00.-

a.:J0L-

(9

~ C\J

Co Co:J :J0 0'- '-

(9 (9

metals

metalloids

non-metals

Transition metals(Groups 3 to 12)

~..~~~. Patterns and trendsin the periods and groups of theperiodic table

Under normal conditions,hydrogen and oxygen arecolourless, odourless gases.If you ignite a mixture of Period 1hydrogen and oxygen, it burns Period 2explosively, forming water. P . d 3Water's physical and chemical erlo

properties are different fromIthose of the two original

elements. What if you had neverheard of hydrogen, oxygen, oreven water? As long as you understood some basic facts about the organizationof the periodic table, you could predict that hydrogen and oxygen shouldcombine to form a new compound with the formula H2O. The periodic tablecould 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 appearin 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 2are called alkaline earth metals, elements in group 17 are called halogens, andelements 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 thatborder this line - metalloids - have some metallic and some non-metallic

properties. Table 1.3 summarizes characteristic physical properties of thesethree groups of elements.

Table 1.3 Properties of Metals, Metalloids, and Non-Metals

Chapter 1 Atoms, Elements, and Compounds. MHR 25

,.Patterns of Electron Arrangements in GroupsThe key to recognizing the group-related pattern of the periodic table is thenumber of electrons in the outer occupied energy level. For example, turn backto Figure 1.22B and examine the group 1 elements. Notice that atoms of eachelement 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 elementhas the same number of electrons in its outer occupied energy level. Group 2elements have two electrons in their outer occupied energy level. This patternholds for groups 1,2, and 13 through 18. Notice that all group 18 elementshave a filled outer energy level. Helium has two electrons, while neon andargon both have eight electrons in their outer energy level.

The outermost occupied energy level of an atom is called its valence energylevel. 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 aregular, 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. Thearrangement of valence electrons of the ions is identical to that of the nearestnoble gas on the periodic table. For example, the sodium ion, Na+, has thesame 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 closestnoble gas with a lower atomic number. Neon is the closest noble gas in theperiodic 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 ofelectrons 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- andF- have the same number of electrons as atoms of argon.

Na+CI-

Ions in ioniccompounds are arrayed inregular, repeating patternscalled crystal lattices.

Table 1.5 Selected Molecular Elements

The Formation of Molecular CompoundsWhen 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 groupsof atoms that share electrons. These groups of atoms are called molecules.

Compounds that are composed of molecules are called molecularcompounds. Carbon dioxide, like most other compounds containingonly non-metal elements, is a molecular compound. Because atoms of thesame element can form bonds, some elements also exist as molecules. Forexample, oxygen, 02, exists as diatomic (two-atom) molecules. Table 1.5shows examples of diatomic elements and of other elements that can existas polyatomic molecules.

Atoms in molecules are joined by covalent bonds. Covalent bonds are verydifferent from ionic bonds. BecaUse they are non-metals, atoms in a molecularcompound tend to gain electrons to complete their valence energy level. Neitheratom gives up electrons easily. When the atoms interact, no electron tramfer islikely to take place. Therefore, ions and a crystal lattice do not form. Instead,.molecules form when atoms share valence electrons in a covalent bond. Someexamples of molecules are shown in Figure 1.29 on the next page.

10053000330313

Chapter 1 Atoms, Elements, and Compounds. MHR 31

Section 1.3 SummaryIn this section, you have examined how the valence electrons of atoms affect theways that elements combine to form ionic and molecular compounds. In thenext chapter, you will learn some rules for naming compounds and writingtheir formulas. Then, you will investigate some properties of substances tosee 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 levelof an atom important?

(b) How is the number of valence electrons in the atom of an elementrelated 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 ofelectrons as an atom of argon. Use chemical symbols to representthese cations and anions.

~

Xa~

88

8Yb.8

..:Zc:..

5. Draw electron dot diagrams to predict how bonding occurs between

atoms of the following elements.

(a) magnesium and oxygen

(b) nitrogen and hydrogen

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 existsas 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 expect to 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 youcannot, go back to the sections indicated in parentheses after each part.

(a) Give three examples of ways in which early NorthAmerican Aboriginal peoples used chemicals toimprove the quality of their lives. (1.1)

(g) How is the arrangement of electrons in atomsrelated to the placement of elements in theperiodic table? (1.3)

(b) Give three examples of ways in which you usechemicals to improve the quality of your life. (1.1)

(h) Describe the special relationship betweenvalence energy levels of the noble gases andthe formation of chemical bonds. (1.3)

(c) Identify three features ofWHMIS requirementsthat improve safety for people working withhazardous chemicals. (1.1)

(i) Identify the type of element that fonns cationsand the type that fonns anions. Give an exampleof an atom in each group, and describe how eachatom fonns an ion. (1.3)(d) What conclusions did John Dalton make about

the structure of the atom? (1.2)(;) Use electron dot diagrams to illustrate the

fonnation of ionic and molecular compounds (1.3)(e) Summarize the experimental evidence thatsuggested that electrons are particles with bothcharge and mass. (1.2) (k) Describe how the arrangement of particles

differs in ionic and molecular substances. (1.3)(f) List the names and summarize the work of

scientists identified in this chapter as contributingto our understanding of atomic structure. (1.2)

Chapter 1 Atoms, Elements; and Compounds. MHR 37

Key Terms

stable octetvalence energy levelvalence electronelectron dot diagramioncationanion

isotopeenergy levelnucleonatomic numbermass number

periodgroup

ionic compoundbinary ionic compoundionic bondcrystal latticemoleculemolecular compoundcovalent bond

gas discharge tubecathode rayelectronnucleus

protonsubatomic particleneutron

10. Compare and contrast anions and cations. (1.3)Understanding Key ConceptsSection numbers are provided if you need to review. 11. Explain in tenns of electron behaviour what

happens when an alkali metal reacts vigorouslywith a halogen. (1.3)

1. What types of safety information are found ina Material Safety Data Sheet (MSDS)? (1.1)

12. Expl'ain how the number of valence electrons inatoms of aluminium is related to the position ofaluminium on the periodic table. (1.3)

2. Distinguish between ilie terms "evidence" and"inference" using examples from Ruilierford'sgold foil experiment. (1.2)

3. List two phenomena iliat Dalton's atomic ilieoryfailed to explain. Justify your answers. (1.2)

Developing Skills13. Draw an electron dot diagram to represent

an atom of each of the following elements:

(a) calcium (d) argon

(b) rubidium (e) arsenic

(c) iodine (f) boron

4. Describe how Rutherford's gold foil experimentshowed that there were flaws in the Thomsonmodel of the atom. (1.2)

5. What is an isotope? Define the teffi1 and explainwhy the existence of isotopes provides evidencefor the neutron. (1.2) 14. Identify the ion that an atom of each of the

following elements is likely to form in anionic compound.

(a) Na (d) Mg

(b) Br (e) Li

(c) 0 (f) N

6. According to Bohr's model of the atom, howmany electrons can exist in each of the firsttwo energy levels of an atom? (1.2)

7. What are valence electrons and why are theyimportant in the fonnation of compounds? (1.3)

15. Use electron dot diagrams to represent theformation of ionic compounds from thefollowing elements:

(a) potassium and bromine

(b) magnesium and fluorine

(c) beryllium and oxygen

(d) lithium and oxygen

8. How does the formation of an ionic bond differfrom 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 bromideion, Br-.

38 MHR . Unit 1 Energy and Matter in Chemical Change

16. Copy the following table into your notebookand complete it.

20. Unknown element Z exists as a diatomicmolecule. In the molecule, the two atomsof 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 expectto find this element? Explain your answer.

Thinking Critically21. If the Thomson model of the atom had been

accurate, how would the results of Rutherford'sgold foil experiment have been different? Explainyour answer in detail.

17. Use electron dot diagrams to represent thesharing of electrons in one molecule of eachof 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 sameelectron arrangement as the following ions:

(a) Li+

(b) Ca2+

(c) Br-

(d) N3-

(e) K+

(f) 52-

22. Use the Internet to find infonnation abouthydrogen chloride gas, HCI(g)'

(a) What are the hazards associated with usinghydrogen chloride gas?

. (b) What uses does hydrogen chloride gas have?

(c) What precautions would a chemist need totake when working with this substance?

(d) When hydrogen chloride gas is dissolved inwater, it is called hydrochloric acid, HCl(aq)'What hazards are associated with hydrochloricacid? Are they different from the hazardsassociated with the gas?

(e) What uses does hydrochloric acid have?

(f) What precautions would a chemist need totake when working with this substance?

Problem Solving! Applying19. Unknown element Q reacts with chlorine,

forming an ionic compound. In the ioniccompound, Q exists as ion Q+.

(a) Classify the element as a metal ora non-metal.

(b) Where in the periodic table would youexpect to find this element? Explainyour answer.

Chapter 1 Atoms, Elements, and Compounds. MHR 39