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### Atom - The atom is the basic unit of matter, consisting of a dense central nucleus surrounded by a cloud of negatively charged electrons. Understanding its structure is fundamental to chemistry and physics. #### Atomic Theories & Models - **Dalton's Atomic Theory (1808):** Atoms are tiny, indivisible, indestructible particles; atoms of same element are identical; combine in whole number ratios. - **Thomson's Plum-Pudding Model (1898):** Atom is a sphere of positive electricity with electrons embedded (like plums in pudding). - **Rutherford's Nuclear Model (1909):** Most of atom is empty space; dense, positively charged nucleus at center; electrons orbit nucleus. Nucleus size is ~10^-5 times atom size. - **Bohr's Model (1913):** Electrons orbit nucleus in specific, fixed energy paths (orbits); theory based on quantum mechanics. - **Chadwick's Discovery (1932):** James Chadwick discovered the neutron, completing the picture of the atomic nucleus. #### Subatomic Particles - **Electron:** Negative charge (-1), mass ~1/1836 (or 1/2000) of a proton. Discovered by J.J. Thomson (1897). Location: Orbits the nucleus in shells/energy levels. - **Proton:** Positive charge (+1), mass ~1 amu. Discovered by Ernest Rutherford (1917). Location: Found in the nucleus. - **Neutron:** No charge (neutral), mass ~1 amu. Discovered by James Chadwick (1932). Location: Found in the nucleus. - **Positron:** Positively charged particle (+1e), antiparticle of electron, same mass. Discovered by Carl Anderson (1932). - **Nucleons:** Collective term for protons and neutrons, which constitute the nucleus. Location: Central part of the atom (nucleus). Significance: Account for nearly all the mass of an atom. #### Atomic Mass & Number - **Atomic Number (Z):** Number of protons in the nucleus. Determines the element. In a neutral atom, it also equals the number of electrons. Unique for each element. - **Mass Number (A):** Sum of protons and neutrons in the nucleus. Represents the approximate atomic mass in amu. Calculation: Number of neutrons = Mass Number - Atomic Number (A - Z). - **Molecular Mass:** Summation of atomic masses of all atoms in a molecule (e.g., $HNO_3 = 1 (H) + 14 (N) + 3 \times 16 (O) = 63$ g/mol). - **Mole concept:** 1 mole of a substance Avogadro's no ($6.02 \times 10^{23}$ atoms). - **Avogadro's Constant:** Value: $6.022 \times 10^{23}$ particles/mol. Represents the number of atoms in exactly 12 grams of Carbon-12. Used to convert between moles and number of particles (atoms/molecules). #### Quantum Numbers - **Principle Quantum Number (n):** Defines the energy level (shell, e.g., K, L, M, N). For 4f orbital, n=4. - **Azimuthal/Angular Momentum Quantum Number (l):** Defines the shape of the orbital (subshell). For 4f, l=3 (s=0, p=1, d=2, f=3). - **Magnetic Quantum Number (m):** Defines the orientation of the orbital in space. For l=3, m can be -3, -2, -1, 0, 1, 2, 3. - **Spin Quantum Number (s):** Defines the intrinsic angular momentum of an electron. Can be +1/2 or -1/2. - **Electronic Shells:** Maximum electrons in nth shell: $2n^2$. K-shell (n=1): Max 2 electrons. L-shell (n=2): Max 8 electrons. M-shell (n=3): Max 18 electrons. N-shell (n=4): Max 32 electrons. Orbit or energy level is designated as K, L, M, & N shells. - **Valency:** Definition: The number of electrons an atom can lose, gain, or share to form a stable electron configuration (like noble gases). Example: Aluminium (Al) has 13 electrons (2, 8, 3). Its valency is 3, as it tends to lose 3 electrons. #### Types of Atoms - **Isotopes:** Atoms of the same element (same atomic number) with different mass numbers (different number of neutrons). Have same chemical properties & different physical properties. E.g., Chlorine-35 and Chlorine-37 ($^{35}_{17}Cl$, $^{37}_{17}Cl$). - **Isobars:** Atoms of different elements (different atomic number) that have the same mass number (same total protons + neutrons). Have same physical properties & different chemical properties. - **Isotone:** Atoms of the different elements which have same number of neutrons. #### Ions - **Anion:** -ve charged ion. - **Cation:** +ve charged ion. #### Laws of Chemical Combination - **Law of Conservation of Mass:** Matter can neither be created nor destroyed in a chemical reaction. Total mass of reactants equals total mass of products. Atoms are rearranged, not created or destroyed. - **Law of Constant Proportion (Definite Proportions):** A given chemical compound always contains its component elements in fixed ratios by mass. Regardless of source or preparation, the elemental composition of a pure compound is constant. - **Law of Multiple Proportions:** If two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. Explains compounds like CO and CO2: fixed mass of Carbon, varying masses of Oxygen are in a simple ratio. #### Symbol of Elements - Symbols were given by Berzelius for first time. - Dalton used it for the first time. - Examples include: Hydrogen, Azote, Aluminum, Calcium, Strontium, Barium, Carbon, Beryllium, Iron, Oxygen, Silicon, Zinc, Phosphorus, Yttrium, Copper, Sulfur, Magnesia, Potash, Lead, Mercury, Silver, Lime, Soda, Water, Carbonic oxide, Gold, Platinum.

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