Atoms & Molecules (Class 9)

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### Atoms - **Definition:** Smallest particle of an element that can exist independently and retain its chemical properties. - **Atomic Theory (Dalton):** - All matter is made of atoms. - Atoms are indivisible and indestructible. - Atoms of a given element are identical in mass and properties. - Atoms of different elements have different masses and properties. - Atoms combine in simple whole-number ratios to form compounds. - Atoms are neither created nor destroyed in chemical reactions. ### Subatomic Particles - **Electrons (e⁻):** - Discovered by J.J. Thomson. - Negatively charged particle. - Mass is negligible (approx. 1/1837th of a proton). - Orbits the nucleus. - **Protons (p⁺):** - Discovered by E. Rutherford. - Positively charged particle. - Mass is approximately 1 atomic mass unit (amu). - Located in the nucleus. - **Neutrons (n⁰):** - Discovered by J. Chadwick. - No charge (neutral). - Mass is approximately 1 amu. - Located in the nucleus. | Particle | Charge | Mass (amu) | Location | |----------|--------|------------|----------| | Proton | +1 | 1 | Nucleus | | Neutron | 0 | 1 | Nucleus | | Electron | -1 | $\approx$ 0 | Orbits | ### Atomic Structure - **Nucleus:** - Central part of an atom. - Contains protons and neutrons (collectively called nucleons). - Extremely dense and positively charged. - Accounts for almost all the mass of an atom. - **Electron Shells/Orbits:** - Electrons revolve around the nucleus in fixed paths called orbits or shells. - Each shell has a fixed energy. - Energy levels are denoted by K, L, M, N... or 1, 2, 3, 4... - Maximum number of electrons in a shell is given by $2n^2$, where 'n' is the shell number. - K-shell (n=1): $2(1)^2 = 2$ electrons - L-shell (n=2): $2(2)^2 = 8$ electrons - M-shell (n=3): $2(3)^2 = 18$ electrons - **Valence Shell:** Outermost shell of an atom. - **Valence Electrons:** Electrons present in the valence shell. These determine an atom's chemical reactivity. ### Atomic Number and Mass Number - **Atomic Number (Z):** - Number of protons in an atom. - Defines the identity of an element. - For a neutral atom, Atomic Number = Number of Protons = Number of Electrons. - **Mass Number (A):** - Total number of protons and neutrons in the nucleus. - Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N). - Number of Neutrons (N) = Mass Number (A) - Atomic Number (Z). - **Notation:** An element X is represented as $^A_Z X$. ### Isotopes and Isobars - **Isotopes:** - Atoms of the same element with the same atomic number (Z) but different mass numbers (A). - They have the same number of protons but different numbers of neutrons. - Example: Hydrogen has three isotopes - Protium ($^1_1 H$), Deuterium ($^2_1 H$), Tritium ($^3_1 H$). - **Isobars:** - Atoms of different elements with different atomic numbers (Z) but the same mass number (A). - Example: Argon ($^{40}_{18} Ar$) and Calcium ($^{40}_{20} Ca$). ### Ions - **Definition:** An atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge. - **Cation:** Positively charged ion (formed by losing electrons). Example: $Na^+$, $Mg^{2+}$. - **Anion:** Negatively charged ion (formed by gaining electrons). Example: $Cl^-$, $O^{2-}$. ### Molecules - **Definition:** A group of two or more atoms chemically bonded together, held by attractive forces. - Can be formed by: - Atoms of the same element (e.g., $O_2$, $N_2$, $Cl_2$). - Atoms of different elements (e.g., $H_2O$, $CO_2$, $NH_3$). - **Atomicity:** The number of atoms constituting a molecule. - Monatomic: He, Ne, Ar (1 atom) - Diatomic: $O_2$, $N_2$, $Cl_2$ (2 atoms) - Triatomic: $O_3$ (Ozone) (3 atoms) - Polyatomic: $S_8$, $P_4$ (more than 3 atoms) ### Writing Chemical Formulae - **Valency:** The combining capacity of an element. It is the number of electrons an atom loses, gains, or shares to achieve a stable electron configuration (usually 8 valence electrons, octet rule). - **Steps:** 1. Write the symbols of the elements/ions. 2. Write their valencies below the symbols. 3. Crossover the valencies (interchange them and write as subscripts). 4. Ignore the positive/negative signs of ions. 5. Reduce subscripts to the simplest whole-number ratio if possible. - **Example: Magnesium Chloride** - Symbols: Mg, Cl - Valencies: Mg²⁺ (2), Cl⁻ (1) - Crossover: $Mg_1 Cl_2$ - Formula: $MgCl_2$