Comprehensive Study Notes — Atoms, Molecules & Tissues Summary & Study Notes

⚛️ Key discoveries & sub-atomic particles

Electrons ($e^-$) were identified by J.J. Thomson; they are negatively charged and light compared to nucleons. Protons ($p^+$) were inferred from canal rays (E. Goldstein) and established as positive particles with mass ≈ 1 u. Neutrons ($n$) (J. Chadwick, 1932) are neutral nucleons with mass ≈ 1 u and occur in nuclei (except the most common hydrogen isotope).

🧪 Experimental evidence

Experiments with electric charge and discharge, and Rutherford’s α-particle scattering on thin gold foil, showed that atoms are not solid spheres. Rutherford’s observations led to the conclusion that most of an atom’s volume is empty space and that mass and positive charge are concentrated in a tiny nucleus whose radius is roughly $10^{-5}$ times the atomic radius.

🧭 Atomic models (brief)

• Thomson’s model: electrons embedded in a uniformly positive sphere ("plum pudding"). Explains electrical neutrality but fails to account for scattering results.
• Rutherford’s nuclear model: tiny, dense, positively charged nucleus with electrons orbiting around it; explains scattering but cannot explain atomic stability.
• Bohr’s model: electrons occupy discrete allowed orbits (energy levels); electrons in these orbits do not radiate energy. This introduces quantized energy levels and helps explain atomic spectral lines.

🪐 Electron shells & distribution rules

• Shells/energy levels are labeled K, L, M, N... or $n=1,2,3,4\ldots$.
• Maximum electrons in shell $n$ given by formula: $2n^2$ (so K: $2$, L: $8$, M: $18$, N: $32$).
• Outermost shell can hold up to 8 electrons (octet rule approximation for many elements).
• Electrons fill inner shells first (stepwise filling).

🔗 Valency (combining capacity)

Valence electrons are the electrons in the outermost shell. The valency of an element is its tendency to lose, gain or share electrons to attain a full outer shell (often an octet). Examples from first eighteen elements: Li, Na — valency 1; Mg — valency 2; Al — valency 3; F — tends to gain 1 electron → valency 1.

🔢 Atomic number, mass number, isotopes & isobars

• Atomic number ($Z$) = number of protons; defines the element.
• Mass number ($A$) = total number of nucleons (protons + neutrons).
• Notation for a nuclide: $^A_Z X$ (where $X$ is element symbol).
• Isotopes: atoms with same $Z$ but different $A$ (e.g., protium $^1_1$H, deuterium $^2_1$H, tritium $^3_1$H). Chemical properties are similar; physical properties differ.
• Isobars: different elements (different $Z$) with same mass number $A$ (e.g., $^{40}${20}2040​Ca and $^{40}${18}Ar).

✅ Quick summary

The modern picture: atoms consist of a tiny nucleus (protons and neutrons) containing most of the mass, surrounded by electrons in quantized energy levels. Atomic number determines identity; mass number measures nucleon count. Isotopes and electron configurations explain many chemical and physical properties.

🧪 Laws of chemical combination

Law of Conservation of Mass: mass is neither created nor destroyed in a chemical reaction. Law of Constant (Definite) Proportions: a given compound always contains the same elements in the same mass ratio (e.g., in water the mass ratio H:O is $1:8$).

🧫 Dalton’s atomic theory (foundation)

Dalton explained these laws by proposing that matter is made of indivisible atoms (each element’s atoms identical). Key postulates linked atomic behavior to observed stoichiometry and fixed mass ratios in compounds.

⚖️ Atomic mass & the atomic mass unit

• The modern standard defines 1 unified atomic mass unit ($1,u$) as $\frac{1}{12}$ of the mass of one atom of carbon-12. In symbols: $1,u = \frac{1}{12},$mass$(^{12}\textrm{C})$.
• Relative atomic masses are measured against this standard (examples: H ≈ $1,u$, C ≈ $12,u$, O ≈ $16,u$, Cl ≈ $35.5,u$).

🔬 Molecules, ions & formulae

• A molecule is the smallest particle of an element or compound that can exist independently and retain chemical identity (e.g., $O_2$, $H_2O$).
• Ions are charged species: cations (positive) and anions (negative). Polyatomic ions are groups of atoms carrying net charge (e.g., $\rm NO_3^{-}$, $\rm SO_4^{2-}$, $\rm NH_4^{+}$).
• Writing formulas: use valencies (charges) to balance positive and negative charges; metal written first in ionic compounds. For polyatomic ions with subscripts >1, use parentheses, e.g., $\rm Ca(OH)_2$, $\rm (NH_4)_2SO_4$.

🧮 Molecular mass & formula unit mass

• Molecular mass = sum of atomic masses of atoms in a molecule (units: $u$). Example: $\rm H_2O$ → molecular mass $= 2\times 1 + 16 = 18,u$.
• Formula unit mass applies to ionic compounds (e.g., $\rm NaCl$): sum of atomic masses in the empirical formula (e.g., $23 + 35.5 = 58.5,u$ for $\rm NaCl$).

🧾 Practical rules & examples

• Use valency/criss-cross charges to obtain empirical formulas (Mg$^{2+}$ + Cl$^{-}$ → $\rm MgCl_2$).
• Convert mass ratios to simplest whole-number atom ratios to deduce molecular formulas (example exercise: from masses and atomic masses deduce $H_2O$ ratio H:O = $2:1$).

✅ Summary

Dalton’s atomic idea plus precise mass standards ($1,u = \frac{1}{12}$ mass of $^{12}$C) and the concept of molecules and ions form the basis for writing chemical formulae, calculating molecular/formula unit masses, and understanding stoichiometry in reactions.

🧬 What is a tissue?

A tissue is a group of cells with similar structure and a common function. In multicellular organisms, tissues are organized assemblies that carry out specialized activities essential for life.

🧱 Epithelial tissue

Epithelial tissue covers body surfaces, lines cavities and forms glands. It has closely packed cells with little extracellular matrix, functions in protection, absorption, secretion, and sensation, and shows polarity (apical and basal surfaces).

🦴 Connective tissue

Connective tissue supports and connects other tissues and organs. It typically has cells embedded in an abundant extracellular matrix (fibres like collagen and ground substance). Types include loose connective tissue, dense connective tissue, adipose tissue, cartilage, bone and blood; functions include support, transport, protection, and storage.

💪 Muscle tissue

Muscle tissue is specialized for contraction and generating force. Three types exist: skeletal (voluntary movement, striated), cardiac (heart muscle, striated with intercalated discs), and smooth (involuntary, non-striated, in walls of organs and vessels).

🧠 Nervous tissue

Nervous tissue is composed of neurons (signal-conducting cells) and neuroglia (support cells). It is specialized for rapid communication and control through electrical impulses and neurotransmitters; it forms the brain, spinal cord and peripheral nerves.

🩺 Integration & function

Tissues combine to form organs; organs form organ systems. Proper tissue structure (cell type, arrangement, extracellular matrix) underlies physiological function and is central to understanding development, repair and disease.