Unit 6: The Structure of Matter

Chapter 5

Section 1: Compounds and Molecules Objectives • • • • Distinguish between compounds and mixtures.

Relate the chemical formula of a compound to the relative numbers of atoms or ions present in the compound.

Use models to visualize a compound’s chemical structure.

Describe how the chemical structure of a compound affects its properties.

Compounds vs. Mixtures

• • Compounds are formed when two or more elements chemically combine, forming a substance with very different properties than the individual elements that make it up.

Compounds are different from mixtures because the atoms are joined together by chemical bonds. The substances in a mixture are just placed together and keep their own properties.

Chemical Formulas

• • • A compound always has the same chemical formula. This means that water is always H always C 6 H 12 O 6 . That never changes. If the formula changes, the substance changes.

2 O, hydrochloric acid is always HCl, and glucose is What are some chemical formulas you know?

A chemical formula tells two important things about a compound: what types of atoms are in it and how many of each type there are.

Chemical Structure

• • The chemical structure of a compound is used to show the arrangement of the atoms in a compound. The properties of the compound are determined in large part by its chemical structure, which tells how far apart the atoms are and the angles between them.

• • • A compound with a network structure is a very strong compound. This means that the atoms are held together in a very strong, rigid repeating pattern. Quartz is an example. This explains why rocks that contain quartz are usually harder than rocks that do not.

Typically, the bonds that hold solids together are stronger than those that hold liquids together, which are stronger than those that hold gases together.

Section 2: Ionic and Covalent Bonding Objectives • • • • Explain why atoms sometimes join to form bonds.

Explain why some atoms transfer their valence electrons to form ionic bonds, which other atoms share valence electrons to form covalent bonds.

Differentiate between ionic, covalent, and metallic bonds.

Compare the properties of substances with different types of bonds.

How Do Bonds Form?

• • • When two atoms join, a bond forms between them. This simply means that there is an attraction between the valence electrons of the two atoms. As with the structure of a compound, the types of bonds that hold the compound together also affect its properties.

Generally speaking, atoms join to form bonds so that each atom has a stable (completely full or completely empty) outer energy level.

• For example, when two hydrogen atoms come near each other, the positive and negative charges in each pull one another closer. Soon, they are close enough that their electrons clouds overlap and their electrons are shared between them. The bond that is formed is strong enough that the only way to break it apart is to add energy.

Ionic Bonds

• • • • Of the three types of bonds that can be formed between atoms, ionic bonds are the strongest. Ionic bonds form between metals and nonmetals that have oppositely charged ions. Metals usually form positive ions (meaning they give up electrons) and nonmetals form negative ions (or take on extra electrons).

Ionic bonds are formed by the transfer of these electrons. If O has 6 valence electrons and Ca has 2, Ca (a metal) will give away its 2 electrons, and oxygen (a nonmetal) will gain them. Then, Ca will have an empty outer energy level and O will have a full outer energy level. Ionic compounds will also conduct electricity when dissolved in water because the charges are free to move around.

Metallic Bonds

• • • Metallic bonds are formed between two metals.

Metallic compounds are able to conduct electricity in their solid form. Metallic compounds are usually flexible and can bend without breaking because the electrons are free to move about from atom to atom.

Covalent Bonds

• • • A covalent bond is formed between two nonmetals. This type of bond occurs when instead of atoms giving away or taking on electrons, they simply share their valence electrons.

Essentially, this means that each atom takes turns being stable, as only one at a time can have a full outer energy level. The example of 2 hydrogen atoms from earlier shows a covalent bond.

Multiple Bonds

• • • • When atoms share only one pair of electrons, it is called a single bond and is shown as -.

When they share two pairs of electrons, it is called a double bond and is shown as =.

When they share three pairs of electrons, it is a triple bond and there’s no key to show what that looks like, so use your imagination.

The bigger the bond, the stronger it is because it takes more energy to break it.

Polyatomic Ions

• • • • Many common compounds are made up of groups of atoms that travel together and act like a single atom. These are called polyatomic ions. For example, SO 4 2 is the sulfate ion. Sulfate is a group of atoms that moves together and can bond with H to make sulfuric acid, or NH 4 to make ammonium sulfate. CO 3 is the carbonate ion, which bonds with Na to make sodium carbonate, or H to make baking soda.

OH is the hydroxide ion, which can bond with Na, Li, Ca, and numerous other alkali metals.

Section 3: Compound Names and Formulas Objectives

• • • • Name simple ionic and covalent compounds.

Predict the charge of a transition metal cation in an ionic compound.

Write chemical formulas for simple ionic compounds.

Distinguish a covalent compound’s empirical formula from its molecular formula.

Naming Ionic Compounds

• • Ionic compounds are formed by the strong attractions between cations (positive ions) and anions (negative ions), so their names include both ions.

Usually, the cation is named first in the compound. That means theatom that loses electrons (usually the metal) comes first. Also, the name of the cation doesn’t change. For example, a calcium compound always starts with “calcium”. Its name is not altered.

• • When naming the negative anion portion of the compound, it may be altered.

For example, the nonmetal anion will usually have an altered suffix. For example, most oxygen compounds will end in “oxide”. Fluorine compounds will end in “fluoride”.

Transition Metals

   Transition metals do not always follow these rules, so keep in mind that these are not all inclusive.

Since transition metals can form ions with different charges, the name of a transition metal compound will often have roman numerals in it to show which ion it contains.

To determine the charge of a transition metal cation, you must first know the charge of the nonmetal to which it is bonded. If it’s bonded with O, which has a charge of -2, the charge of the metal must be +2.

Writing Ionic Formulas

• • In order to write the formula for a compound, you must know the charges of the ions formed.

To do so, – – List the symbols for each ion Write the symbol for the cation first – Find the least common multiple of the ions’ charges – Write the chemical formula, indicating with subscripts how many of each ion are needed to make a neutral compound

Examples

• Write the formula for aluminum fluoride.

– Al (+3) – F (-1) – The least common multiple of 1 and 3 is __.

– Remember that the overall charge of a compound is 0.

– That means there must be 3 fluorine atoms to balance out the 3 positive aluminum charges.

– Therefore, the formula is AlF 3 .

• Write the formula for lithium oxide.

– Li (+1) – O (-2) – The least common multiple of 1 and 2 is __.

– Since the atom’s overall charge must be 0, there need to be 2 negative charges and 2 positive charges.

– Therefore, lithium oxide is Li 2 O.

• Write the formula for titanium (III) nitride.

– Ti (+3) – N (-3) – The least common multiple of 3 and 3 is ___.

– That means there must be __ positive charges and __ negative charges.

– Therefore, the formula for titanium (III) nitride is ________.

• Write the formula for cobalt (III) hydroxide.

– Co (+3) – OH (-1) – The least common multiple of 3 and 1 is ___.

– There must be ___ positive charges and ___ negative charges.

– Therefore, the formula for cobalt (III) hydroxide must be _____________.

Naming Covalent Compounds

• • • For two-element covalent compounds, numerical prefixed will be used to tell how many atoms of each element are present. If there is only one atom of the first elements present, it does not get a prefix.

You must know these prefixes!!!

Prefixes Used to Name Covalent Compounds

3 4 5 6 7

Number of Atoms

1 2 8 9 10

Prefix

Mono Di Tri Tetra Penta Hexa Hepta Octa Nona Deca-

Examples

• • • Boron trifluoride is a compound with one B atom and three F atoms.

Dinitrogen tetroxide has two N atoms and four O atoms.

In many cases, the “a” at the end of the prefix is dropped to make the name easier to say. This is the case when the word that follows it starts with a vowel.

Empirical Formulas

• • • A compound’s simplest formula is its empirical formula. The empirical formula for a compound is found by knowing the mass of each element in the compound and the smallest whole number ratio of atoms that are in the compound. The empirical formula for water is H always 2:1. 2 O. That tells us that the ratio of hydrogen to oxygen is

• If you know the mass of a substance and you know the masses of each of the elements in it, you can form a ratio that will tell you how many atoms of each element are in the compound, and therefore, you can determine its empirical formula.

• • • For example, a 142 g sample of compound X contains 62 g of P and 80 g of O. Since P has a mass of 30.97 g/mol and O has a mass of 16 g/mol, P makes up 2 mol of the compound X and O makes up 5 mol of compound X.

Therefore, the empirical formula of compound X must be P 2 O 5 .

• • • Molecular formulas are determined from empirical formulas. A compound’s molecular formula tells how many atoms are in one molecule of the compound. In some cases, a compound’s molecular formula is the same as its empirical formula, like water. However, the molecular formula for acetic acid is two times its empirical formula.

Section 4: Organic and Biochemical Compounds Objectives

• • • Describe how carbon atoms bond covalently to form organic compounds.

Identify the names and structures of groups of simple organic compounds and polymers.

Identify what makes up the polymers that are essential to life.

Organic Compounds

• • • An organic compound is a covalently bonded compound of of molecules that contain carbon, and almost always, hydrogen. They are called organic because they are found in almost all living organisms. Some other atoms, such as oxygen, nitrogen, sulfur, and phosphorus are also found in organic compounds.

There are so many organic compounds that there is an entire branch of chemistry devoted to it – organic chemistry.

• • • • Since carbon has 4 valence electrons, it can form four different bonds in a compound. The bond is almost always with a hydrogen atom. If the carbon compound has only single bonds, it is called an alkane.

If the compound has any double bonds, it’s called an alkene.

If the compound has any triple bonds, it’s called an alkyne.

Alkane name

Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane

Common Alkanes

Molecular formula

CH 4 C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12 C 6 H 14 C 7 H 16 C 8 H 18 C 9 H 20 C 10 H 22

Polymers

• • Polymers are large molecules that make up complex substances like rubber, wood, and plastic.

There are numerous polymers found in the body as well. Starch, protein, and DNA are all examples.

Biochemical Compounds

• • • • • • Compounds that are naturally occurring and vital to living organisms such as carbohydrates, proteins, lipids, and nucleic acids are called biochemical compounds. Carbohydrates are the energy source for your body.

Hair, fingernails and muscles are made of proteins.

Lipids insulate your organs and nerves.

Nucleic acids are substance that carry genetic information.

Each of these biochemical compounds are made up of polymers with complex structures.