SE Unit 1 Topic 1 Notes- Hydrogen Bonding _ Structure of Water

Water is a polar molecule, characterized by its molecular structure where one oxygen atom is bonded to two hydrogen atoms.
The polarity leads to hydrogen bonding, which is crucial for many of water's unique properties.

Hydrogen bonds are weak attractions between a partially positive hydrogen atom in one molecule and a highly electronegative atom in another molecule, typically oxygen or nitrogen.
These bonds are responsible for the cohesive and adhesive properties of water, which enable various biological functions.

SYI-1 Living systems are organized hierarchically and interact at various levels.
SYI-1.A Essential Knowledge:
- SYI-1.A.1: The polarity and hydrogen bonding properties of water affect its functions in biological systems.
- SYI-1.A.2: Living organisms depend heavily on the properties of water, such as its ability to dissolve substances and facilitate chemical reactions.
- SYI-1.A.3: Hydrogen bonds result in water's cohesive, adhesive properties and surface tension.

Matter: Anything that occupies space and possesses mass (e.g., rocks, metals, gases).
Elements: Substances that cannot be broken down further; there are 92 naturally occurring elements in nature.
Compounds: Substances consisting of two or more different elements combined in fixed ratios (e.g., H2O, NaCl).

Of the 92 elements, only 20-25% are essential for life, with CHON (Carbon, Hydrogen, Oxygen, Nitrogen) making up 96% of living matter.
Trace elements: Needed in very small quantities but are critical for biological functions.

Atomic number refers to the number of protons in an atom.
Atomic mass includes the total number of protons and neutrons, averaged across isotopes.

The Bohr model illustrates electrons orbiting around the nucleus in distinct energy levels.
Each electron shell has a maximum capacity: 1st shell (2 electrons), 2nd shell (8 electrons), and 3rd shell (up to 18 electrons).
Lithium is an example with 3 protons, 3 electrons, and 1 valence electron.

Chemical Bonds: Interactions that hold atoms together via shared or transferred electrons.
Electronegativity: A measure of an atom’s ability to attract electrons; it varies in the periodic table.

Covalent Bonds: Formed by the sharing of electrons between atoms, can be single, double, or triple bonds.
- Nonpolar covalent bonds: Electrons are shared equally.
- Polar covalent bonds: Electrons are shared unequally, leading to partial charges (e.g., water, H2O).

Ionic Bonds: Formed through the transfer of electrons which creates charged ions (cations and anions), establishing electrostatic attractions (e.g., NaCl).

Water molecules exhibit cohesion due to hydrogen bonding, allowing them to stick together, leading to surface tension which supports structures like water striders.
Cohesion aids in the movement of water and nutrients against gravity in plants.

The attraction of water molecules to other substances (adhesion) helps water cling to plant cell walls, resisting gravity.

Capillary Action: The movement of water upward in plants due to cohesion and adhesion, essential for nutrient transport.

High Specific Heat: Water’s ability to absorb and release heat helps moderate environmental temperatures and stabilizes marine environments.
Evaporative Cooling: Water's high heat of vaporization helps regulate temperature for ecosystems and organisms.

When water freezes, it expands and becomes less dense, allowing ice to float and insulate aquatic life beneath it.

Water, due to its polarity, acts as a versatile solvent, dissolving various substances and facilitating biochemical reactions.
Water can dissolve ionic compounds by surrounding charged ions, allowing substances to mix homogeneously.