9th Class Chemistry Chapter 3 Notes explain the development of the periodic table, from Dobereiner’s triads to the modern periodic law. This chapter also covers groups, periods, blocks of elements, and periodic trends like atomic radius and ionization energy.
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These 9th Class Chemistry Chapter 3 Notes follow the latest Punjab Board syllabus, making them perfect for matric students preparing for their annual exams. Every concept is explained with definitions, tables, and examples for easy understanding.
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.Early Attempts to Classify Elements
Dobereiner’s Triads
A triad is a group of three elements with similar chemical properties, where the atomic mass of the middle element is the average of the other two. For example:
- Lithium (7), Sodium (23), Potassium (39)
- Average of Li and K = (7+39)/2 = 23, matching Sodium’s atomic mass
Newlands’ Law of Octaves
In 1864, British chemist Newlands proposed that there is a repetition in chemical properties of every eighth element when arranged by increasing atomic mass.
Example: The properties of sodium (Na) are similar to those of lithium (Li), since sodium is the eighth element after lithium.
Why Newlands’ Work Was Not Recognized
Newlands’ law of octaves failed to gain recognition for two main reasons:
- No space was left for undiscovered elements.
- Noble gases were not known at that time.
Mendeleev’s Periodic Table
In 1869, Russian chemist Mendeleev arranged 63 known elements in order of increasing atomic mass. Elements were placed in horizontal rows called periods, while elements with similar properties fell into vertical columns called groups.
Mendeleev’s Periodic Law
The properties of elements are periodic functions of their atomic masses.
Demerits of Mendeleev’s Periodic Table
Despite its advantages, Mendeleev’s table had a few weaknesses:
- It failed to explain the position of isotopes.
- It placed some elements in the wrong order based on atomic mass.
This section of 9th Class Chemistry Chapter 3 Notes is important since it explains why the periodic table needed further improvement.
Modern Periodic Law
Moseley modified Mendeleev’s periodic law by proving that element properties are a periodic function of atomic number, not atomic mass.
Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers (Z).
What is a Periodic Function?
A periodic function is a common property of elements due to which other properties change at regular intervals. Since atomic number determines element arrangement, it is considered the key periodic function in 9th Class Chemistry Chapter 3 Notes.
Groups and Periods in the Modern Periodic Table
Groups
Groups are the vertical columns in a periodic table. There are 18 groups, numbered from 1 to 18, left to right.
Periods
Periods are the horizontal rows of elements. There are 7 periods, numbered from top to bottom.
Elements of Group 1
| Name | Symbol |
|---|---|
| Hydrogen | H |
| Lithium | Li |
| Sodium | Na |
| Potassium | K |
| Rubidium | Rb |
| Cesium | Cs |
| Francium | Fr |
Elements of 2nd Period
The second period is called the normal period and contains 8 elements: Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine, and Neon.
Noble Gases
Noble gases are the gaseous elements of Group 18. They include:
- Helium (He)
- Neon (Ne)
- Argon (Ar)
- Krypton (Kr)
- Xenon (Xe)
- Radon (Ra)
Blocks of Elements
Blocks of elements are groups of elements with similar subshell electronic configuration, based on the completion of a particular subshell. The modern periodic table is divided into four blocks:
- s-block
- p-block
- d-block
- f-block
Understanding blocks is a key part of 9th Class Chemistry Chapter 3 Notes, since it connects directly to electronic configuration studied in the previous chapter.
Periodicity of Properties
Periodicity of properties refers to the repetition of similar properties after regular intervals in the periodic table. Properties showing periodicity include:
- Atomic radius (atomic size)
- Ionization energy
- Electron affinity
- Electronegativity
- Valency
Atomic Radius
Atomic radius is half the diameter of an atom, measured in picometers (pm). It is determined by measuring the distance between the nuclei of two atoms and dividing by two.
Trends:
- In a period, atomic radius decreases from left to right.
- In a group, atomic radius increases from top to bottom.
Shielding Effect
Shielding effect is the decrease in attractive force exerted by the nucleus on valence shell electrons due to inner electrons.
Trends:
- In a group, shielding effect increases from top to bottom.
- In a period, shielding effect decreases from left to right.
Ionization Energy
Ionization energy is the amount of energy required to remove the most loosely bound electron from the valence shell of an isolated gaseous atom.
Trends:
- In a period, ionization energy increases from left to right.
- In a group, ionization energy decreases from top to bottom.
Electron Affinity
Electron affinity is the amount of energy released when an electron is added to the outermost shell of an isolated gaseous atom.
Trends:
- In a period, electron affinity increases from left to right.
- In a group, electron affinity decreases from top to bottom.
Electronegativity
Electronegativity is the ability of an atom to attract the shared pair of electrons in a molecule.
Trends:
- In a period, electronegativity increases from left to right.
- In a group, electronegativity generally decreases from top to bottom.
This periodic trends section of 9th Class Chemistry Chapter 3 Notes is one of the most commonly tested topics in board exams.
Why Electron Affinity and Electronegativity Follow the Same Trend
Both electron affinity and electronegativity increase from left to right in a period. This happens because a higher effective nuclear charge (Zeff) shortens the distance between the shared electron pair and the nucleus, increasing the nucleus’s power to attract electrons.
Practical Applications from the Chapter
Some practical concepts covered in 9th Class Chemistry Chapter 3 Notes include:
- Effect of impurities on melting point: Impurities lower the melting point of a solid, even if the impurity itself melts at a higher temperature.
- Sublimation examples: Freeze-dried coffee uses sublimation, where ice is removed from frozen coffee under vacuum.
- Separating mixtures: Naphthalene and potassium chloride can be separated using sublimation or crystallization.
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Why These Notes Matter for Exam Preparation
9th Class Chemistry Chapter 3 Notes are designed to cover both conceptual and practical portions of the chapter. Students should focus on:
- History of periodic table development
- Differences between groups and periods
- Blocks of elements and their significance
- Periodic trends: atomic radius, ionization energy, electron affinity, electronegativity
- Practical applications like sublimation and melting point behavior
Consistent revision of these 9th Class Chemistry Chapter 3 Notes ensures better performance in both short-question and long-question sections of the Punjab Board exam.
FAQs
Q1: What topics are covered in 9th Class Chemistry Chapter 3 Notes?
These notes cover the history of the periodic table, Dobereiner’s triads, Newlands’ law of octaves, Mendeleev’s periodic table, modern periodic law, groups, periods, blocks of elements, and periodic trends.
Q2: What is the difference between a group and a period?
Groups are vertical columns in the periodic table, numbered 1 to 18. Periods are horizontal rows, numbered 1 to 7. Elements in the same group share similar chemical properties.
Q3: What is the modern periodic law?
The modern periodic law states that the properties of elements are periodic functions of their atomic numbers. This law was proposed by Moseley, replacing Mendeleev’s atomic mass-based law.
Q4: How does atomic radius change in a period and a group?
Atomic radius decreases from left to right in a period due to increasing nuclear charge. It increases from top to bottom in a group because more electron shells are added.
Q5: What are noble gases?
Noble gases are the gaseous elements of Group 18, including helium, neon, argon, krypton, xenon, and radon. They are generally unreactive due to their complete outer electron shells.
Q6: Why did Mendeleev’s periodic table need improvement?
Mendeleev’s table failed to explain the position of isotopes and placed some elements in the wrong order based on atomic mass. Moseley later solved this by using atomic number instead.