AP BIO CHAP 2 UNIT 1
Concept 2.1: Matter consists of chemical elements in pure form and in combinations called compounds
Hydrogen, Oxygen, Nitrogen, Carbon, and Phosphorus are the most important elements that primarily makeup life (96%)
- Most of the remaining 4% consists of calcium, phosphorus, potassium, and sulfur
- Trace elements are those required by an organism in minute quantities
organisms are composed of matter
matter is anything that takes up space and has mass
An element is a substance that cannot be broken down to other substances by chemical reactions
A compound is a substance consisting of two or more elements in a fixed ratio
Concept 2.2: An element’s properties depend on the structure of its atoms
Each element consists of unique atoms
An atom is the smallest unit of matter that still retains the properties of an element
Atoms are composed of subatomic particles
Relevant subatomic particles include:
Neutrons (no electrical charge)
Protons (positive charge)
Electrons (negative charge)
Neutrons and protons form the atomic nucleus
Electrons form a cloud around the nucleus
Neutron mass and proton mass are almost identical and are measured in daltons
All atoms of an element have the same number of protons but may differ in number of neutrons
Isotopes are two atoms of an element that differ in number of neutrons
Radioactive isotopes decay spontaneously, giving off particles and energy
Energy Levels of Electrons
Energy is the capacity to cause change
Potential energy is the energy that matter has because of its location or structure
The electrons of an atom differ in their amounts of potential energy
An electron’s state of potential energy is called its energy level, or electron shell
Electron Distribution and Chemical Properties
The chemical behavior of an atom is determined by the distribution of electrons in electron shells
The periodic table of the elements shows the electron distribution for each element
Valence electrons are those in the outermost shell, or valence shell
The chemical behavior of an atom is mostly determined by the valence electrons
Elements with a full valence shell are chemically inert i.e.-He (noble gases)
Electron Orbitals
An orbital is the three-dimensional space where an electron is found 90% of the time
Each electron shell consists of a specific number of orbitals
Concept 2.3: The formation and function of molecules depend on chemical bonding between atoms
Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms
These interactions usually result in atoms staying close together, held by attractions called chemical bonds
A covalent bond is the sharing of a pair of valence electrons by two atoms
In a covalent bond, the shared electrons count as part of each atom’s valence shell
A molecule consists of two or more atoms held together by covalent bonds
A single covalent bond, or single bond, is the sharing of one pair of valence electrons
A double covalent bond, or double bond, is the sharing of two pairs of valence electrons
The notation used to represent atoms and bonding is called a structural formula
For example, H–H
This can be abbreviated further with a molecular formula
For example, H2
Electronegativity is an atom’s attraction for the electrons in a covalent bond
The more electronegative an atom, the more strongly it pulls shared electrons toward itself
- In a nonpolar covalent bond, the atoms share the electron equally
- In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally
Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule
ion - A charged atom (or molecule)
A cation is a positively charged ion
An anion is a negatively charged ion
An ionic bond is an attraction between an anion and a cation
A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
Van der Waals interactions are attractions between molecules that are close together as a result of these charges
A molecule’s shape is usually very important to its function
A molecule’s shape is determined by the positions of its atoms’ valence orbitals
Biological molecules recognize and interact with each other with a specificity based on molecular shape
Concept 2.4: Chemical reactions make and break chemical bonds
Chemical reactions are the making and breaking of chemical bonds
The starting molecules of a chemical reaction are called reactants
The final molecules of a chemical reaction are called products
Chemical equilibrium is reached when the forward and reverse reaction rates are equal