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The Chemical Context of Life (An element's properties depend on the…
The Chemical Context of Life
Matter consists of chemical elements in pure form and in combinations called compounds
Matter is anything that takes up space and has mass
Elements are substances that cannot be chemically broken down to other types of matter
A compound is made up of two or more elements combined in a fixed ratio
Essential elements are those needed for an organism to live and reproduce
Some elements, such as iron (Fe) and iodine (I), may be required in very minute quantities and are called trace elements
An element's properties depend on the structure of its atoms
Each element has its own type of atom, the smallest unit of matter retaining the properties of that element
Uncharged neutrons and positively charged protons are packed tightly together to form the atomic nucleus of an atom. Negatively charged electrons form a large cloud around the positively charged nucleus
Each element has a characteristic atomic number, or number of protons in each of its atoms. The term atomic mass refers to the total mass of an atom
The mass number is equal to the number of protons and neutrons in the nucleus and approximates the mass of an atom of that element in daltons
Although the number of protons is constant, the number of neutrons can vary among the atoms of an element, creating different isotopes that have slightly different masses but the same chemical behaviors
Some isotopes are unstable; the nuclei of radioactive isotopes spontaneously decay, giving off particles and energy
Energy is define as the capacity to cause change - to do work. Potential energy is energy stored in matter as a consequence of its position or structure
Electrons can be located in different electron shells, each with a characteristic energy level and distance from the nucleus
The formation and function of molecules depend on chemical bonding between atoms
These interactions usually result in attractions called chemical bonds, which hold the atoms close together
When two atoms share a pair of valence electrons, a covalent bond is formed. A molecule consists of two or more atoms held together by covalent bonds
A structural formula, such as H-H, uses a dash to indicate the shared electrons of a single bond. In an oxygen molecule (O2), two pairs of valence electrons are shared between oxygen atoms, forming a double bond
The valence, or bonding capacity, of an atom usually equals the number of unpaired electrons in its valence shell
Electronegativity is the attraction of a particular atom for shared electrons. If the bond is said to be a nonpolar covalent bond, the electrons remain equally shared. If the bond is said to be a polar covalent bond, the electrons are unequally shared
An ionic bond may hold these ions together because of the attraction of their opposite charges. A hydrogen bond is covalently bonded to an electronegative atom that has a partial positive charge and can be attracted to a different nearby electronegative atom
Chemical reactions make and break chemical bonds
Chemical reactions involve the making or breaking of chemical bonds
Matter is conserved in chemical reactions; the same number and kinds of atoms are present in both reactants and products, although the rearrangement of electrons and atoms causes the properties of these molecules to be different
Chemical equilibrium is reached when the forward and reverse reactions proceed at the same time, and the relative concentrations of reactants and products no longer change