How Elements Form Compounds

1. How Elements Form Compounds Some atoms lose or gain electrons to become stable charged particles called ions When atoms loses electrons, they form positively charged ions called cations When atoms gain electrons, they form negatively charged ions called anions.

2. Sodium chloride is a relatively harmless compound because the sodium and chlorine atoms have stable ions . The compound formed is called an ionic compound because it is made up of positive and negative ions that have resulted from the transfer of from a metal to a nonmetal. The positive and negative ions are attracted to each other because they have opposite charges.

4. When ionic compounds are placed in water, the ions separate and are surrounded by water molecules. They are electrolytes. They are also conductive

5. Ionic Compounds There are over 100 elements in the PT Thousands of different compounds are formed when these elements combine. How can we name these compounds? How can we write formulas to represent them?

6. We have seen from past discussions that The PT and a knowledge of the electronic structure could be used to predict ionic charge of elements Ionic charges (or valences) of some elements in the PT

8. Naming Ionic Compounds The name of the metal first, followed by name of the of the nonmetal. The ending of the name of the nonmetal changes and ends with “ide”

9. Names and Ionic Charges of some nonmetals

10. Names and Formulas for Atoms with More Than One Ionic Charge Some metals are able to form more than one kind of ion. For example, the element copper forms two completely different compounds when it reacts with chlorine One of the compound is white: the other is yellow

11. Ionic charge on the copper in the white compound is 1+ . Its chemical formula is CuCl The ionic charge on the copper in the yellow compound is 2+, its formula is CuCl2

12. We have come across compounds such as • Calcium carbonate • Sodium bicarbonate • Calcium hydroxide, and copper sulfate • These names do not fit the naming so far What are these compounds?

13. Polyatomic ions • They are pure substances • Involve combinations of metals with polyatomic ions • Groups of atoms that tend to stay together and carry an overall ionic charge

14. When a compound containing this ion is dissolved in water, the positive metal ion and the nitrate ion separate from each other but the nitrate ion itself stays together as a unit surrounded by water molecules An example is The nitrate ion

15. Writing Formulas for Polyatomic Compounds The ionic charges of polyatomic ions makes it possible for them to form ionic compounds Common Polyatomic ions and Their Ionic charges

16. When a polyatomic ion such as nitrate or sulfate combines with other elements We follow the same rules for writing formulas

17. What is the formula for the ionc compound formed by sodium and a sulfate ion? Rule 1: write the symbols of the metal and of the polyatomic group Na SO4 Rule 2: write the ionic charges 1+ 2- Na SO4

18. Crisscross rule: crisscross the ionic charges 1+ 2- Na SO4 Note that polyatomic ions do not ”reduce” . Formula cannot be simplified Na1SO2 because SO4 is a group The formula is Na2SO4

19. Try this: what is the formula of lead(IV) carbonate?

20. There are many types of polyatomic ions, but one special group is known as the Oxyacids Oxyacids are compounds formed when hydrogen combines with polyatomic ions that contain oxygen. Ionic charge for hydrogen in these compounds is 1+

21. Molecular Compounds Imagine that you find an unlabelled container of solid white crystals in the kitchen. You are sure the crystals are either salt or sugar A simple taste test will tell you what the crystals are. But imagine you find the same crystals in the lab. A taste is too dangerous. What do you do? Dissolve the crystals in water and test for conductivity. If it conducts electricity, the compound must contain ions Salt or sodium chloride is an ionic compound

22. In ionic compounds, metals with 1, 2, or 3 electrons in their outer shell lose electrons to nonmetals, which often have 5, 6, or 7 electrons in their outer shell. If the solution does not conduct electricity, it must be a different kind of compound

23. Most compounds you encounter every day do not contain ions. Rather, they contain neutral groups of atoms called molecules. Sugar is a molecular compound. It is made up of molecules in which nonmetal atoms, such as hydrogen and oxygen share electrons to form stable arrangements.

24. Water and carbon dioxide are also molecular compounds, whether in a gas, a liquid, or a solid state, the particles in ionic and molecular compounds are different as shown Salt is an example of an ionic compound made up of ions of opposite charge. Ice (H2O) is an example of a molecular compound made up of neutral molecules

25. Hydrogen gas is a molecule formed when two hydrogen atoms combine. Each hydrogen atom has one electron. For the two hydrogen atoms to become stable, both must gain an electron. They do this by sharing a pair of electrons, one from each atom

26. The result is a covalent bond--- a shared pair of electrons held between two nonmetal atoms that holds the atoms together in a molecule. Many nonmetals form molecules in this way. For example chlorine gas is a molecule that consists of two chlorine atoms held together with a covalent bond. Each chlorine atom has 7 electrons in its outer orbit and needs to gain electron to be stable

27. Many nonmetallic elements exist as covalently bonded molecules. Table below lists elements that form diatomic molecules.

28. Molecular compounds are all around us a bottle of soda contains water molecules, sucrose, glucose, or fructose

29. Writing formulas for Molecular Compounds Formulas can be written using a method similar to the one used for ionic compounds. The number of electrons that metals and nonmetals transfer to become stable ions can be a clue to the formula of an ionic compound. Similarly, the number of electrons that a nonmetal needs to share to become stable is a clue to the number of covalent bonds it can form The combining capacity of a nonmetal is a measure of the number of covalent bonds that it will need to form a stable molecule

30. Table 1: Combining Capacities of Nonmetal Atoms

31. Carbon has four electrons in its outer(valence) orbit. If it lost 4 electrons, it would form a positive ion. If it gained 4 electrons, it would have the electron arrangement of neon and would form a negative ion It turns out that carbon cannot form either ion. Instead it “gains” 4 electrons by sharing: carbon has a combining capacity of 4. For example, when carbon shares one of its outer orbit electrons with each of four different hydrogen atoms, as shown in figure, the result is methane CH4, the major component of natural gas

32. As a result of forming covalent bonds through sharing electrons, the atoms end up with a stable arrangement in their orbit similar to that of a noble gas. You can use the combining capacity to write the formulas of molecular compound s without having to consider the electronic structure

33. How would you write the formula for a compound formed between Carbon and Sulfur? Rule 1: Write the symbols, with the left hand element from Table 1 with the combining capacities • 2 • C S Rule 2: Crisscross the combining capacities to produce subscripts • 2 • C S The formula is C2S4

34. Rule 3: Reduce the subscripts if possible The formula C2S4 is reduced to C1S2 Rule 4: Any “1” subscript is not needed. The correct formula is CS2

35. Naming Molecular Compounds Many molecular compounds have simple names. The compound H2S is called hydrogen sulfide, much as if it is ionic. Other molecular compounds have names that are very familiar to us even though they do not follow a system Common names have been used for centuries for water (H2O): ammonia (NH3), hydrogen peroxide (H2O2) and methane (CH4)

36. The names of molecular compounds often contain prefixes. These prefixes are used to count the number of atoms when the same two elements form different combinations. For example , the gas that you exhale is carbon dioxide (CO2) while the poisonous combination of carbon and oxygen that can be formed in automobiles is carbon monoxide The prefixes “di” and “mono” differentiate between the two molecules

37. Table 2: Prefixes in Molecular Compounds

38. References • Science power 11 Mcgraw-Hill Ryerson • Nelson Science 11