Periodic Classification of Elements Class 10 Notes

Key Terms for Periodic Classification

Elements

Pure substances consisting of only one type of atom, which cannot be broken down into simpler substances by chemical means.

Atomic Number (Z)

The number of protons in the nucleus of an atom. It is the fundamental property used to classify elements in the Modern Periodic Table.

Groups

The 18 vertical columns in the Modern Periodic Table. Elements in the same group have the same number of valence electrons and similar chemical properties.

Periods

The 7 horizontal rows in the Modern Periodic Table. Elements in the same period have the same number of electron shells.

Valency

The combining capacity of an element. For metals, it's equal to the number of valence electrons. For non-metals, it is 8 minus the number of valence electrons.

Valence Electrons

The electrons present in the outermost shell of an atom, which determine the chemical properties of the element.

Atomic Radius

The distance from the centre of the nucleus to the outermost shell of an atom. It is a measure of the size of an atom.

Metallic Character

The tendency of an atom to lose electrons and form positive ions (cations). It increases down a group and decreases across a period.

Non-metallic Character

The tendency of an atom to gain electrons and form negative ions (anions). It decreases down a group and increases across a period.

Metalloids

Elements that exhibit properties intermediate between those of metals and non-metals. Examples: Boron (B), Silicon (Si), Germanium (Ge).

Early Attempts at Classification: A Comparison

The Modern Periodic Table: A New Foundation

The limitations of Mendeleev's table were resolved with the introduction of the Modern Periodic Table. In 1913, Henry Moseley showed that the atomic number (Z) of an element is a more fundamental property than its atomic mass. This led to the Modern Periodic Law, which states: 'The physical and chemical properties of elements are the periodic functions of their atomic number.'

This new basis of classification resolved the major issues of the previous table. Arranging elements by increasing atomic number automatically fixed the problem of anomalous pairs like Cobalt (Z=27) and Nickel (Z=28). Since isotopes of an element have the same atomic number but different masses, they are placed in the same slot in the Modern Periodic Table, solving another key problem.

The Modern Periodic Table consists of 18 vertical columns called groups and 7 horizontal rows called periods.

• Groups: Elements in the same group have the same number of valence electrons. For example, all Group 1 elements (Li, Na, K) have 1 valence electron, making them highly reactive alkali metals.
• Periods: The period number indicates the number of electron shells being filled. For instance, elements in Period 3 (Na, Mg, Al, Si, P, S, Cl, Ar) have electrons in three shells (K, L, M).

Achievements and Limitations of Mendeleev's Table

• {"id":1,"point":"Systematic Arrangement: First to systematically classify all known elements into groups and periods based on properties."}
• {"id":2,"point":"Prediction of New Elements: Left gaps for undiscovered elements and predicted their properties. For example, he predicted 'Eka-aluminium' (later discovered as Gallium), 'Eka-boron' (Scandium), and 'Eka-silicon' (Germanium)."}
• {"id":3,"point":"Correction of Atomic Masses: Corrected the atomic masses of several elements like Beryllium, Gold, and Platinum based on their positions in the table."}
• {"id":4,"point":"Noble Gases: When noble gases were discovered, they could be placed in a new group without disturbing the existing order."}
• {"id":5,"point":"Limitation: Position of Hydrogen: Hydrogen resembles both alkali metals (Group 1) and halogens (Group 17), so its position was not fixed."}
• {"id":6,"point":"Limitation: Anomalous Pairs: Some elements with higher atomic mass were placed before elements with lower atomic mass to maintain similar properties (e.g., Co before Ni)."}
• {"id":7,"point":"Limitation: Position of Isotopes: Isotopes of an element have different atomic masses but similar chemical properties. Mendeleev's table had no place for them."}

Trends in the Modern Periodic Table

Exam Traps and Scoring Tips

A very common question asks you to justify a trend. Don't just state that 'atomic size decreases across a period'. You must explain why. The key reasons are increasing nuclear charge (for periods) and addition of new shells (for groups). Use these phrases in your answers for full marks. Another trap is determining the position of an element from its electronic configuration. For example, if the configuration is 2, 8, 5:

• Period: The number of shells is 3 (K, L, M), so it's in Period 3.
• Group: The number of valence electrons is 5. Since it's more than 2, the group number is 10 + 5 = 15. It's in Group 15.

Quick Revision Check

• Q: State the Modern Periodic Law and name the scientist who proposed it. A: The Modern Periodic Law states that the properties of elements are a periodic function of their atomic number. It was proposed by Henry Moseley.
• Q: An element 'X' has electronic configuration 2, 8, 7. What is its group number, period number, and valency? A: Period = 3 (3 shells). Group = 17 (10 + 7 valence electrons). Valency = 8 - 7 = 1.
• Q: Why does metallic character increase down a group? A: Metallic character increases down a group because new electron shells are added. This increases the atomic radius and reduces the effective nuclear charge on the valence electrons, making them easier to lose.
• Q: Name two elements predicted by Mendeleev that were discovered later. A: Eka-aluminium (discovered as Gallium) and Eka-silicon (discovered as Germanium).

Frequently Asked Questions

Frequently Asked Questions