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Chapter 2 The Chemical Context of Life (2.2 An element's properties…
Chapter 2 The Chemical Context of Life
2.1 Matter consists of chemical elements in pure form and in combinations called compounds
Elements and Compounds
element = substance that can't be broken down to other substances by chemical reactions, stays the same
compound = substance made of two or more different elements combined in a fixed ratio
example: NaCl (table salt) is a 1:1 ratio
The Elements of Life
essential elements = 20-25% of 92 naturally occurring elements, necessary for a healthy life and reproduction
examples: humans need 25 vs plants need 17
trace elements = required in very small amounts and vary for different organisms
Carbon, Hydrogen, Nitrogen and Oxygen make up 96% of living matter
toxic elements; some naturally occurring elements can be toxic
example: arsenic can be lethal
Introduction
matter = anything that moves or takes up space
organisms are made of matter
2.2 An element's properties depend on the structure of its atoms
Introduction
elements contain atoms that are different to the atoms of another element
atom = smallest unit of matter that still retains the properties of its element
Subatomic Particles
subatomic particles are the smallest parts that make up atoms
3 relevant ones: protons (positive charge), electrons (negative charge), and neutrons (no charge)
protons and neutrons in the nucleus, electrons in electron cloud
dalton/ amu (atomic mass unit) = unit of measurement for atoms, molecules, and subatomic particles
Atomic Number and Atomic Mass
atomic number = element's unique number of protons
mass number = total number of protons and neutrons in the nucleus of an atom
atomic mass = varies slightly from mass number because its the average of all the different isotopes an element has
Isotopes
isotope = different atomic forms of same element due to more neutrons creating greater mass
example: 3 naturally occurring isotopes of Carbon, two are stable and one is unstable (radioactive)
radioactive isotope = nucleus decays spontaneously, giving off particles and energy
this leads to a change of protons and it eventually becomes another element
Radioactive Tracers
uses radioactive isotopes as a diagnostic tool
example: tracking growth of cancer
incorporates radioactive isotopes into biologically active molecules and traces them during metabolism
Radiometric Dating
process of measuring the ratio of different isotopes and calculating the age of a fossil by the amount of half-lives that have passed
half-life = time it takes parent isotope to decay by 50%
Energy Levels of Electrons
energy = capacity to do work or cause change
potential energy = energy possessed by matter due to location or structure
Electron Distribution and Chemical Properties
valence shell = outermost electron shell
valence electrons = electrons in the valence (outermost) shell
an atom's chemical behavior depends on the amount of valence electrons
chemically inert/ unreactive: elements with a full valence shell, such as the ones on far right of periodic table
2.3 The formation and function of molecules depend on chemical bonding between atoms
Introduction
chemical bonds = interactions that hold atoms close together so they can share or transfer valence electrons
strongest chemical bonds = covalent bonds or ionic bonds in dry compounds
Covalent Bonds
covalent bond = sharing of valence electrons by two atoms
example: two hydrogen atoms share and now both have a completed valence shell (H2) and became a hydrogen molecule
molecule = two or more atoms in a covalent bond
single bond = pair of shared electrons; double bond = two pairs
electronegativity = based on how strongly an atom pulls the shared electrons of a covalent bond to itself
nonpolar covalent bond = electrons are shared equally because atoms have the same electronegativity
example: single bond of H2 is nonpolar
polar covalent bond = bond in which electrons are shared unequally due to one atom being more electronegative
example: H20 is polar
Ionic Bonds
ions = oppositely charged atoms due to the more electronegative element stripping away the electron it needs from a partner
cations = positively charged ion
anion = negatively charged ion
ionic bond = the attraction between cations and anions
electrons being transferred allows the bond to form the attraction of the oppositely charged ions
ionic compounds = compounds formed by an ionic bond, aka as salts
example: NaCl or table salt is an ionic compound
Weak Chemical Interactions
Hydrogen Bonds
hydrogen bonds = attraction between hydrogen atom and an electronegative atom (usually oxygen or nitrogen)
van der Waals interactions
example: gecko climbing on glass or wall. (Unevenly distributed electrons) ever-changing regions of positive or negative charge that allow all atoms and molecules to stick together
Molecular Shape and Function
a molecule's characteristic size and shape is key to its function
shape is crucial; it determines how biological molecules recognize and respond to one another with specificity
2.4 Chemical reactions make and break chemical bonds
chemical reactions = making and breaking of chemical bonds that lead to changes in the composition of matter
example: 2 H2 + O2 = 2 H20; breaks covalent bonds of H2 and O2 and forms a new bond of H20
reactants -----> chemical reaction ----> products
reactants = starting materials; products = resulting materials
reactions can't create or destroy atoms, can only rearrange
chemical equilibrium = point at which the forward and reverse reactions offset one another exactly