The Structure of Matter Chapter 4 The Structure of Matter

4.1 Compounds & Molecules A compound is different from the elements it contains. NaCl, Sodium chloride, is totally different than the metal, sodium, or the gas, chlorine.

Chemical bonds distinguish compounds from mixtures The forces that hold compounds together are called chemical bonds. During a chemical reaction chemical bonds are broken & atoms are re-arranged to form a new substance.

A compound has a specific formula The formula for water is H2O The formula for table sugar is C12H22O11. Compounds are always made of specific elements in specific numbers. A molecule of water is always H2O. [two hydrogen atoms bonded to one oxygen atom.]

Chemical structure illustrates the bonding within a compound The way the atoms are arranged in a compound determine many of its properties. [water is polar; oil is non-polar] Two terms are used to specify the positions of atoms relative to one another. 1. Bond length: the average distance between the nuclei of the 2 bonded atoms 2. Bond angle: the angle formed by two bonds to the same atom

Models of Compounds A model helps you to “visualize” something. 1. Ball & stick model: ball is the atom & the stick is the bond. 2. Structural formulas: use symbols & lines to depict the structure. 3. Space filling model: can’t “see” the bonds. [Fig. 4.4 P.111]

Structure Different structures give compounds different properties. Network structure: SiO2 is an ionic compound which has a strong, rigid structure. All the angles are the same (109.5o). This is why rocks containing SiO2 are hard & inflexible. It takes a lot of energy to break these bonds. (high melting point)

Some network bonds are ionic Some networks are made of bonded ions (charged particles). NaCl is made of a network of tightly packed positive Na+ and negative Cl- ions. The result is strong, rigid bonds that have high melting & boiling points. Ionic bonds occur between a positive metal ion & a negative nonmetal ion.

Some compounds are molecules Molecules are made when nonmetals combine with nonmetals. The bonds can be weak or strong. Molecules of gases have little attraction to each other, so they spread out & fill any size container.

Strength of attraction between molecules Attraction forces are greatest in solids. Attraction forces are less in liquids, and even less in gasses. (see data table 4.2 on p. 113) The higher melting & boiling points of water are due to the strong bonds within the water molecule. Each water molecule is attracted to the molecule next to it, due to the polar nature of the molecule. Teacher demo p.113

4.2 Ionic & Covalent Bonding Atoms bond when their valence electrons interact. When atoms gain or lose electrons they form ionic compounds. When atoms share electrons they form molecules.

4.2 Ionic & Covalent Bonding Bonds between atoms act like flexible springs, because they can bend & stretch without breaking

Ionic Bonds Ionic bonds occur between ions of opposite charge. Metals lose electrons to become positive ions, called cations. Nonmetals gain electrons to become negative ions, called anions.

Ratio’s NaCl means 1 atom of Na is bonded to 1 atom of Cl. The Na+ is attracted to the Cl- (opposites attract). Within a network there are millions of positive ions being attracted to the millions of negative ions. NaCl is the formula unit. The formula unit varies according to the compound. Ex.: CaF2

Ionic compounds conduct electricity when melted Electric current is the result of moving charges. When in solid form, ionic compounds do not conduct electricity because the ions are locked in position (crystal). However, in liquid form, the ions are free to move about and they do conduct electricity

Metallic Bonds Metals conduct electricity, are malleable & ductile. Electrons move freely between metal atoms. “sea of electrons” The atoms are packed so closely that the valence electrons overlap each other (sea of electrons). This frees up the electrons to move from atom to atom. Remember, electricity is the result of moving charges.

Covalent Bonds Covalent bonds form molecules (nonmetals bonding to nonmetals) Covalent compounds can be solid, liquid or gas. Most covalent substances have low melting points. Covalent bonds do not conduct electricity.

Covalent bonds share electrons Electrons are shared so that both atoms achieve the “magic number” of chemistry – 8, valence electrons. If the shared electrons are equally attracted to each nucleus, they produce a nonpolar covalent bond. The sharing is represented by a single line between the atoms. See p.119

Atoms may share more than 1 pair of electrons In a double covalent bond, 2 pairs of electrons are shared. In a triple covalent bond, 3 pairs electrons are shared. Triple bonds are stronger than double bonds. Triple & double bonds are shorter than single bonds.

Electrons are not always equally shared In some instances electrons are attracted more to one nucleus than another. This results in a polar molecule. Usually electrons are more attracted to elements on the upper right side of the Periodic Table. Water is a good example of a polar molecule.

Polyatomic ions Some compounds have both ionic and covalent bonds; such compounds contain polyatomic ions. Polyatomic ions are groups of covalently bonded atoms that have either lost or gained electrons. Polyatomic ions act just like normal ions. Parentheses are used to denote when more than one ion is bonded to another. The corresponding subscript indicates that everything inside the parentheses is effected by that subscript.

Naming the polyatomic ions -ite & -ate are suffixes that indicate the presence of oxygen. These endings do NOT tell you how many oxygen atoms are present. If there is –ate at the end of the name, there is 1 more oxygen atom than if there is an –ite at the end. The charge is the same for each ion Some have neither –ite or –ate, these don’t follow the rules.

4.3 Compound Names & Formulas Naming ionic compounds – cations & anions form compounds with strong bonds. Both elements’ names are represented in the name. Ex.: BaF2 is Barium Fluoride. In many cases, the name of the cation is simply the name of the element. When the anion is made of 1 element, the anion has a name similar to the name of the element but the ending is different. See fig. 4.5 on p. 124

Some cation names reflect the charge According to what we have learned, FeO and Fe2O3 should both be called iron oxide. Iron (Fe) is a transition metal. Transition metals can lose 1 or 2 or 3 valence electrons. To distinguish between the two a Roman numeral is added in parentheses. The Roman numeral tells the charge Iron (III) oxide tells us iron is Fe3+ (a cation that lost 3 electrons).

Determining the charge of transition metals Total charge of every compound must be zero. In Fe2O3 there are 3 oxygen atoms. Oxygen & all elements in column 6 have oxidation #’s of 2- 3 oxygen atoms times – 2 charge = -6 To total zero for the whole compound, iron must have a charge of +6. There are 2 iron atoms, so each must have a +3 charge Therefore we are working with iron (III)

Naming Covalent Bonds With two element covalent bonds, numerical prefixes tell us the number of atoms present in the compound. If there is only 1 atom of the 1st element listed, there is no prefix. For 2 atoms use the prefix di-, for 3 atoms tri- etc. The element furthest to the right on the Periodic Table is named 2nd & ends in -ide

Naming Covalent Compounds Mono- One Di- Two Tri- Three Tetra- Four Penta- Five Hexa- Six Hepta- Seven Octa- Eight Nona- Nine Deca- Ten

Naming Covalent Compounds BF3 Boron trifluoride N2O4 Dinitrogen tetroxide

A compound’s simplest formula is its empirical formula Empirical formula: the smallest whole number ratio of the atoms in a compound. We use the molecular formula to denote exactly how many atoms are in one molecule of a compound. Some compounds have the same empirical formula, but different molecular formulas. For example, formaldehyde, CH2O; acetic acid, C2H4O2; & glucose, C6H12O6 – all have the empirical formula of CH2O

4.4 Organic & Biochemical Compounds An organic compound is a covalently bonded compound made of molecules. Organic compounds contain carbon & usually hydrogen. When a compound has only carbon & hydrogen it is called a hydrocarbon. Methane, CH4, has 4 single bonds. Methane is formed when living matter decays. Carbon can also form double bonds when it shares 2 of its electrons. Triple bonds are also possible but carbon can never forms more than 4 bonds.

Alkanes An alkane is a hydrocarbon that has only single bonds – like we mentioned for CH4. Alkanes can have C-H bonds or C-C bonds. Alkanes names end in –ane.

Alkenes Alcohols Hydrocarbons that have at least 1 double bond, C=C Alkenes names end in –ene. Alcohols Alcohols are organic compounds that contain hydrogen, carbon & oxygen. Alcohols have hydroxyl or –OH groups

Polymers A polymer is a large organic molecule made of many smaller bonds. Small organic molecules bond to form long chains called polymers. Polymers are found in your body, wood, rubber & plastic. Rubber, wood, cotton are examples of natural polymers. Man-made polymers are usually either plastic or fibers. The structure determines its properties, just as for other compounds. Polymers are likened to a bowl of spaghetti, tangled but can slide over one another.

Biochemical Compounds A biochemical compound is any organic compound that is important to living things. Some examples include carbohydrates for energy & proteins that form all the organs in our body. Carbohydrate: any organic compound that is made of carbon, hydrogen & oxygen & that provides nutrients to the cells of living things.

Proteins A protein is a biological polymer made of bonded amino acids. An amino acid is any of 20 different naturally occurring organic molecules that combine to form proteins. Amino acids (there are 20) are made C, H, O, & N. How they combine determines the protein. Proteins are long chains of amino acids.

DNA is a polymer w/complex structure DNA determines entire genetic make up & is made of C, H, O, N & P. DNA has the shape of a twisted ladder known as a double helix. Every cell in your body has a copy of your genetic make up.