The Chemical Context of Life Biology 2 Chapter 2 The Chemical Context of Life

Matter Anything which has mass and takes up space.

Element A substance that cannot be broken down to other substances by chemical reactions. 92 elements occurring in nature Symbols derived from Latin or German names

Compound A substance consisting of two or more different elements combined in a fixed ratio.

Trace Elements Those elements required by an organism in only minute quantities. Example: Iodine is an essential ingredient of a hormone produced by the thyroid gland.

Subatomic Particles Protons (+) and neutrons (0) make up the atomic nucleus at the center of the atom. The electrons (-) move at nearly the speed of light and form a cloud around the nucleus.

Dalton John Dalton, the British scientist who helped develop atomic theory around 1800.

Dalton Same as the atomic mass unit (amu) Protons and neutrons have masses of close to 1 dalton (or 1.7 x 10 to the negative 24 power). Electrons have only about 1/1000 of the mass of a proton or neutron.

Atomic Number The number of protons which is unique to that element Unless otherwise indicated, an atom is neutral in electrical charge, which means that its protons must be balanced by an equal number of electrons.

Mass Number The sum of protons plus neutrons in the nucleus of an atom. Subtract the number of protons from the mass number to find the number of neutrons.

Isotopes The different atomic forms of an element. Have more or fewer neutrons than protons.

Radioactive Isotope One in which the nucleus decays spontaneously, giving off particles and energy. When decay leads to a change in the number of protons, it transforms the atom to an atom of a different element.

Radioactive Isotopes May be useful in establishing a date of fossils or as diagnostic tools in medicine.

Energy The capacity to cause change or do work. An atom’s electrons vary in the amount of energy they possess.

Potential Energy The energy that matter possesses because of its location or structure. Example: Water running downhill has the energy to turn turbines and generate electricity. Electrons which are farther from the nucleus have greater potential energy.

Energy Levels The different states of potential energy that electron's have in an atom. An electron’s energy level is correlated with its average distance from the nucleus

Electron Shells A representation of the average distances of the electrons from the nucleus of an atom. The first shell is closest to the nucleus, and electrons in this shell have the lowest potential energy.

Electron Configuration The chemical behavior of an atom is determined by the distribution of electrons in the atom’s electron shells.

Electron shell diagrams

Valence Electrons The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell which are called valence electrons.

Valence Shell The outermost electron shell

Orbital The three dimensional space where an electron is found 90 % of the time. Each electron shell consists of a number of orbitals of distinctive shapes and orientations. Think of an orbital as a component of an electron shell.

Orbitals No more than 2 electrons can occupy a single orbital. The reactivity of atoms arises from the presence of unpaired electrons in one or more orbitals of the valence shells.

Orbitals cont.

Chemical Bonds Atoms with incomplete valence shells can interact with certain other atoms in such a way that each partner completes its valence shell The atoms either share or transfer valence electrons.

Covalent Bonds The sharing of a pair of valence electrons by two atoms.

Molecule Two or more atoms held together by covalent bonds. Smallest particle of a compound.

Single Bond A pair of shared electrons

Structural Formula The notation which represents both atoms and bonding.

CO2 Molecular Formula H2O CH3COOH C12H22O11 NaCl C3H4O3 C6H8O6 Indicates simply the atoms and number of atoms in a molecule. C3H4O3 C12H22O11 CH3COOH NaCl CO2 C6H8O6

Electronegativity The attraction of a particular kind of atom for the electrons of a covalent bond. The more electronegative an atom, the more it pulls shared electrons toward itself.

Nonpolar Covalent Bond Two atoms of the same element which equally share electrons.

Polar Covalent Bond Bonds which vary in their polarity, depending on the relative electronegativity of the two atoms.

Ioni Ionic Bond The transfer of an electron

Ion A charged atom of an element. When an atom has more or less electrons than protons.

Cation A positively charged ion. An atom with fewer electrons than protons

Anions A negatively charged ion. Has more electrons that protons.