Week 1 Lecture 2 (Textbook Based)

Element

A substance that cannot be chemically broken down into other substances

Atom

Smallest unit of an element that retains its properties

composed of protons, neutrons, and electrons

Subatomic particles

Protons (+1 charge, mass ≈1), neutrons (neutral, mass ≈1), electrons (−1 charge, negligible mass)

Atomic number

Number of protons in an atom’s nucleus, determines the element’s identity

Atomic mass

Approximate sum of protons and neutrons in an atom

Electron behavior

Electrons are attracted to protons in the nucleus but repelled by each other

rapid movement keeps them from collapsing into the nucleus

Isotopes

Atoms of the same element with identical proton numbers but different neutron counts

Radioactive isotope

Isotope with an unstable nucleus that breaks down spontaneously, emitting radiation

Radioactive decay

Process by which unstable isotopes release particles

Applications of radioactivity

Used in dating fossils, cancer treatment, and medical imaging

Risks of radioactivity

High-energy emissions can damage DNA and cells

Periodic table

Organizes elements by atomic number and recurring chemical properties

about 94 naturally occurring, ~28 synthetic

Big Four elements in the human body

Oxygen, carbon, hydrogen, nitrogen—together >96% of human body mass

Other important elements in the body

Calcium, phosphorus, potassium, sulfur, sodium, chlorine, magnesium

Trace elements

Required in very small amounts but critical for survival (e.g., iron, iodine, zinc)

Molecule

Two or more atoms held together by chemical bonds

Chemical reaction

Process where reactants are transformed into products by forming or breaking bonds

Bond energy

Energy required to break a bond

if products have lower bond energy than reactants, excess energy is released

Valence electrons

Electrons in the outermost shell that determine bonding capacity

Covalent bond

A bond formed by sharing one or more pairs of electrons between atoms

strong and directional

Single covalent bond

Sharing of one pair of electrons (e.g., H2 molecule)

Double covalent bond

Sharing of two pairs of electrons (e.g., O2 molecule)

Carbon bonding

Carbon has four valence electrons, allowing it to form four covalent bonds, enabling diverse structures

Ionic bond

A bond formed when electrons are transferred from one atom to another, producing oppositely charged ions that attract

Ions

Charged atoms or molecules formed by electron transfer

cation = positive (lost e−), anion = negative (gained e−)

Example of ionic bond

NaCl forms when sodium donates an electron to chlorine, producing Na+ and Cl−

Hydrogen bond

Weak attraction between a slightly positive hydrogen atom in one molecule and a slightly negative atom (O or N) in another molecule

Polarity

Unequal sharing of electrons in polar covalent bonds produces partial charges that enable hydrogen bonding

Importance of hydrogen bonds

Though weak individually, large numbers of them give water and biomolecules critical properties

Why are there unique properties of water?

Result from polarity and hydrogen bonding

Property 1: Cohesion

Water molecules stick together due to hydrogen bonding

• critical for transport in plants

• Causes surface tension

Property 2: High heat capacity

Water absorbs large amounts of heat before temperature rises

• Maintain body temperature for humans

Property 3: Low density as ice

Hydrogen bonds form a lattice keeping molecules farther apart in solid form, making ice float

• protects fish under the lake

Property 4: Good solvent

Water dissolves polar and ionic substances, enabling biochemical reactions and transport

Surface tension

Enhanced cohesion at water’s surface resists disruption (allows some organisms to “walk” on water)

Capillary action

Water’s adhesion to surfaces and cohesion with itself allows it to move upward through narrow spaces (e.g., plant xylem)

Hydrogen bonding and temperature

Heat input breaks hydrogen bonds, slowing temperature increase compared to other liquids

Salt and freezing

High salinity interferes with hydrogen bond lattice formation, lowering water’s freezing point

Nonpolar exclusion

Nonpolar molecules (oils, fats) do not dissolve in water

they aggregate instead