Chemistry of Life: Atoms, Bonds, and Water

Matter is defined as anything that occupies space and has mass; it is composed of elements.
An element is a substance that cannot be chemically broken down into simpler forms. There are $92$ naturally occurring elements.
Essential elements required for life include Carbon ( $C$ ), Hydrogen ( $H$ ), Oxygen ( $O$ ), and Nitrogen ( $N$ ), which account for $96\%$ of body mass.
Trace elements are required in quantities less than $0.01\%$ , such as Iron ( $Fe$ ) and Iodine ( $I$ ). Iodine deficiency ( $150\,\mu g/day$ ) can lead to a condition called a goiter as the thyroid glands can’t work properly and produce hormones.
Compounds (e.g., $NaCl$ ) consist of two or more elements in a fixed ratio and exhibit emergent properties distinct from their component elements.

Atoms are the smallest units of an element, composed of protons ( $+$ ), neutrons (neutral), and electrons ( $-$ ).
Protons and neutrons form the nucleus, while electrons occupy orbitals or shells.
The Atomic Number is the unique number of protons in an atom. The Mass Number is the sum of protons and neutrons.
Atomic mass is measured in Daltons ( $d8$ ); one Dalton equals $1.67 \times 10^{24}\,g$ .
Isotopes are variants of an element with the same number of protons but different numbers of neutrons (e.g., $^{12}C$ , $^{13}C$ , and radioactive $^{14}C$ used in radiocarbon dating).
Radiometric dating, such as using isotopes of uranium, has determined moon rocks are approximately $4.5 \text{ billion}$ years old.

Electron shells fill from the innermost to the outermost ( $1\text{st}$ shell: $2$ electrons; $2\text{nd}$ shell: $8$ ; outermost: up to $32$ electrons in $16$ pairs).
Reactivity is driven by unpaired electrons in the valence shell.
Covalent Bonds occur when atoms share electrons to form molecules.
- Nonpolar: Equal sharing of electrons.
- Polar: Unequal sharing due to differences in electronegativity, creating slightly charged regions (e.g., $H_2O$ ).
Ionic Bonds involve the complete transfer of electrons, creating Cations ( $Na^+$ ) and Anions ( $Cl^-$ ).
Hydrogen Bonds are weak intermolecular attractions between a slightly positive hydrogen atom and an electronegative atom like Oxygen.

Cohesion and Adhesion: Hydrogen bonding causes water to stick to itself and other surfaces, enabling high surface tension and capillary action up to $100\,m$ in plants.
Temperature Moderation: Water has a high specific heat and high heat of vaporization, stabilizing environmental temperatures and facilitating evaporative cooling.
Expansion Upon Freezing: Ice is approximately $10\%$ less dense than liquid water because hydrogen bonds lock molecules into a lattice, allowing ice to float and insulate aquatic life.
Universal Solvent: Water's polarity allows it to dissolve ionic compounds and polar molecules, defining substances as hydrophilic (water-loving) or hydrophobic (water-fearing).