Ch. 2 The nature of molecules and the properties of water
2.1 All matter is composed of atoms
• Any substance that has mass and occupies space is defined as matter
• All matter is composed of extremely small particles called atoms
• The Bohr atom has electrons orbiting a central nucleus formed of two kinds of
subatomic particles: protons and neutrons
2.1.2 electrons determine the chemical behavior of atoms
• The key to the chemical behavior of an atom lies in the number and arrangement of its
electrons in their orbitals.
• Electron orbitals:
o The lowest energy level is level K. It’s occupied by a single s orbital referred to as
o The next highest energy level, L, is occupies by four orbitals: one s orbital (2s)
and three p orbitals (2p). each orbital holds two paired electrons and the L level
is populated by a total of eight electrons.
2.1.3 atoms contain discrete energy levels
• Because electrons are attracted to the positively charged nucleus, it takes work to keep
them in their orbitals.
• Energy is the ability to do work
• Electrons have potential energy related to their position. To oppose the attraction of the
nucleus and move the electron to a more distant orbital requires an input of energy,
which results in an electron with greater PE.
• Moving an electron closer to the nucleus has the oppose effect: energy is released,
usually as radiant energy and the electron ends up with less PE.
• When an atom absorbs energy, an electron moves to a higher energy level, farther from
the nucleus. When an electron falls to lower energy levels, closer to the nucleus, energy
• The discrete energy levels correspond to quanta which means a specific amount of
• Because the amount of energy an electron possesses is related to its distance from the
nucleus, electrons that are the same distance from the nucleus have the same energy,
even if they occupy different orbitals. Such electrons are said to occupy the same
• The different energy levels are denoted as K, L, M, N and so on.
• During some chemical reactions, electrons are transferred from one atom to another. In
such reactions, the loss of an electron is called oxidation, and the gain of an electron is
2.2 the elements in living systems have low atomic masses
• Only 12 elements are common in organisms:
o Carbon (C)
o Hydrogen (H) o Oxygen (O)
o Nitrogen (N)
o Phosphorus (P)
o Sulfur (S)
o Sodium (Na)
o Potassium (K)
o Calcium (Ca)
o Magnesium (Mg)
o Iron (Fe)
o Chlorine (Cl)
• 4 most common elements: carbon, hydrogen, oxygen and nitrogen
2.3 molecules are collections of atoms held together by chemical bonds
• a molecule is a group of atoms held together in a s table association by energy
• when a molecule contains atoms of more than one element it is called a compound
• the energy or force holding two atoms together in a molecule is called a chemical bond.
Chemical bonds determine the shapes of the large biological molecules.
• There are three principal kinds of chemical bonds:
o Ionic bonds: where the force is generated by the attraction of oppositely charged
o Covalent bonds: where the force results from the sharing of electrons
o Hydrogen bonds: where the force is generated by the attraction of opposite
partial electric charges
o Covalent bond is the strongest while hydrogen is weakest
2.3.2 covalent bonds build stable molecules
• The strength of a covalent bond depends on the number of shared electrons. This
double bonds, which satisfy the octet rule, are stronger than single bonds, in which pnly
one electron pair is shared. The strongest of the bonds is a tr