I Under normal conditions, hydrogen and oxygen are colourless, odourless gases. If you ignite a mixture of Period 1 hydrogen and oxygen, it burns Period 2 explosively, forming water. P. d 3 Water's physical and chemical erlo ~ C\J Co Co :J :J 0 '- 0 '- (9 (9 metals metalloids non-metals Transition metals (Groups 3 to 12) properties are different from those of the two original elements. What if you had never heard of hydrogen, oxygen, or even water? As long as you understood some basic facts about the organization of the periodic table, you could predict that hydrogen and oxygen should combine to form a new compound with the formula H2O. The periodic table could also help you predict some properties of this new compound. The periodic table, shown in Figure 1.22A, arranges elements into periods (horiwntal rows) and groups (vertical column). Across periods, elements appear in order of their atomic number. Down groups, elements have similar properties. Groups of elements are also called "families." Several of the groups have names. For example, elements in group 1 are called alkali metals, elements in group 2 are called alkaline earth metals, elements in group 17 are called halogens, and elements in group 18 are called noble gases or inert gases. Figure 1.22A also shows the three major sections of the periodic table. The dark "staircase" line separates metals from non-metals. Elements that border this line - metalloids - have some metallic and some non-metallic properties. Table 1.3 summarizes characteristic physical properties of these three groups of elements. Table 1.3 Properties of Metals, Metalloids, and Non-Metals 00.- a. :J 0 L- (9 ~..~~~. Patterns and trends in the periods and groups of the periodic table Chapter 1 Atoms, Elements, and Compounds. MHR 25
,. Patterns of Electron Arrangements in Groups The key to recognizing the group-related pattern of the periodic table is the number of electrons in the outer occupied energy level. For example, turn back to Figure 1.22B and examine the group 1 elements. Notice that atoms of each element in group 1 have only one electron in their outer occupied energy level. Now examine group 2. As with group 1, you will notice that each element has the same number of electrons in its outer occupied energy level. Group 2 elements have two electrons in their outer occupied energy level. This pattern holds for groups 1,2, and 13 through 18. Notice that all group 18 elements have a filled outer energy level. Helium has two electrons, while neon and argon both have eight electrons in their outer energy level. The outermost occupied energy level of an atom is called its valence energy level. The electrons in the valence energy level are called valence electrons. Table 1.4 summarizes some of the properties of groups 1,2,17, and 18. Try the Practice Problems below to apply what you have learned. Chapter 1 Atoms, Elements, and Compounds. MHR 27
The ions of an ionic compound such as sodium chloride are arranged in a regular, repeating pattern. Scientists call this repeating pattern a crystal lattice. The ions are held rigidly in place by strong ionic bonds. Figure 1.28 shows. how chloride and sodium ions are arranged in a crystal of sodium chloride. Each ion in sodium chloride has a completed valence energy level. The arrangement of valence electrons of the ions is identical to that of the nearest noble gas on the periodic table. For example, the sodium ion, Na+, has the same electron configuration as a neon atom. In general, in ionic compounds, a metal cation has the same number of electrons as an atom of the closest noble gas with a lower atomic number. Neon is the closest noble gas in the periodic table to sodium, magnesium, and aluminium. The cations N a +, Mg2 +, and Al3+ all have the same number of electrons as atoms of neon. Similarly, CI- has the same electron configuration as an atom of argon. In general, in ionic compounds, a non-metal anion has the same number of electrons as an atom of the closest noble gas with a higher atomic number. Argon is the closest noble gas to oxygen and fluorine. The anions 02- and F- have the same number of electrons as atoms of argon. CI- Na+ Ions in ionic compounds are arrayed in regular, repeating patterns called crystal lattices. The Formation of Molecular Compounds When coal burns in oxygen, atoms of two non-metals - carbon and oxygen - form bonds. The carbon dioxide gas that results shows no indication of being made of ions. Neither does it appear to be one large array of particles. Instead, samples of carbon dioxide behave as though they are made of separate groups of atoms that share electrons. These groups of atoms are called molecules. Compounds that are composed of molecules are called molecular compounds. Carbon dioxide, like most other compounds containing only non-metal elements, is a molecular compound. Because atoms of the same element can form bonds, some elements also exist as molecules. For example, oxygen, 02, exists as diatomic (two-atom) molecules. Table 1.5 shows examples of diatomic elements and of other elements that can exist as polyatomic molecules. Atoms in molecules are joined by covalent bonds. Covalent bonds are very different from ionic bonds. BecaUse they are non-metals, atoms in a molecular compound tend to gain electrons to complete their valence energy level. Neither atom gives up electrons easily. When the atoms interact, no electron tramfer is likely to take place. Therefore, ions and a crystal lattice do not form. Instead,. molecules form when atoms share valence electrons in a covalent bond. Some examples of molecules are shown in Figure 1.29 on the next page. Table 1.5 Selected Molecular Elements 10053000330313 Chapter 1 Atoms, Elements, and Compounds. MHR 31
Section 1.3 Summary In this section, you have examined how the valence electrons of atoms affect the ways that elements combine to form ionic and molecular compounds. In the next chapter, you will learn some rules for naming compounds and writing their formulas. Then, you will investigate some properties of substances to see if you can use the theory you have learned to explain what you observe. Check Your Understanding 1. Find rubidium (Rb) in d1e periodic table, and answer d1e following questions. (a) How many electrons are in an atom of rubidium? (b) How many occupied energy levels does a rubidium atom have? (c) How many electrons are in d1e valence energy level of a rubidium atom? (d) Write d1e symbol for d1e ion d1at you would expect rubidium to form in an ionic compound. 2. (a) Why is knowing the number of electrons in the valence energy level of an atom important? (b) How is the number of valence electrons in the atom of an element related to the position of the element in the periodic table? 3. What is the total charge on each of the following ions or sets of ions? (a) one fluoride ion (b) one sulfide anion (c) two nitride anions 4. Think of two cations and two anions that have the same number of electrons as an atom of argon. Use chemical symbols to represent these cations and anions. 5. Draw electron dot diagrams to predict how bonding occurs between atoms of the following elements. (a) magnesium and oxygen (b) nitrogen and hydrogen ~ Xa~ 88 8Yb. 8.. :Zc:.. 6. Apply Examine the electron dot diagrams on the left. They represent atoms of three hypothetical elements. (a) Which element or elements are non-metals? How do you know? (b) Predict which element will not combine with oxygen. Give reasons to explain your answer. 7. Thinking Critically In an ionic compound, an unknown element exists as ions with a 2- charge. (a) Classify this element as a metal or a non-metal. (b) Where, specifically, in the periodic table would you expecto find this element? Why? 36 MHR. Unit 1 Energy and Matter in Chemical Change
Now that you have completed this chapter, try to do the following. If you cannot, go back to the sections indicated in parentheses after each part. (a) Give three examples of ways in which early North American Aboriginal peoples used chemicals to improve the quality of their lives. (1.1) (b) Give three examples of ways in which you use chemicals to improve the quality of your life. (1.1) (c) Identify three features ofwhmis requirements that improve safety for people working with hazardous chemicals. (1.1) (d) What conclusions did John Dalton make about the structure of the atom? (1.2) (e) Summarize the experimental evidence that suggested that electrons are particles with both charge and mass. (1.2) (f) List the names and summarize the work of scientists identified in this chapter as contributing to our understanding of atomic structure. (1.2) (g) How is the arrangement of electrons in atoms related to the placement of elements in the periodic table? (1.3) (h) Describe the special relationship between valence energy levels of the noble gases and the formation of chemical bonds. (1.3) (i) Identify the type of element that fonns cations and the type that fonns anions. Give an example of an atom in each group, and describe how each atom fonns an ion. (1.3) (;) Use electron dot diagrams to illustrate the fonnation of ionic and molecular compounds (1.3) (k) Describe how the arrangement of particles differs in ionic and molecular substances. (1.3) Chapter 1 Atoms, Elements; and Compounds. MHR 37
Key Terms gas discharge tube cathode ray electron nucleus proton subatomic particle neutron isotope energy level nucleon atomic number mass number period group stable octet valenc energy level valenc electron electron dot diagram ion cation anion ionic compound binary ionic compound ionic bond crystal lattice molecule molecular compound covalent bond Understanding Key Concepts Section numbers are provided if you need to review. 1. What types of safety information are found in a Material Safety Data Sheet (MSDS)? (1.1) 2. Distinguish between ilie terms "evidence" and "inference" using examples from Ruilierford's gold foil experiment. (1.2) 3. List two phenomena iliat Dalton's atomic ilieory failed to explain. Justify your answers. (1.2) 4. Describe how Rutherford's gold foil experiment showed that there were flaws in the Thomson model of the atom. (1.2) 5. What is an isotope? Define the teffi1 and explain why the existence of isotopes provides evidence for the neutron. (1.2) 6. According to Bohr's model of the atom, how many electrons can exist in each of the first two energy levels of an atom? (1.2) 7. What are valence electrons and why are they important in the fonnation of compounds? (1.3) 8. How does the formation of an ionic bond differ from the formation of a covalent bond? (1.3) 9. Use diagrams in your answers to these que~tions. (1.3) (a) How are electron dot diagrams useful? (b) Draw an electron dot diagram of selenium, Se. (c) Draw an electron dot diagram of a bromide ion, Br-. 10. Compare and contrast anions and cations. (1.3) 11. Explain in tenns of electron behaviour what happens when an alkali metal reacts vigorously with a halogen. (1.3) 12. Expl'ain how the number of valence electrons in atoms of aluminium is related to the position of aluminium on the periodic table. (1.3) Developing Skills 13. Draw an electron dot diagram to represent an atom of each of the following elements: (a) calcium (b) rubidium (c) iodine (d) argon (e) arsenic (f) boron 14. Identify the ion that an atom of each of the following elements is likely to form in an ionic compound. (a) Na (b) Br (c) 0 (d) Mg (e) Li (f) N 15. Use electron dot diagrams to represent the formation of ionic compounds from the following elements: (a) potassium and bromine (b) magnesium and fluorine (c) beryllium and oxygen (d) lithium and oxygen 38 MHR. Unit 1 Energy and Matter in Chemical Change
16. Copy the following table into your notebook and complete it. 20. Unknown element Z exists as a diatomic molecule. In the molecule, the two atoms of Z are connected by a single covalent bond. (a) Classify the elemen,t as a metal or a non-metal. (b) Where in the periodic table would you expect to find this element? Explain your answer. 17. Use electron dot diagrams to represent the sharing of electrons in one molecule of each of the following substances: (a) water, H2O (b) oxygen, 02 (c) nitrogen, N2 (d) carbon dioxide, CO2 (e) methane, C~ (f) hydrogen bromide, HBr 18. Identify the noble gas that has the same electron arrangement as the following ions: (a) Li+ (b) Ca2+ (c) Br- (d) N3- (e) K+ (f) 52- Thinking Critically 21. If the Thomson model of the atom had been accurate, how would the results of Rutherford's gold foil experiment have been different? Explain your answer in detail. 22. Use the Internet to find infonnation about hydrogen chloride gas, HCI(g)' (a) What are the hazards associated with using hydrogen chloride gas?. (b) What uses does hydrogen chloride gas have? (c) What precautions would a chemist need to take when working with this substance? (d) When hydrogen chloride gas is dissolved in water, it is called hydrochloric acid, HCl(aq)' What hazards are associated with hydrochloric acid? Are they different from the hazards associated with the gas? (e) What uses does hydrochloric acid have? (f) What precautions would a chemist need to take when working with this substance? Problem Solving! Applying 19. Unknown element Q reacts with chlorine, forming an ionic compound. In the ionic compound, Q exists as ion Q+. (a) Classify the element as a metal or a non-metal. (b) Where in the periodic table would you expect to find this element? Explain your answer. Chapter 1 Atoms, Elements, and Compounds. MHR 39