ATOMS AND MOLECULES

 Key Concepts

• Valency: Valency refers to the combining capacity of an element, which is determined by the number of electrons in its outer shell. Techniques to remember atomic masses and valencies can enhance understanding .

• Atoms: The concept of atoms was introduced by ancient philosophers. Mahārṣhi Kanāda proposed that matter is composed of indivisible particles called paramāṇu (atoms). Later, Democritus named these particles atomos, meaning indivisible .

• Atomic Theory: John Dalton's atomic theory states that:

  1. All matter is made of very tiny particles called atoms.
  2. Atoms are indivisible and cannot be created or destroyed .
  3. Atoms of a given element are identical in mass and properties, while atoms of different elements differ in mass and properties.
  4. Atoms combine in simple whole-number ratios to form compounds .

Important Laws

• Law of Conservation of Mass: This law states that during a chemical reaction, the total mass of the reactants equals the total mass of the products. For example, if 20 grams of reactants yield 20 grams of products, mass is conserved .

• Law of Constant Proportions: This law states that a chemical compound always contains its component elements in fixed ratio by mass, regardless of the sample size or source. For instance, water (H₂O) always consists of hydrogen and oxygen in a 1:8 ratio by mass .

Examples and Applications

• Example of Conservation of Mass: When 12 grams of carbon reacts with 32 grams of oxygen, the mass of carbon dioxide produced is 44 grams, demonstrating that mass is conserved .

• Example of Constant Proportions: If 6 grams of carbon react with oxygen, the required mass of oxygen can be calculated based on the fixed ratio of 3:8 .

Atomic Structure and Symbols

• Modern Atomic Theory: While Dalton's theory laid the foundation, later discoveries showed that atoms can be divided into subatomic particles (protons, neutrons, electrons). This highlights the limitations of Dalton's original postulates .

• Element Symbols: Each element is represented by a unique symbol, typically derived from its name. For example, hydrogen is represented as H, while sodium (from its Latin name "Natrium") is represented as Na .

Memory Aids

• To remember the first 20 elements, a mnemonic can be used: "H H He Li Be B C N O F Ne" for the first ten, and "Na Mg Al Si P S Cl Ar" for the next ten .

This summary encapsulates the essential concepts, laws, and examples related to atoms and molecules, providing a comprehensive overview for exam preparation.

Study Notes

1. Preferences of Characters

• Abdul's Preferences: Abdul dislikes dark items, particularly Pepsi and cold drinks. It's advised not to ask him for these items .

2. Atomic Mass

• Definition: The atomic mass of an element is the mass of a single atom of that element .
• Calculation: The International Union of Pure and Applied Chemistry (IUPAC) provides atomic masses. For example, carbon-12 (C-12) is used as a standard to define atomic mass units (amu) .

3. Atomic Mass Units (AMU)

• Definition: 1 AMU is defined as 1/12 of the mass of a carbon-12 atom .
• Example: The atomic mass of hydrogen is approximately 1 AMU, while other elements have their specific atomic masses based on this standard .

4. Tricks for Remembering Atomic Masses

• Even Numbered Elements: For even-numbered elements, multiply the atomic number by 2 to find the atomic mass (e.g., Helium, atomic number 2, has an atomic mass of 4 AMU) .
• Odd Numbered Elements: For odd-numbered elements, multiply the atomic number by 2 and add 1 (e.g., Lithium, atomic number 3, has an atomic mass of 7 AMU) .

5. Types of Ions

• Cations: Positively charged ions formed when an atom loses electrons (e.g., H⁺) .
• Anions: Negatively charged ions formed when an atom gains electrons (e.g., Cl⁻) .

6. Molecular Structure

• Definition: A molecule is a group of two or more atoms bonded together by attractive forces .
• Types of Molecules:
  • Molecules of Elements: Composed of the same type of atoms (e.g., O₂) .
  • Molecules of Compounds: Composed of different types of atoms (e.g., H₂O) .

7. Atomicity

• Definition: Refers to the number of atoms present in a single molecule of an element .
• Types:
  • Monoatomic: Contains one atom (e.g., Helium) .
  • Diatomic: Contains two atoms (e.g., O₂) .
  • Polyatomic: Contains three or more atoms (e.g., P₄, S₈) .

8. Molecular Mass

• Definition: The sum of the atomic masses of all atoms in a molecule .

Example Calculation: For H₂O, the molecular mass is calculated as:$${\text{Molecular Mass of H}}_2\text{O}=(2\times 1)+(1\times 16)=18\text{ AMU}$$

9. Formula Unit Mass

• Definition: Similar to molecular mass but used for ionic compounds .

These notes cover key concepts, definitions, and examples from the provided excerpts, making it easier to study for the exam.

Key Concepts in Chemistry

Ionic Charges and Valencies

• Sodium (Na) has a charge of +1, Magnesium (Mg) +2, Zinc (Zn) +2, Iron (Fe) +2 or +3, Copper (Cu) +2, Aluminum (Al) +3, and Iron(III) +3. Negative charges are seen in ions like Chloride (Cl⁻), Bromide (Br⁻), Iodide (I⁻), Oxide (O²⁻), Sulfide (S²⁻), and Nitride (N³⁻) .

• Polyatomic Ions: Examples include Ammonium (NH₄⁺), Hydroxide (OH⁻), and Nitrate (NO₃⁻). Knowing the charges of these ions is crucial for naming compounds .

Valency Definition

• Valency is defined as the combining capacity of an element, indicating how many other atoms it can bond with. For example, Hydrogen has a valency of 1, while Sodium also has a valency of 1 .

Naming Compounds

• The naming of compounds is based on the valencies of the elements involved. For example, when forming a compound, the charges are crossed and multiplied to determine the formula. For instance, in Hydrogen Chloride (HCl), both elements have a valency of 1, leading to the formula HCl .

Chemical Formulas

• To write formulas for compounds, identify the elements and their valencies, then cross-multiply to balance the charges. For example:
  • Hydrogen Sulfide (H₂S): Hydrogen (1) and Sulfide (2) → H₂S .
  • Carbon Tetrachloride (CCl₄): Carbon (4) and Chlorine (1) → CCl₄ .

Mole Concept

• Mole: A mole is a unit that represents $6.022\times 10^{23}$ particles (Avogadro's number). For example, if you ask for a dozen bananas, you expect 12; similarly, 1 mole of any substance contains $6.022\times 10^{23}$ entities .

• Molar Mass: The mass of 1 mole of a substance in grams is numerically equal to its atomic mass in atomic mass units (u). For instance, if the atomic mass of Hydrogen is 1 u, then the molar mass is 1 g .

Calculating Moles

• To find the number of moles, use the formula:

  $$\text{Number of moles}=\frac{\text{Mass of substance (g)}}{\text{Molar mass (g/mol)}}$$

  For example, for 32 g of Oxygen (molar mass = 16 g/mol), the number of moles is $\frac{32}{16}=2$ .

• Alternatively, if given the number of particles, use:

  $$\text{Number of moles}=\frac{\text{Number of particles}}{6.022\times 10^{23}}$$

  For example, if you have $6.022\times 10^{23}$ particles, that equals 1 mole .

Practice Problems

• Practice writing formulas for various compounds based on their valencies. For example, Copper(II) Bromide (CuBr₂) and Ammonium Sulfate (NH₄)₂SO₄ .

• Understand the difference between molecular and formula unit mass. Molecular mass is used for molecular compounds, while formula unit mass is for ionic compounds .