CHAPTER 6 CHEMICAL BONDS STABLE ELECTRON CONFIGURATIONS When

CHAPTER 6: CHEMICAL BONDS

STABLE ELECTRON CONFIGURATIONS When is an atom unlikely to react? When the highest occupied energy level of an atom is filled with electrons, the atom is stable and not likely to react.

STABLE ELECTRON CONFIGURATIONS The chemical properties of an element depend on the number of valence electrons. An electron dot diagram is a model of an atom in which each dot represents a valence electron. The symbol in the center represents the nucleus and all other electrons in the atom.

STABLE ELECTRON CONFIGURATIONS

STABLE ELECTRON CONFIGURATIONS Noble gases are the most stable elements. The highest occupied energy level of a noble gas atom is completely filled. The noble gases have stable electron configurations with eight valence electrons (two valence electrons in the case of helium) Elements tend to react to achieve electron configurations similar to those of noble gases.

IONIC BONDS What is one way in which elements can achieve stable electron configurations? Some elements achieve stable electron configurations through the transfer of electrons between atoms.

IONIC BONDS Transfer of Electrons A sodium atom has one more electron than a neon atom. Removing one electron would give sodium a stable configuration.

IONIC BONDS Transfer of Electrons A chlorine atom has one fewer electron than an argon atom. Adding one electron would give chlorine a stable configuration.

IONIC BONDS When sodium reacts with chlorine, an electron is transferred from each sodium atom to a chorine atom. Each atom ends up with a more stable electron arrangement than it had before the transfer.

IONIC BONDS Formation of Ions When at atom gains or loses an electron, the number of protons is no longer equal to the number of electrons. The charge on the atom is not balanced, and the atom is not neutral. An atom that has a net positive or negative electric charge is called an ion. The charge on an ion is represented by a plus or minus sign.

IONIC BONDS A sodium ion has 11 protons and 10 electrons. The sodium ion has a charge of +1. The symbol for the ion is written Na+. An ion with a positive charge is a cation. A cation uses the element name, as in the sodium ion.

IONIC BONDS A chlorine ion has 17 protons and 18 electrons. This ion has a charge of -1. The symbol for the ion is written Cl-. An ion with a negative charge is called an anion. Anions like the Cl- ion are named by using the beginning of the element name and the ending –ide. The, Cl- is called a chloride ion.

IONIC BONDS Formation of Ionic Bonds A particle with a negative charge will attract a particle with a positive charge. A chemical bond is the force that holds atoms or ions together as a unit. An ionic bond is the force that holds cations and anions together. An ionic bond forms when electrons are transferred from one atom to another.

IONIC BONDS Ionization Energy Cations form when electrons gain enough energy to escape from atoms The energy allows electrons to overcome the attraction of the protons in the nucleus. The amount of energy required to remove an electron is called ionization energy. The lower the ionization energy, the easier it is to remove an electron from an atom.

IONIC BONDS Ionization Energy

IONIC COMPOUNDS How does the structure of an ionic compound affect its properties? Ionic compound particles are arranged in a lattice structure called crystals. The properties of an ionic compound can be explained by the strong attractions among ions within a crystal lattice.

IONIC COMPOUNDS Compounds that contain ionic bonds are ionic compounds, which can be represented by chemical formulas. A chemical formula is a notation that shows what elements a compound contains and the ratio of the atoms or ions of those elements in the compound. The chemical formula for sodium chloride, Na. Cl, indicates one sodium ion for each chloride ion in sodium chloride.

IONIC COMPOUNDS What is the chemical formula for magnesium chloride?

IONIC COMPOUNDS What is the chemical formula for magnesium chloride? For each magnesium atom, you need two chlorine atoms, one for each electron. The ratio is one magnesium atom to two chlorine atoms, or Mg. Cl 2.

IONIC COMPOUNDS Properties of Ionic Compounds Ionic compounds have high melting points � Na. Cl has a melting point of 801ºC Solid ionic compounds are poor conductors of electric current. When melted, they are good conductors of electric current. Ionic compound crystals shatter when struck with a hammer

ASSESSMENT QUESTIONS 1. When is an atom stable? a. b. c. d. When its electrons are evenly distributed through its energy levels When its lowest occupied energy level is filled with electrons When its highest unoccupied energy level is filled with electrons When its highest occupied energy level is filled with electrons Answer: D

ASSESSMENT QUESTIONS 2. Which description applies to an element that has two valence electrons. a. b. c. d. Reactive metal Nonreactive metal Reactive nonmetal Nonreactive nonmetal Answer: A

ASSESSMENT QUESTIONS 3. How do sodium and chlorine both achieve stable electron configurations when they react? a. b. c. d. An electron is transferred from the sodium atom to the chlorine atom. An electron is transferred from the chlorine atom to the sodium atom. Both atoms gain one electron. Both atoms lose one electron. Answer: A

ASSESSMENT QUESTIONS 4. The ratio of aluminum ions to chloride ions in aluminum chloride (Al. Cl 3) is 3: 1. True False Answer: False, 1: 3

COVALENT BONDS How are atoms held together in covalent bonds? A covalent bond is a chemical bond in which two atoms share a pair of valence electrons. The attractions between the shared electrons and the protons in each nucleus hold that atoms together in a covalent bond.

COVALENT BONDS A hydrogen atom has one electron. If it had two electrons it would have the same electron configuration as a helium atom. Two hydrogen atoms can achieve a stable electron configuration by sharing their electrons and forming a covalent bond. When two atoms share one pair of electrons, the bond is called a single bond.

COVALENT BONDS There are several ways to show a covalent bond. In the electron dot model, the bond is shown by a pair of dots in the space between the symbols. In the structural formula, the pair of dots is replaced by a line.

COVALENT BONDS Molecules of Elements Two hydrogen atoms bonded together form a unit called a molecule. A molecule is a neutral group of atoms that are joined together by one or more covalent bonds.

COVALENT BONDS The hydrogen molecule is neutral because it contains two protons (one from each atom) and two electrons (one from each atom). A chemical formula can be used to describe the molecules of an element as well as a compound. The element hydrogen has the chemical formula H 2.

COVALENT BONDS Many nonmetal elements exist as diatomic molecules. Diatomic means “two atoms. ” There are seven elements that are diatomic. In addition to the elements shown on the right, oxygen (O 2) and hydrogen (H 2) are the other diatomic elements.

COVALENT BONDS Multiple Covalent Bonds When two atoms two pairs of electrons, the bond is called a double bond. When two atoms share three pairs of electrons, the bond is called a triple bond. In a nitrogen molecule, N 2, the two nitrogen atoms are connected by a triple bond. In an oxygen molecule, O 2, the two oxygen atoms are connected by a double bond.

UNEQUAL SHARING OF ELECTRONS What happens when atoms don’t share electrons equally? When two atoms form a covalent bond, sometimes one of the atoms has a greater attraction for electrons than the other. If this is the case, the bond is a polar covalent bond, which means the electrons are shared unequally.

UNEQUAL SHARING OF ELECTRONS The atom with the greater attraction for electrons has a partial negative charge and the other atom has a partial positive charge. Except for noble gases, elements on the right of the periodic table tend to have a greater attraction for electrons than element on the left. Element at the top of a group tend to have a greater attraction for electrons than elements at the bottom of a group.

UNEQUAL SHARING OF ELECTRONS Polar Covalent Bonds In a molecule of an element, the atoms that form covalent bonds are from the same element, so they have the same ability to attract an electron. The electrons are shared equally. In a molecule of a compound, electrons may not be shared equally.

UNEQUAL SHARING OF ELECTRONS A covalent compound forms when hydrogen reacts with chlorine. A chlorine atom has a greater attraction for electrons than a hydrogen atom does. In a hydrogen chloride molecule, the shared electrons spend more time near the chlorine atom than near the hydrogen atom.

UNEQUAL SHARING OF ELECTRONS Polar and Nonpolar Molecules Can you assume that a molecule that contains a polar covalent bond is polar? When a molecule has only two atoms, it will be polar When molecules have more than two atoms, it depends on the molecule

UNEQUAL SHARING OF ELECTRONS In a carbon dioxide (CO 2) molecule, the polar bonds between the carbon atom and the oxygen atom cancel out because the molecule is linear. In a water (H 2 O) molecule, the polar bonds between the oxygen atom and the hydrogen atoms do not cancel out because the molecule is bent.

ASSESSMENT QUESTIONS 1. What attractions hold two atoms in a molecule together? a. b. c. d. Attraction between ions with opposite charges Attraction between the nuclei of the atoms and shared electrons Attraction between each nucleus and the electrons of the other atom Attraction between the molecule and other molecules Answer: B

ASSESSMENT QUESTIONS 2. What determines whether a molecule is polar? a. b. c. d. Type of atoms and shape of molecule Mass of atoms and number of valence electrons Type and mass of atoms Ionization energy and number of covalent bonds Answer: A

DESCRIBING IONIC COMPOUNDS What information do the name and formula of an ionic compound provide? The name of an ionic compound must distinguish the compound from other ionic compounds containing the same elements. Both of these compounds contain copper and chlorine, but they are not the same. There must be two different names to tell them apart.

DESCRIBING IONIC COMPOUNDS What information do the name and formula of an ionic compound provide? The formula of an ionic compound describes the ratio of the ions in the compound.

DESCRIBING IONIC COMPOUNDS This table lists eight common anions. The name of an anion is formed by adding –ide to the stem of the name of the nonmetal.

DESCRIBING IONIC COMPOUNDS Metals with Multiple Ions Many transition metals form more than on type of ion. We use Roman numerals to tell them apart. The Roman numeral is equal to the charge on each ion.

DESCRIBING IONIC COMPOUNDS Polyatomic Ions A covalently bonded group of atoms that has a positive or negative charge and acts as a unit is a polyatomic ion. Most simple polyatomic ions are anions.

DESCRIBING IONIC COMPOUNDS Writing Formulas for Ionic Compounds Use the name of an ionic compound to write its formula. The symbol of the cation is first, followed by the symbol of the anion. Use subscripts to show the ratio of the ions in the compound. Parentheses are used with the subscript for polyatomic ions. The formula for iron(III) hydroxide is Fe(OH)3.

DESCRIBING IONIC COMPOUNDS Because all compounds are neutral, the total charges on the cations and anions must add up to zero. Suppose an atom that gains two electrons, such as sulfur, reacts with an atom that loses one electron, such as sodium. � There must be two sodium ions (Na+) for each sulfide ion (S 2 -). � The formula for sodium sulfide is Na 2 S. The 2 - charge on one sulfide ion is balanced by the 1+ charge on two sodium ions.

DESCRIBING IONIC COMPOUNDS What is the formula for the ionic compound calcium chloride? 1. List the symbols and charges for the cation and anion. Ca has a charge of 2+ Cl has a charge of 1 - 2. Determine the ratio of ions in the compound. It takes two 1 - charges to balance the 2+ charge. There will be two chloride ions for each calcium ion.

DESCRIBING IONIC COMPOUNDS What is the formula for the ionic compound calcium chloride? 3. Write the formula for calcium chloride. Ca. Cl 2

DESCRIBING IONIC COMPOUNDS Write the formula for the compound calcium oxide. It takes one calcium ion with a charge of 2+ to balance out one oxygen ion with a charge of 2 -. The formula is Ca. O.

DESCRIBING IONIC COMPOUNDS Write the formula for the compound copper(I) sulfide. It takes two copper(I) ions, each with a charge of 1+ to balance out a sulfide ion with a charge of 2 -. The formula is Cu 2 S.

DESCRIBING IONIC COMPOUNDS Write the formula for the compound sodium sulfate. It takes two sodium ions, each with a charge of 1+ to cancel out one sulfate ion with a charge of 2 -. The formula is Na 2 SO 4.

DESCRIBING IONIC COMPOUNDS What is the name of the compound whose formula is Na. OH? In the formula, Na represents the sodium ion, and OH represents the hydroxide ion. The name of the compound is sodium hydroxide.

DESCRIBING MOLECULAR COMPOUNDS What information do the name and formula of a molecular compound provide? The name and formula of a molecular compound describes the type and number of atoms in a molecule of the compound.

DESCRIBING MOLECULAR COMPOUNDS Naming Molecular Compounds The general rule is that the element farthest to the left appears first in the name. If both elements are in the same group, the bottommost element appears first. The name of the second element is changed to end in –ide, as in carbon dioxide.

DESCRIBING MOLECULAR COMPOUNDS There may be more than one molecular compound that can exist with the same two elements. The Greek prefixes in the table are used to name molecular compounds.

DESCRIBING MOLECULAR COMPOUNDS Two compounds that contain nitrogen and oxygen have the formulas N 2 O 4 and NO 2. The name of N 2 O 4 is dinitrogen tetraoxide. The name of NO 2 is nitrogen dioxide. The prefix mono- is never used for the first element in the compound.

DESCRIBING MOLECULAR COMPOUNDS Writing Molecular Formulas To write the formula for a molecular compounds, write the symbols for the elements in the order the elements appear in the name. � The prefixes indication the number of atoms of each element in the molecule. � The prefixes appear as subscripts in the formulas. � If there is no prefix for an element name, there is only one atom of that element in the molecule.

DESCRIBING MOLECULAR COMPOUNDS What is the formula for diphosphorus tetrafluoride? Phosphorus has the symbol P. Fluorine has the symbol F. Di- indicates there are two phosphorus atoms Tetra- indicates there are four fluorine atoms The formula for the compound is P 2 F 4.

ASSESSMENT QUESTIONS 1. Which of these formulas describes a binary ionic compound? a. b. c. d. O 2 Mg. Cl 2 NO 2 Fe(OH)3 Answer: B

ASSESSMENT QUESTIONS 2. What is the correct name for CCl 4? a. b. c. d. Carbon(IV) chloride Carbon tetrachlorine Carbon tetrachloride Monocarbon tetrachloride Answer: C