chapter 2

• An element is a substance that cannot be broken down by chemical means into other substances.
• The hydrogen bonds that hold water molecules together give water a collection of important unique properties.
• Living things such as this newt and plant are mostly composed of six elements: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Sulfur.
• Atoms make up all matter.
• An atom is composed of three smaller particles: protons, neutrons, and electrons.
• Atomic weight is the average mass.
• Some covalent bonds, called double bonds, share four electrons between atoms, for example, oxygen gas (O2).
• Protons and neutrons are close together in the atomic nucleus, which is the center of the atom.
• Each bond in water has two electrons.
• Unpaired electrons form bonds with other atoms, making atoms most stable when their outer shells have no vacancies.
• Energy shells, or orbitals, contain the atom’s electrons.
• When atoms share electrons, as in a methane molecule, covalent bonds are formed.
• Protons are positively charged, neutrons are neutral, and electrons are negatively charged.
• An element’s atomic number indicates how many protons are in each atom of that element.
• An atom’s mass number is the total number of protons and neutrons in its nucleus.
• The number of neutrons may vary among atoms of the same element, and an isotope is any of these different forms of the element.
• The slight positive charge on the hydrogen atom of one water molecule attracts the slight negative charge on the oxygen of an neighboring water molecule, creating a hydrogen bond.
• Some atoms have such different electronegativities that one atom completely pulls an electron away from the other, forming an ionic bond.
• Each isotope of an element has a different mass, because the number of neutrons differs.
• The atom that loses an electron becomes positively charged, and the atom that gains an electron becomes negatively charged, attracting the atoms to each other and forming an ionic bond.
• The oxygen and hydrogen atoms of a water molecule have very different electronegativities, with oxygen attracting electrons more strongly than hydrogen.
• Electronegativity measures an atom’s ability to attract electrons.
• In an ionic bond, both atoms achieve full outer energy shells, and there are no longer vacancies in either atom.
• Methane is held together by nonpolar covalent bonds, because this type of bond forms when atoms have similar electronegativities.
• Since electrons spend more time near oxygen, the oxygen atom has a slightly negative charge.
• Since electrons spend less time near hydrogen, hydrogen atoms have a slight positive charge.
• The charge difference between oxygen and hydrogen in water gives the bonds polarity, and water is held together by polar covalent bonds.
• The periodic table arranges atoms by electronegativity.
• Electronegativity differences determine chemical bonds.
• In water (H2O), the oxygen atom pulls the electrons closer to it, because it attracts them more strongly than the hydrogen atoms can.
• Electronegativity can be used to predict bonding types.
• Ionic bonds form when one atom donates one or more electrons to another atom, with an electronegativity difference between atoms of very large magnitude (>1.7).
• Covalent bonds form when two atoms share pairs of electrons, with an electronegativity difference between atoms of small magnitude (<0.4).
• Hydrogen bonds form when an atom with a partial negative charge attracts a hydrogen atom with a partial positive charge.
• Waxes are a class of lipid that is particularly hydrophobic and is used in nature to form waterproof seals, such as the cuticle that stops leaves from drying out.
• Cohesion is the tendency of water molecules to stick to one another, which gives liquid water high surface tension.
• Water molecules also form hydrogen bonds with other molecules, a property called adhesion.
• Water is an excellent solvent, as it dissolves hydrophilic (“water-loving”) substances.
• Water only dissolves selected molecules, such as saturated fat (solid at room temperature).
• Water regulates temperature due to its ability to form hydrogen bonds, making it resist changes in temperature and cooling and heating up very slowly.