Atoms, Molecules and Chemical Reactions

Atom: The smallest unit of an ordinary chemical element that retains its distinctive chemical properties. Modern atomic theory establishes that atoms are divisible and composed of three fundamental subatomic particles: Protons (positively charged nucleons, mass $\approx 1.673 \times 10^{-24}\text{ g}$), Neutrons (electrically neutral nucleons, mass $\approx 1.675 \times 10^{-24}\text{ g}$), and Electrons (negatively charged extranuclear particles, mass $\approx 9.109 \times 10^{-28}\text{ g}$).

Rutherford’s celebrated alpha particle scattering experiment demonstrated that nearly all the mass and positive charge of an atom is concentrated within an extraordinarily minute central core called the Nucleus. Surrounding this dense nucleus is a vast expanse of empty space where negatively charged electrons orbit at relativistic speeds in discrete shells governed by electrodynamic principles.

Examiner Trap: Students frequently hypothesize that electrons contribute significantly to atomic mass or reside inside the nucleus. Always remember: electron mass is negligible (1/1837th of a proton). Total atomic mass is strictly concentrated within the nuclear nucleons (protons and neutrons).

Every physical structure in the observable cosmos — oceans, biological organisms, and distant galaxies — is constructed from just these three subatomic particles. Particle physicists at CERN have further revealed that protons and neutrons are composed of fundamental fractional-charge entities called Quarks (up and down quarks held by gluons)!

Atomic Number ($Z$): The total number of protons located within the atomic nucleus ($Z = p$). In a neutral atom, it also equals the total orbiting electrons, serving as the elemental fingerprint.
Mass Number ($A$): The integer sum of nuclear nucleons: protons plus neutrons ($A = p + n = Z + N$). Therefore, neutron count is given by $N = A - Z$.

Electrons occupy concentric quantum energy levels designated as $K, L, M, N$ shells. Under the Bohr-Bury scheme, each shell accommodates a maximum of $2n^2$ electrons (where $n$ is shell level):
• $K$ shell ($n=1$): Max $2(1)^2 = 2$ electrons.
• $L$ shell ($n=2$): Max $2(2)^2 = 8$ electrons.
• $M$ shell ($n=3$): Max $2(3)^2 = 18$ electrons.
Crucially, the outermost valence shell can hold no more than 8 electrons regardless of principal quantum limit.

For the chlorine nuclide $_{17}^{35}\text{Cl}$:
1. Protons $Z = 17$; in neutral atom, Electrons = 17.
2. Mass number $A = 35 \implies$ Neutrons $N = 35 - 17 = 18$.
3. Electronic Shell Allocation: K shell = 2, L shell = 8, M shell = 7 (2, 8, 7). Needing just 1 electron to satisfy the noble gas octet, chlorine avidly gains an electron to form the stable chloride anion ($\text{Cl}^-$).

Exam Trap: Never confuse isotopes with isobars!
• Isotopes: Nuclides of the same element having identical atomic numbers ($Z$) but differing mass numbers ($A$) due to unequal neutrons (e.g. Protium $_1^1\text{H}$, Deuterium $_1^2\text{H}$, Tritium $_1^3\text{H}$).
• Isobars: Nuclides of distinct elements having identical mass numbers ($A$) but differing atomic numbers ($Z$) (e.g. Argon $_{18}^{40}\text{Ar}$ and Calcium $_{20}^{40}\text{Ca}$).

Deuterium combined with oxygen forms Heavy Water ($ ext{D}_2 ext{O}$), widely utilized as an essential thermal neutron moderator and coolant in commercial CANDU and PHWR nuclear energy reactors.

Molecule: The smallest discrete particle of an element or compound capable of independent stable existence ($\text{O}_2, \text{H}_2\text{O}, \text{CO}_2$).
Ions: Electrically charged species generated when neutral atoms transfer electrons. Metallic atoms lose electrons to form positive Cations ($M^{n+}$); non-metallic atoms capture electrons to form negative Anions ($X^{m-}$).

Noble gases like Helium ($2$), Neon ($2, 8$), and Argon ($2, 8, 8$) are chemically unreactive because their outermost valence shells are completely saturated. The Octet Rule states that atoms chemically interact, transfer, or share electrons specifically to attain this ultra-stable eight-electron valence configuration.

A Radical is a group of atoms of different elements covalently bonded together that behaves as a single charged unit during chemical transformations:
• Monovalent Radicals (Charge $\pm 1$): Hydroxide ($\text{OH}^-$), Nitrate ($\text{NO}_3^-$), Ammonium ($\text{NH}_4^+$), Bicarbonate ($\text{HCO}_3^-$).
• Divalent Radicals (Charge $-2$): Sulphate ($\text{SO}_4^{2-}$), Carbonate ($\text{CO}_3^{2-}$), Sulphite ($\text{SO}_3^{2-}$).
• Trivalent Radicals (Charge $-3$): Phosphate ($\text{PO}_4^{3-}$).

Common Fallacy: Students erroneously presume that cations gain protons to become positive. Chemical reactions involve only extranuclear valence electrons; nuclear protons remain completely unchanged. A cation is positive solely because it surrendered negative electrons!

Biological neural conduction, muscle contraction, and cardiac rhythms rely entirely upon trans-membrane flux of sodium ($\text{Na}^+$) and potassium ($\text{K}^+$) cations. Oral Rehydration Salts (ORS) replenish these depleted electrolytes during severe dehydration.

Valency: The combining capacity of an atom with other atoms to form chemical compounds. Quantitatively defined as the number of hydrogen or chlorine atoms with which one atom of the element combines. For example, in $\text{HCl}$, Chlorine has valency 1; in $\text{H}_2\text{O}$, Oxygen has valency 2; in $\text{NH}_3$, Nitrogen has valency 3; in $\text{CH}_4$, Carbon has valency 4.

Standard method for writing chemical formulas:
1. Write the cation or metallic symbol on the left and the anion or radical on the right.
2. Inscribe their respective valencies as top superscripts.
3. Cross over the valency numbers diagonally so that each becomes the opposite species’ bottom subscript.
4. Divide by highest common factor to reduce to simplest whole-number ratio (e.g. $\text{Ca}_2\text{O}_2 \implies \text{CaO}$).

• Aluminium Oxide: $\text{Al}^{3+}$ and $\text{O}^{2-} \implies$ $\text{Al}_2\text{O}_3$.
• Calcium Hydroxide: $\text{Ca}^{2+}$ and $\text{OH}^- \implies$ $\text{Ca(OH)}_2$.
• Ferric Sulphate: $\text{Fe}^{3+}$ and $\text{SO}_4^{2-} \implies$ $\text{Fe}_2(\text{SO}_4)_3$.

Examiner Mandate: Polyatomic radicals present in quantities greater than one must invariably be enclosed within parentheses before writing the numerical subscript: e.g. $\text{Ca(OH)}_2$. Omitting parentheses, as in $\text{CaOH}_2$, communicates 1 calcium, 1 oxygen, and 2 hydrogens — an entirely invalid chemical construct.

Certain transition elements demonstrate variable valency by involving inner electrons in chemical combinations:
• Iron ($\text{Fe}$): Lower valency 2 (Ferrous, $\text{FeCl}_2$) and higher valency 3 (Ferric, $\text{FeCl}_3$).
• Copper ($\text{Cu}$): Lower valency 1 (Cuprous, $\text{Cu}_2\text{O}$) and higher valency 2 (Cupric, $\text{CuO}$).
The lower state traditionally takes suffix -ous; the higher state takes -ic.

A Chemical Reaction involves the rearrangement of atoms via breaking existing reactant bonds and synthesizing new product bonds. The original substances are termed Reactants and newly formed substances are termed Products. Symbolic arrows ($\to$) denote reaction progression, while gas evolution ($\uparrow$) and precipitation ($\downarrow$) denote physical state transitions.

Formulated by Antoine Lavoisier in 1789: In any closed chemical reaction, matter is neither created nor destroyed; total mass of reactants identically equals total mass of products. Consequently, an equation must be balanced so that the total count of each atomic species is equal across both sides.
Example: Balancing hydrogen and oxygen to synthesize water: $$2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$$ (Reactants: 4 H, 2 O; Products: 4 H, 2 O).

Exam Trap: Balancing an equation must exclusively be accomplished by adjusting leading stoichiometric multipliers (coefficients). Altering internal chemical subscripts is catastrophic: writing $\text{H}_2\text{O}_2$ instead of $2\text{H}_2\text{O}$ transforms potable water into toxic bleaching disinfectant!

Vital planetary processes are balanced chemical equations in action: atmospheric carbon fixation via Photosynthesis ($6\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$) and cellular energy liberation via Aerobic Respiration ($ \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$).