Comprehensive Study Notes on Atomic Structure and Periodic Properties

Historical and Philosophical Origins of Atomic Theory

• Conceptualization of Matter: Since ancient times, philosophers have theorized about the nature of matter. Empedocles suggested that everything is composed of four fundamental elements: earth, air, fire, and water. Later, in the $5^{\text{th}}$ century, the Greek philosophers Leucippus and Democritus proposed that matter is built from small particles, varying in size and shape, which they termed "atomos," meaning "uncuttable."
• Acharya Kanada's Parmanu: Indian sage and philosopher Acharya Kanada (born $600\,\text{BC}$ in Prabas Kshetra, Gujarat) suggested that matter consists of indivisible, eternal, and indestructible particles called "parmanu" (ultimate particles). He noted that parmanu cannot exist in a free state but combine to form larger particles. His research is documented in the "Vaisheshika Sutra," earning him the title of the father of atomic theory. His name, "Kanada," originated from "Kan," meaning the smallest particle in Sanskrit.
• Evolution of the Term "Atom": The term derives from the Greek word "atomos." Atoms are the smallest units of matter that retain the properties of an element and act as the basic building blocks of all substances in the universe.

Dalton's Atomic Theory (1803)

• John Dalton's Postulates: John Dalton proposed a foundational theory describing matter through specific characteristics:
  • All matter is made of tiny, indivisible particles called atoms.
  • Atoms of a given element are identical in size, mass, and other properties.
  • Atoms of different elements differ in size, mass, and properties.
  • Atoms cannot be subdivided, created, or destroyed.
  • In chemical reactions, atoms combine, separate, or rearrange, but never change into atoms of another element.
  • Atoms of different elements combine in simple whole-number ratios to form chemical compounds (e.g., $H_2O$ consists of 2 hydrogen atoms and 1 oxygen atom).
  • The atom is the smallest particle that participates in a chemical reaction.

Discovery of Subatomic Particles

• Divisibility of the Atom: Late $19^{\text{th}}$ and early $20^{\text{th}}$ century research revealed that atoms are divisible and consist of fundamental subatomic particles: protons, neutrons, and electrons.
• Discovery of Electrons ($e^-$): Discovered by J. J. Thomson in 1897 using a cathode ray discharge tube experiment.
  • Experiment: A high-voltage potential applied to gas at very low pressure in a glass tube emitted cathode rays from the negative cathode. These rays were deflected toward the positive plate in an electric field, proving they were negatively charged.
  • Properties: Negative charge of $1.6 \times 10^{-19}\,\text{coulomb}$; mass of $9.11 \times 10^{-31}\,\text{kg}$ (approximately $\frac{1}{1837}$ of a hydrogen atom's mass); constant charge-to-mass ratio.
• Discovery of Protons ($p^+$): E. Goldstein discovered "canal rays" or "anode rays" in 1886.
  • Experiment: Positive radiations were observed passing through a perforated cathode in a discharge tube. These were identified as positive ions; in hydrogen gas, these were protons.
  • Properties: Positive charge equal to the electron ($1.6 \times 10^{-19}\,\text{coulomb}$); mass of approximately $1.67 \times 10^{-27}\,\text{kg}$; localized in the nucleus.
• Discovery of Neutrons ($n^0$): James Chadwick discovered neutrons in 1932 by bombarding a thin sheet of beryllium with alpha ($\alpha$) particles.
  • Properties: Electrically neutral; mass slightly greater than a proton ($1.674 \times 10^{-27}\,\text{kg}$); located in the nucleus with protons.

Atomic Models

• Thomson's Plum Pudding Model (1904): Thomson proposed an atom consists of a positively charged sphere with negatively charged electrons embedded in it, similar to seeds in a watermelon or raisins in a plum pudding. The atom is electrically neutral as the charges balance. Its drawback was the inability to explain subsequent experimental results.
• Rutherford's Gold Foil Experiment (1911): Ernest Rutherford bombarded a thin gold foil with positively charged alpha ($\alpha$) particles.
  • Observations: Most particles passed through without deflection; a small number were deflected at small angles; a very few ($\sim 1$ in 20,000) retraced their path ($180^{\circ}$ deflection).
  • Conclusions: The atom is largely empty space; positive charge and mass are concentrated in a tiny central region called the nucleus; electrons revolve around the nucleus at high speeds like planets around the Sun (Planetary Model).
  • Drawbacks: Could not explain stability, as accelerating electrons should radiate energy and spiral into the nucleus.
• Bohr's Atomic Model (1913): Niels Bohr improved upon Rutherford's model.
  • Postulates: Electrons revolve in fixed circular orbits called shells or energy levels ($n = 1, 2, 3, 4$ or $K, L, M, N$). Each orbit has a discrete energy level; electrons do not lose energy while revolving in the ground state. They jump to higher orbits (excited state) when absorbing energy.

Atomic and Mass Numbers

• Atomic Number ($Z$): Defined as the number of protons in the nucleus. It uniquely identifies an element. For neutral atoms: $Z = \text{Number of protons} = \text{Number of electrons}$.
• Mass Number ($A$): The sum of the number of protons ($p$) and neutrons ($n$) in the nucleus.
  • Formula: $A = Z + n$
  • Neutron Calculation: $n = A - Z$
• Symbolic Representation: An element $X$ is denoted as ${}_Z^A X$. For example, Carbon-12 is ${}_6^{12}C$.

Isotopes and Their Properties

• Definition: Atoms of the same element with the same atomic number but different mass numbers due to differing neutron counts.
• Properties: Identical chemical properties (same electron count) but different physical properties (mass, density, boiling point).
• Common Examples:
  • Hydrogen: Protium (${}_1^1H$: 1p, 0n), Deuterium (${}_1^2H$: 1p, 1n), Tritium (${}_1^3H$: 1p, 2n).
  • Carbon: $C-12$ (6n), $C-13$ (7n), $C-14$ (8n).
  • Oxygen: ${}^{16}O$, ${}^{17}O$, ${}^{18}O$.
  • Chlorine: ${}^{35}Cl$, ${}^{37}Cl$.

Electronic Configuration

• Bohr-Bury Scheme: Governs electron distribution in shells.
  • Maximum Electrons: Determined by $2n^2$. $K(n=1) = 2$, $L(n=2) = 8$, $M(n=3) = 18$, $N(n=4) = 32$.
  • Sequential Filling: Inner shells fill completely before outer shells.
  • Octet Rule: The outermost shell cannot exceed 8 electrons (or 2 for the first shell, known as the Duplet Rule).
• Example (Potassium, $Z=19$): Configuration is $2, 8, 8, 1$. Although the $M$ shell can hold 18, it takes only 8 to allow the last electron into the $N$ shell to maintain stability.

Valency and Global Trends in the Periodic Table

• Definition: Valency is the measure of an element's combining capacity, determined by the number of valence electrons (electrons in the outermost shell).
• Calculation:
  • If valence electrons $\le 4$, Valency = valence electrons.
  • If valence electrons $> 4$, Valency = $8 - \text{valence electrons}$.
  • Example: Oxygen has 6 valence electrons, so Valency = $8 - 6 = 2$.
• Periodic Trends (Atomic Numbers 1-20): Valency increases from 1 to 4 across a period, then decreases to 0 (Noble Gases).
  • Group 1 (Alkali Metals): Valency 1 (Highly reactive).
  • Group 2 (Alkaline Earth Metals): Valency 2.
  • Group 17 (Halogens): Valency 1 (Very active non-metals).
  • Group 18 (Noble Gases): Valency 0 (Stable/Inert).
• Variable Valency: Capability of elements (especially transition metals) to show multiple valencies. Examples from Table 4.6:
  • Copper: Cuprous ($Cu^+$), Cupric ($Cu^{2+}$).
  • Iron: Ferrous ($Fe^{2+}$), Ferric ($Fe^{3+}$).
  • Lead: Plumbous ($Pb^{2+}$), Plumbic ($Pb^{4+}$).

Ions and Radicals

• Cations (Positive Ions): Formed when elements lose electrons (electropositive).
  • Examples: $Na^+$, $Li^+$, $K^+$, $Mg^{2+}$, $Ca^{2+}$, $Zn^{2+}$.
• Anions (Negative Ions): Formed when elements gain electrons (electronegative).
  • Examples: $Cl^-$, $F^-$, $O^{2-}$, $S^{2-}$, $H^-$ (Hydride).
• Radicals: Groups of atoms behaving as a single unit with a charge.
  • Positive: Ammonium ($NH_4^+$).
  • Negative: Sulphate ($SO_4^{2-}$, valency 2), Carbonate ($CO_3^{2-}$, valency 2), Nitrate ($NO_3^-$, valency 1).

Questions & Discussion

• In-Text Questions - 1:
  • Q: Helium has mass $4\,u$ and 2 protons. Neutrons? A: $4 - 2 = 2\,\text{neutrons}$.
  • Q: Element with $Z=14$, $A=28$. Symbol and particles? A: Silicon (${}_{14}^{28}Si$); 14 electrons, 14 neutrons.
• In-Text Questions - 2:
  • Q: Valency of $Mg(12), C(6), Li(3), Cl(17), Al(13)$? A: $Mg=2, C=4, Li=1, Cl=1, Al=3$.
• Assess Yourself MCQs:
  • Q: Number of electrons in outermost shell for stability? A: 8 (Option d).