period 2 have 2 electron shells
(b) Groups
- Definition: Vertical columns in the periodic table
- Total: 18 groups (numbered 1 to 18)
- Key Feature: Elements in the same group have similar chemical properties due to same number of valence electrons
- Also called: Families of elements
Example: All elements in Group 1 have 1 valence electron
Quick Reference: Periods vs Groups
| Aspect |
Periods (Rows) |
Groups (Columns) |
| Direction |
Horizontal |
Vertical |
| Number |
7 |
18 |
| Electron Shells |
Same for all elements |
Different (increases down group) |
| Valence Electrons |
Different (increases across period) |
Same for all elements |
| Property Trend |
Properties change gradually |
Properties are similar |
7. Electronic Configuration and Position of Elements
The arrangement of electrons in shells (electronic configuration) determines an element's position in the periodic table and its chemical behavior.
How Electronic Configuration Determines Position
- Period Number = Number of electron shells
- Group Number (for main group elements) = Number of valence electrons
- Block (s, p, d, f) = Type of orbital being filled
Example 1: Highly Reactive Metal
Element: Sodium (Na)
Atomic Number: 11
Electronic Configuration: 2, 8, 1
Analysis: Has 1 electron in outer shell → tends to lose it easily → highly reactive metal
Position: Group 1, Period 3
Example 2: Chemically Stable
Element: Neon (Ne)
Atomic Number: 10
Electronic Configuration: 2, 8
Analysis: Has full outer shell (octet) → stable configuration → chemically inert
Position: Group 18, Period 2
Valence Electrons: The Key to Chemical Behavior
Valence electrons (electrons in the outermost shell) play the most important role in determining:
- Chemical reactivity of the element
- Type of bonds formed (ionic or covalent)
- Oxidation states
- Physical properties like conductivity
8. Valency
Valency is the combining capacity of an element. It represents the number of electrons an atom loses, gains, or shares during a chemical reaction to achieve a stable electronic configuration.
Metals
- Usually lose electrons to form positive ions (cations)
- Valency = Number of electrons lost
- Example: Sodium (Na) loses 1 electron → Valency = 1
- Metals have 1, 2, or 3 valence electrons typically
Non-Metals
- Usually gain or share electrons
- Valency = 8 - (Number of valence electrons)
- Example: Chlorine (Cl) has 7 valence electrons, gains 1 → Valency = 1
- Non-metals have 4, 5, 6, or 7 valence electrons typically
Determining Valency
For main group elements (Groups 1, 2, 13-18):
- Groups 1, 2, 13: Valency = Group number (for Groups 1, 2) or Group number - 10 (for Group 13)
- Groups 14-18: Valency = 8 - (Group number - 10) OR simply: Valency = 18 - Group number
- Transition elements: Can show variable valency due to incomplete inner d-orbitals
9. Important Periodic Trends
Periodic trends are patterns in properties of elements that occur across periods and down groups in the periodic table.
(a) Atomic Size (Atomic Radius)
Across a Period (Left to Right)
Atomic size decreases
Reason: Nuclear charge increases, pulling electrons closer to the nucleus. Number of electron shells remains the same.
Down a Group (Top to Bottom)
Atomic size increases
Reason: Number of electron shells increases, outweighing the effect of increased nuclear charge.
(b) Metallic Character
Across a Period (Left to Right)
Metallic character decreases
Reason: Tendency to lose electrons decreases as nuclear attraction increases.
Down a Group (Top to Bottom)
Metallic character increases
Reason: Increased atomic size makes it easier to lose electrons.
(c) Non-Metallic Character
Across a Period (Left to Right)
Non-metallic character increases
Reason: Tendency to gain electrons increases as nuclear charge increases.
Down a Group (Top to Bottom)
Non-metallic character decreases
Reason: Increased atomic size makes it harder to gain electrons.
(d) Reactivity of Elements
| Element Type |
Trend Across Period |
Trend Down Group |
Reason |
| Metals |
Reactivity decreases |
Reactivity increases |
Depends on ease of losing electrons |
| Non-Metals |
Reactivity increases |
Reactivity decreases |
Depends on ease of gaining electrons |