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The Octet Rule

The Octet Rule. Noble gases are unlike any other group of elements on the periodic table because of their extreme stability. Each noble gas has eight valence electrons, except for helium, which has two.

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The Octet Rule

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  1. The Octet Rule • Noble gases are unlike any other group of elements on the periodic table because of their extreme stability. • Each noble gas has eight valence electrons, except for helium, which has two. • The modern model of how atoms react to form compounds is based on the fact that the stability of a noble gas results from the arrangement of its valence electrons.

  2. The Octet Rule • The octet rule says that atoms can become stable by having eight electrons in their outer energy level, as shown in the noble gas, Neon, (or two electrons in the case of some of the smallest atoms).

  3. The Octet Rule • Elements become stable by achieving the same configuration of valence electrons as one of the noble gases, a noble gas configuration.

  4. Forming Chemical Bonds • When energy is added to or taken away from a system, one phase can change into another. • The force that holds two atoms together is called a chemical bond. • Chemical bonds form because of attractions between oppositely charged atoms, called ions, or between electrons and nuclei.

  5. Numbers in compounds • In the formula for an ionic compound, the symbol of the cation is written before that of the anion. • Subscripts, or small numbers written to the lower right of the chemical symbols, show the numbers of ions of each type present in a formula unit.

  6. Electrons Can Be Transferred • Sodium is in Group IA or 1A or 1, so it has one valence electron. • Na has one electron to lose to become like a noble gas in configuration. 1+ • Chlorine is in Group VII or 7A or 17and has seven valence electrons. • Cl has needs one electron to become like a noble gas in configuration.

  7. Electrons Can Be Transferred • How can the valence electrons of atoms rearrange to give each atom a stable configuration of valence electrons? • If the one valence electron of sodium is transferred to the chlorine atom, chlorine becomes stable with an octet of electrons.

  8. Electrons Can Be Transferred • Because the chlorine atom now has an extra electron, it has a negative charge. • Also, because sodium lost an electron, it now has an unbalanced proton in the nucleus and therefore has a positive charge.

  9. Electrons Can Be Transferred • Now that each atom has an octet of outer-level electrons, they are no longer neutral atoms; they are charged particles called ions. • An ionis an atom or group of combined atoms that has a charge because of the loss or gain of electrons.

  10. Electrons Can Be Transferred • A compound that is composed of ions is called an ionic compound. • Note that only the arrangement of electrons has changed. Nothing about the atom’s nucleus has changed. • Na 1+ and Cl 1- • Na1Cl1 • NaCl

  11. Formation of an Ionic Compound • Remember that objects with opposite charges attract each other. • The strong attractive force between ions of opposite charge is called anionic bond. • The force of the ionic bond holds ions together in an ionic compound.

  12. Formation of an Ionic Compound • Unprotected aluminum metal reacts with oxygen in air, forming the white coating you can observe on aluminum objects such as lawn furniture. • Explain the formation of an ionic compound from the elements aluminum and oxygen.

  13. Formation of an Ionic Compound 1. Analyze the Problem • You are given that aluminum and oxygen react to form an ionic compound. • Aluminum is a group 3A element with three valence electrons, and oxygen is a group 6A element with six valence electrons. • To acquire a noble gas configuration, each aluminum atom must lose three electrons and each oxygen atom must gain two electrons.

  14. Formation of an Ionic Compound 2. Solve for the Unknown • Remember that the number of electrons lost must equal the number of electrons gained. • The smallest number evenly divisible by the three electrons lost by aluminum and the two gained by oxygen is six. • Three oxygen atoms are needed to gain the six electrons lost by two aluminum atoms.

  15. Formation of an Ionic Compound 3. Evaluate the Answer • The overall charge on one unit of this compound is zero.

  16. Forming Chemical Bonds • A cation, or positive ion, is formed when an atom loses one or more electrons. • An anion, or negative ion, is formed when an atom gains one or more electrons.

  17. Determining Charges of Ions • To determine the electron configuration, refer to the periodic table. • Write the formula of the ion the atom is most likely to form. • Next, identify that ion as a cation or an anion.

  18. Properties of ionic compounds and lattice energy • In a solid ionic compound, the positive ions are surrounded by negative ions, and the negative ions by positive ions. • The resulting structure is called a crystal lattice and contains a regular, repeating, three-dimensional arrangement of ions.

  19. Properties of ionic compounds and lattice energy • The energy required to separate one mole of the ions of an ionic compound is called lattice energy, which is expressed as a negative quantity. • The greater (that is, the more negative) the lattice energy is, the stronger is the force of attraction between the ions. • Lattice energy tends to be greater for more-highly-charged ions and for small ions than for ions of lower charge or large size.

  20. Properties of ionic compounds and lattice energy • Based on the properties of the following “unknowns,” each is classified as either ionic or not ionic.

  21. Properties of ionic compounds and lattice energy • Between each of the following pairs of ionic compounds, only one would be expected to have the higher (more negative) lattice energy. 1. LiF or KBr (LiF would have the higher lattice energy) 2. NaCl or MgS (MgS would have the higher lattice energy)

  22. Names and Formulas for Ionic Compounds • The simplest ratio of the ions represented in an ionic compound is called a formula unit. • The overall charge of any formula unit is zero. • In order to write a correct formula unit, one must know the charge of each ion.

  23. The charges of monatomic ions, or ions containing only one atom, can often be determined by referring to the periodic table or table of common ions based on group number. Names and Formulas for Ionic Compounds

  24. Names and Formulas for Ionic Compounds • For example, ions of group 1A typically have a charge of 1+. • Those of group 2A have a charge of 2+. • Those of group 7A have a charge of 1–.

  25. Names and Formulas for Ionic Compounds • The charge of a monatomic ion is equal to its oxidation number. • The oxidation number, or oxidation state, of an ion in an ionic compound is numerically equal to the number of electrons that were transferred to or from an atom of the element in forming the compound.

  26. Naming ionic compounds • In naming ionic compounds, name the cation first, then the anion. • Monatomic cations use the element name. • Monatomic anions use the root of the element name plus the suffix -ide. • If an element can have more than one oxidation number, use a Roman numeral in parentheses after the element name, for example, iron(II) to indicate the Fe2+ ion. • For polyatomic ions, use the name of the ion.

  27. Naming ionic compounds • Certain polyatomic ions, called oxyanions, contain oxygen and another element. • If two different oxyanions can be formed by an element, the suffix -ate is used for the oxyanion containing more oxygen atoms, and the suffix -ite for the oxyanion containing fewer oxygens. • In the case of the oxyanions of the halogens, the following special rules are used.

  28. Naming ionic compounds • four oxygens, per + root + -ate (example: perchlorate, ClO4–) • three oxygens, root + -ate (example: chlorate, ClO3–)

  29. Naming ionic compounds • two oxygens, root + -ite (example: chlorite, ClO2–) • one oxygen, hypo- + root + -ite (example: hypochlorite, ClO–)

  30. Naming ionic compounds • The formulas below are followed by their ionic compound. 1. NaBrO3 (sodium bromate) 2. Mg(NO3)2 (magnesium nitrate) 3. NH4ClO4 (ammonium perchlorate) 4. Al(ClO)3 (aluminum hypochlorite)

  31. Additional Assessment Questions Question 2 What structure has positive ions surrounded by negative ions, and the negative ions surrounded by positive ions? Answer a crystal lattice

  32. Additional Assessment Questions Question 3 What is the correct formula for the ionic compound aluminum sulfate? Answer Al2(SO4)3

  33. Practice Problems Question 1 Determine the correct formula for the ionic compound composed of the following pairs of ions.

  34. Practice Problems Question 1a aluminum and carbonate Answer 1a Al2(CO3)3

  35. Practice Problems Question 1b magnesium and carbonate Answer 1b MgCO3

  36. Practice Problems Question 1c calcium and chlorate Answer 1c Ca(ClO3)2

  37. Practice Problems Question 2 Name the following compounds.

  38. Practice Problems Question 2a Co(OH)2 Answer 2a cobalt hydroxide

  39. Practice Problems Question 2b Ag2CrO4 Answer 2b silver chromate

  40. Practice Problems Question 2c Na3PO4 Answer 2c sodium phosphate

  41. Explaining the Properties of Ionic Compounds • Ionic compounds are composed of well-organized, tightly bound ions. • These ions form a strong, three-dimensional crystal structure. • Ionic compounds are crystalline solids at room temperature.

  42. Explaining the Properties of Ionic Compounds • Ionic compounds usually have to be heated to high temperatures in order to melt them because the attractions between ions of opposite charge are strong.

  43. Electrolytes • Another physical property of ionic compounds is their tendency to dissolve in water and conduct electricity in the liquid (melted) state. • Any compound that conducts electricity when melted or dissolved in water is an electrolyte. • In order to conduct electricity, ions must be free to move because they must take on or give up electrons.

  44. Basic Assessment Questions Question 1 What is the difference between sodium and chlorine? Answer Sodium is a metal that can be cut with a knife and has a silvery luster where it has been cut. Chlorine is a pale green, poisonous gas that kills living cells.

  45. Ionic Compounds • The submicroscopic structure of ionic compounds helps explain why they share certain macroscopic properties such as high melting points, brittleness, and the ability to conduct electricity when molten or when dissolved in water. • You have learned that ionic compounds are made up of oppositely charged ions held together strongly in well-organized units.

  46. Ionic Compounds • Because of their structure, they usually are hard solids at room temperature and are difficult to melt. Look at the structure of magnesium oxide.

  47. Ionic Compounds • When ionic compounds melt or dissolve in water, their three-dimensional structure breaks apart, and the ions are released from the structure.

  48. Ionic Compounds • These charged ions are now free to move and can conduct an electrical current.

  49. Binary Ionic Compounds • Formulas are part of the language that is used to communicate information about substances. • As a first step in studying this new language, you will learn how to name and write formulas for ionic compounds.

  50. Binary Ionic Compounds • Sodium chloride (NaCl) contains only sodium and chlorine, and potassium iodide (KI) contains only potassium and iodine. • Each is an example of a binary compound, which is a compound that contains only two elements.

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