Chemical Bonding PRENTICE HALL PHYSICAL SCIENCE CHAPTER 6 SECTIONS 1 AND 2 Day 1 – Key Points When is an atom unlikely to react? What is one way in which elements can achieve stable electron configurations? How does the structure of an ionic compound affect its properties? Electron dot diagrams A model of an atom in which each dot represents a valence electron. The symbol in the center represents the nucleus and other electrons. Stable Electron Configurations Noble gases are the most stable elements. • Eight valence electrons (two electrons in the case of helium). • Elements react to achieve electron stability • Eight valence electrons IONIC BONDS ►Bonding through the transfer of electrons. Ions ►An atom that has a net positive or negative electric charge is called an ion. ►The atom losing electrons forms a positive ion (a cation) and is usually a metal. ►The atom gaining electrons forms a negative ion (an anion) and is usually a non-metallic element. Figure 2-3 Ionic Bonding Section 2-1 Sodium atom (Na) Chlorine atom (Cl) Sodium ion (Na+) Chloride ion (Cl-) Transfer of electron Protons +11 Protons +17 Protons +11 Protons +17 Electrons -11 Electrons -17 Electrons -10 Electrons -18 Charge 0 Charge 0 Charge +1 Charge -1 Figure 2-3 Ionic Bonding Section 2-1 Sodium atom (Na) Chlorine atom (Cl) Sodium ion (Na+) Chloride ion (Cl-) Transfer of electron Protons +11 Protons +17 Protons +11 Protons +17 Electrons -11 Electrons -17 Electrons -10 Electrons -18 Charge 0 Charge 0 Charge +1 Charge -1 Ionic Bonds 1. Using Tables Within a period, what happens to the atomic radius as the atomic number of the elements increases? Ionic Bonds 1. Using Tables Within a period, what happens to the atomic radius as the atomic number of the elements increases? Answer: Within a period, the atomic radius decreases as the atomic number increases. Ionic Bonds 2. Using Tables Within Groups 1A, 2A, 6A, and 7A, what happens to the atomic radius of elements as the atomic number increases? Ionic Bonds 2. Using Tables Within Groups 1A, 2A, 6A, and 7A, what happens to the atomic radius of elements as the atomic number increases? Answer: Within these groups, the atomic radius increases as the atomic number increases. Ionic Bonds 3. Inferring How does adding an occupied energy level affect the atomic radius? (Hint: Lithium is a Period 2 element and sodium is a Period 3 element.) Ionic Bonds 3. Inferring How does adding an occupied energy level affect the atomic radius? (Hint: Lithium is a Period 2 element and sodium is a Period 3 element.) Answer: When the next higher energy level is occupied, there is a significant increase in atomic radius. Ionic Bonds 4. Comparing and Contrasting Compare the atomic and ionic radii for potassium (K), and for bromine (Br). Ionic Bonds 4. Comparing and Contrasting Compare the atomic and ionic radii for potassium (K), and for bromine (Br). Answer: The ionic radius for potassium is much smaller than its atomic radius. The ionic radius for bromine is much larger than its atomic radius. Ionic Bonds 5. Making Generalizations What happens to the radius of an atom when the atom loses electrons? When the atom gains electrons? Ionic Bonds 5. Making Generalizations What happens to the radius of an atom when the atom loses electrons? When the atom gains electrons? Answer: With the loss of valence electrons, the radius decreases. With the addition of valence electrons, the radius increases. Ionic Bonds 6. Relating Cause and Effect Explain the difference in size between a metal atom and its cation. Ionic Bonds 6. Relating Cause and Effect Explain the difference in size between a metal atom and its cation. Answer: An energy level that was occupied is no longer occupied, and the size decreases. Ionization Energy Electrons must gain enough energy to escape their attraction to protons The amount of energy used to remove an electron is called ionization energy The lower the ionization energy the easier to remove an electron Chemical Formula a notation that shows what elements a compound contains and the ratio of the atoms or ions of those elements in the compound What is the chemical formula for magnesium chloride? Ionic Compounds – Chemical Formulas What is the chemical formula for magnesium chloride? Ionic Compounds – Chemical Formulas What is the chemical formula for magnesium chloride? Ionic Compounds What is the chemical formula for magnesium chloride? A magnesium atom cannot reach a stable electron configuration by reacting with just one chlorine atom. It must transfer electrons to two chlorine atoms. After the transfer, the charge on the magnesium ion is 2+ and its symbol is Mg2+. Ionic Compounds - Structure The structure and shape of a crystal are related: A. In a sodium chloride crystal, each ion is surrounded by six oppositely charged ions. B Sodium chloride crystals are shaped like cubes Structure depends on the ratio of ions and their relative sizes. Crystals are classified into groups based on the shape of their crystals. Properties of Ionic Compounds The properties of sodium chloride are typical of ionic compounds. • Sodium chloride has a high melting point (801°C). • Solid sodium chloride is a poor conductor of electric current. When melted, it is a good conductor of electric current. • Sodium chloride crystals shatter when struck with a hammer. Assessment Questions 1. When is an atom stable? a. when its electrons are evenly distributed through its energy levels b. when its lowest occupied energy level is filled with electrons c. when its highest unoccupied energy level is filled with electrons d. when its highest occupied energy level is filled with electrons Assessment Questions 1. When is an atom stable? a. when its electrons are evenly distributed through its energy levels b. when its lowest occupied energy level is filled with electrons c. when its highest unoccupied energy level is filled with electrons d. when its highest occupied energy level is filled with electrons ANS: D Assessment Questions 2. Which description applies to an element that has two valence electrons? a. reactive metal b. nonreactive metal c. reactive nonmetal d. nonreactive nonmetal Assessment Questions 2. Which description applies to an element that has two valence electrons? a. reactive metal b. nonreactive metal c. reactive nonmetal d. nonreactive nonmetal ANS: A Assessment Questions 3. How do sodium and chlorine both achieve stable electron configurations when they react? a. An electron is transferred from the sodium atom to the chlorine atom. b. An electron is transferred from the chlorine atom to the sodium atom. c. Both atoms gain one electron. d. Both atoms lose one electron. Assessment Questions 3. How do sodium and chlorine both achieve stable electron configurations when they react? a. An electron is transferred from the sodium atom to the chlorine atom. b. An electron is transferred from the chlorine atom to the sodium atom. c. Both atoms gain one electron. d. Both atoms lose one electron. ANS: A Assessment Questions 4. Why do ionic compounds tend to have high melting points? a. Ionic compounds contain more than one element, which causes a high melting point. b. Ionic compounds cannot absorb energy efficiently because they contain ions. c. An ionic compound contains metal atoms that raise its melting point. d. A strong electrical attraction means ions require a lot of energy to move apart. Assessment Questions 4. Why do ionic compounds tend to have high melting points? a. Ionic compounds contain more than one element, which causes a high melting point. b. Ionic compounds cannot absorb energy efficiently because they contain ions. c. An ionic compound contains metal atoms that raise its melting point. d. A strong electrical attraction means ions require a lot of energy to move apart. ANS: D Assessment Questions 1. The ratio of aluminum ions to chloride ions in aluminum chloride (AlC13) is 3:1. True False Assessment Questions 1. The ratio of aluminum ions to chloride ions in aluminum chloride (AlC13) is 3:1. True False ANS: F, 1:3 Day 2 – Key Questions How are atoms held together in a covalent bond? What happens when atoms don’t share electrons equally? When atoms form a polar covalent bond, the atom with the greater attraction for electrons has a partial negative charge. The other atom has a partial positive How do attractions between polar molecules compare to attractions between nonpolar molecules? COVALENT BONDS ►When 2 elements share one or more electrons ►Occurs between nonmetals ►Form molecules ►strong bond ►ex. CO2 Covalent Bonds There are several ways to show a covalent bond. Covalent Bonds Many nonmetal elements exist as diatomic molecules. Diatomic means “two atoms.” Multiple Covalent Bonds When two atoms share three pairs of electrons, the bond is called a triple bond. When two atoms share two pairs of electrons, the bond is called a double bond. N N Unequal Sharing of Electrons – Polar Covalent Bonds In a molecule of a compound, electrons may not be shared unequally. Except for noble gases, elements on the right of the periodic table tend to have a greater attraction for electrons than elements on the left. Elements at the top of a group tend to have a greater attraction for electrons than elements at the bottom of a group have. A covalent bond in which electrons are not shared equally is called a polar covalent bond. Unequal Sharing of Electrons Shared electrons in a hydrogen chloride molecule spend less time near the hydrogen atom than near the chlorine 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.