Class 9th Science chapter Atoms and Molecules Notes
Atoms and Molecules Notes
1. Laws of Chemical Combination
The two fundamental laws that govern the combination of elements to form compounds are:
1.1 Law of Conservation of Mass
Proposed by: Antoine Lavoisier in 1789.
Statement: The total mass of the reactants in a chemical reaction is always equal to the total mass of the products. Mass can neither be created nor destroyed in a chemical reaction.
Mathematical Expression: Total mass of reactants = Total mass of products
Example: In the combustion of methane (CH₄), methane reacts with oxygen (O₂) to form carbon dioxide (CO₂) and water (H₂O).
CH₄ + 2O₂ → CO₂ + 2H₂O
If we start with 16 g of methane and 64 g of oxygen, the total mass of the products will also be 80 g.
1.2 Law of Constant Proportions
Proposed by: Joseph Proust in 1799.
Statement: A chemical compound always contains the same elements in exactly the same proportion by mass, regardless of its source or method of preparation.
Example: Water (H₂O) always contains hydrogen and oxygen in a mass ratio of 1:8.
Significance: This law supports the idea of fixed composition in compounds.
2. Dalton’s Atomic Theory
In 1808, John Dalton proposed his atomic theory, which marked the beginning of modern chemistry. The key postulates are:
- All matter is composed of tiny indivisible particles called atoms.
- Atoms of the same element are identical in mass and chemical properties.
- Atoms of different elements differ in mass and properties.
- Compounds are formed by the combination of atoms in fixed proportions.
- Atoms cannot be created or destroyed in a chemical reaction, only rearranged.
Drawbacks of Dalton’s Theory
Though revolutionary, Dalton’s atomic theory had its limitations:
- Atoms are divisible into subatomic particles (protons, neutrons, electrons).
- Isotopes of the same element have different masses, which Dalton could not explain.
3. What is an Atom?
Definition: An atom is the smallest unit of matter that retains the properties of an element.
Structure: Atoms consist of a nucleus (made of protons and neutrons) and electrons that revolve around the nucleus in distinct orbits.
Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.
4. How Big are Atoms?
Atoms are extremely small. Their sizes are typically measured in nanometers (1 nm = 10⁻⁹ meters).
Example: The size of a hydrogen atom is approximately 0.1 nm.
5. Modern-Day Symbols of Atoms of Different Elements
Each element is represented by a unique symbol, usually derived from its English or Latin name. Some examples include:
- Hydrogen: H
- Oxygen: O
- Sodium: Na (from Latin “Natrium”)
- Gold: Au (from Latin “Aurum”)
6. Atomic Mass
Definition: Atomic mass is the weighted average mass of all the isotopes of an element, measured in atomic mass units (amu).
Example: The atomic mass of chlorine is approximately 35.5 amu, due to the existence of isotopes Cl-35 and Cl-37.
7. How Do Atoms Exist?
Atoms rarely exist independently in nature. They usually combine with other atoms to form molecules or ions.
Molecules: H₂, O₂, H₂O
Ions: Na⁺, Cl⁻
8. Molecules of Elements
Definition: A molecule of an element consists of two or more atoms of the same element chemically bonded together.
Examples: O₂, H₂, N₂, P₄, S₈
9. Molecules of Compounds
Definition: Molecules of compounds consist of atoms of different elements bonded together in fixed proportions.
Examples: H₂O, CO₂, NH₃
10. What is an Ion?
Definition: Ions are charged particles formed when atoms gain or lose electrons.
Cation: A positively charged ion (e.g., Na⁺)
Anion: A negatively charged ion (e.g., Cl⁻)
11. Writing Chemical Formulae
Chemical formulae represent the composition of molecules and compounds using symbols and numbers.
Steps to write chemical formulae:
- Write the symbol of the element.
- Use subscripts to show the number of atoms of each element.
- For ionic compounds, balance the charges between cations and anions.
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1. What is the Law of Conservation of Mass?
The Law of Conservation of Mass states that in a chemical reaction, the total mass of the reactants is equal to the total mass of the products. Mass can neither be created nor destroyed in a chemical reaction.
2. Who proposed the Law of Constant Proportions?
The Law of Constant Proportions was proposed by Joseph Proust in 1799. It states that a chemical compound always contains the same elements in the same proportion by mass.
3. What are the key postulates of Dalton’s Atomic Theory?
Dalton’s Atomic Theory includes five key postulates:
- All matter is made up of tiny indivisible particles called atoms.
- Atoms of a given element are identical in mass and properties.
- Atoms of different elements differ in mass and properties.
- Compounds are formed by the combination of atoms in fixed ratios.
- Atoms cannot be created or destroyed in chemical reactions.
4. What are the drawbacks of Dalton’s Atomic Theory?
The main drawbacks of Dalton’s Atomic Theory are:
- It could not explain isotopes (atoms of the same element with different masses).
- It assumed atoms were indivisible, but later discoveries showed that atoms consist of subatomic particles (electrons, protons, neutrons).
5. What is an atom?
An atom is the smallest unit of matter that retains the chemical properties of an element. It consists of a nucleus (containing protons and neutrons) and electrons that orbit around the nucleus.
6. How big are atoms?
Atoms are extremely small, typically measuring around 0.1 to 0.5 nanometers in diameter. For example, the size of a hydrogen atom is approximately 0.1 nm.
7. What is the modern-day symbol for Sodium?
The modern-day symbol for Sodium is Na, which comes from its Latin name “Natrium.”
8. What is atomic mass?
Atomic mass is the weighted average mass of an atom, taking into account the relative abundance of its isotopes. It is measured in atomic mass units (amu).
9. How do atoms exist in nature?
Atoms rarely exist independently in nature. They typically combine with other atoms to form molecules or ions through chemical bonding.
10. What are molecules of elements?
Molecules of elements are made up of two or more atoms of the same element bonded together. For example, O₂ (oxygen), H₂ (hydrogen), and N₂ (nitrogen) are molecules of elements.
11. What are molecules of compounds?
Molecules of compounds are composed of atoms of different elements bonded together. Examples include H₂O (water) and CO₂ (carbon dioxide).
12. What is an ion?
An ion is an atom or group of atoms that has gained or lost electrons, resulting in a net positive or negative charge. For example, Na⁺ is a cation (positively charged ion), and Cl⁻ is an anion (negatively charged ion).
13. How do you write the chemical formula of a compound?
To write the chemical formula of a compound:
- Write the symbols of the elements involved.
- Use subscripts to indicate the number of atoms of each element.
- For ionic compounds, ensure that the total positive and negative charges balance each other.
14. What is the molecular formula of water?
The molecular formula of water is H₂O, indicating that it contains two hydrogen atoms and one oxygen atom.
15. What is the molecular formula of carbon dioxide?
The molecular formula of carbon dioxide is CO₂, indicating that it contains one carbon atom and two oxygen atoms.
16. What is the difference between a cation and an anion?
A cation is a positively charged ion formed by the loss of electrons, while an anion is a negatively charged ion formed by the gain of electrons.
17. What is the significance of the Law of Conservation of Mass in chemical reactions?
The Law of Conservation of Mass ensures that the mass of reactants in a chemical reaction is equal to the mass of the products, helping to balance chemical equations and understand that atoms are neither created nor destroyed.
18. What are isotopes?
Isotopes are atoms of the same element that have different numbers of neutrons, and therefore different masses. For example, carbon has isotopes like C-12 and C-14.
19. What is formula unit mass?
Formula unit mass is the sum of the atomic masses of the atoms in the simplest ratio of ions in an ionic compound. For example, the formula unit mass of NaCl is 58.5 amu (23 for Na + 35.5 for Cl).
20. What is molecular mass?
Molecular mass is the sum of the atomic masses of all the atoms in a molecule. It is measured in atomic mass units (amu). For example, the molecular mass of water (H₂O) is 18 amu (2 × 1 for H + 16 for O).
