Electron Configurations. To answer this question, you will explore. Exploring the Topic. Electrons are arranged into shells numbered

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1 LESSON 24 Shell Game Electron Configurations Think About t Recall that the chemistry of the elements is closely related to the number of valence electrons in their atoms. The valence electrons are found in the outermost electron shell of an atom. What does the periodic table indicate about the arrangements To answer this question, you will explore o Subs hells in Atoms f) Electron Configurations e Connecting the Periodic Table to Electron Arrangements o Noble Gas Shorthand Exploring the Topic of electrons? o Subshells in Atoms Electrons are arranged into shells numbered 11 =, 2, 3, and so on. The number of electron shells in an atom is the same as the number of the period where the element is located on the periodic table. Each shell has a maximum number of electrons. For instance, the 11 = 2 shell cannot have more than 8 electrons. Scientific evidence has led chemists to propose that electron shells are further divided into electron subshells. magine magnifying the basic atomic model and finding that each shell is composed of subshells. Notice that the number of subshells that a shell has is equal to n. n = 4 maximum: 32 e- n = 3 maximum: 18 e- [" =, maximum 8 e- n = maximum; 2 e- ;:), r \ t \\\\\.1) J; \ \""'-':/?./ \ ",: o /,( -_/ Each electron '-- shell can have a maximum number of electrons. Electron shells are further divided into subshells. n=4 Lesson 24 Shell Game 117 r;::::.. ')(\1n LCCH ('11,.,.i,...ldYn Dl"'occ

2 ) -O'Q Electron Shells and Subshells 0c;o "" '< n () :3"" c ([) 3 ;:)" (7)" C rt c--< 3 -::-1 ""1J ([)..., Cl (J) n :3"". Number of electrons Number of electrons Shell in the shell.1' Subshell in the subshell n = 1 2 s. o ::J Q Ul Ul a o 3 S: o Ul CD Y1 ;:;. n=2 8 2s 2p 3s Ul n=3 18 3p,, n=4 32 3d 4s 4p 4d 4f

3 The s, p, d, and f Subshells The subshells have special names. They are called the s, p, d, and f subshells. Just like the basic shells, each subshell has a maximum capacity of electrons. The s subshells can have a maximum of 2 electrons, p subshells can have a maximum of 6 electrons, and d subshells can have a maximum of 10 electrons. Finally, f subshells can have a maximum of 14 electrons. Na ssubshell 25 subshell 2p subshell 35 subshell 3p subshell 3d subshell Key 0 s subshells 0 p subshells 0 d subshells 0 f subshells Sodium, Na, has electrons, located in the subshells as shown. Notice that the name of each subshell is labeled using both the basic shell number and the subshell letter (s, 2s, 2p, and so on). Example Electron Arrangements Use the illustration these questions: of the subshells in a sodium atom above to help you answer a. How many total electrons are there in a sodium, Na, atom? Which shells are they in? b. How many valence electrons does sodium have? Which subshell are they in? c. How many electrons are there in the 3s subshell of sodium? n the 3p subshell? Solution The atomic number of sodium is 11. a. There are a total of 11 electrons in a neutral sodium atom. The electrons are in shells, n =, n = 2, n = 3. b. Sodium has one valence electron, located in the 3s subshell. c. There is one electron in the 3s subshell of sodium, and none in the 3p subshell. 'e Electron Configurations t can be time consuming to draw subshell models of the atoms to show the arrangements of the electrons, especially for atoms. with large atomic numbers. Chemists have developed a shorthand notation called an electron configuration to keep track of the electrons in an atom. The electron configurations for the first ten elements are shown here. H s1 C s22s22p2 He s2 N s 2 2s 2 2p3 Li s22s1 0 s22s22p4 Be s22s2 F s 2 2s 2 2p5 B ls22s22pl Ne ls22s 2 2p6 Nitrogen total has a of 7 electrons. 118 Unit Alchemy Section V Moving Electrons

4 Each subshell is written using the shell number and the subshellietter. n addition, the number of electrons in each subshell is indicated with a superscript number. Notice that the superscript numbers add up to the total number of electrons for that atom. The sequence in which electrons fill up the subshells is s, 2s, 2p, 3s, 3p. After the element argon the pattern changes slightly. Li: Electrons in each subshell Subshells 2s 1 Topic: Electron Configuration Visit: Web code: KEY-124 Example 2 Electron Configuration of Sulfur Write the electron configuration for sulfur, S. Solution Sulfur is located in the third row in Group 6A. The atomic number of sulfur is 16, so there are 16 electrons that need to be distributed in subshells beginning with the s subshell. The electron configuration of sulfur is s22s22p 6 3s 2 3p4. e Connecting the Periodic Table to Electron Arrangements An outline of the periodic Periodic Table-subshells table appears here with color-coding to show the Key subshell for the outermost s-block electron of each elemen t. p-block For example, any element d-block located in the green area will have its outermost electron (s) in a p subshell. f-block As you proceed across the periodic table from one element to the next, one additional proton and one additional electron are added, along with one or more neutrons. Each additional electron goes into a specific subshell. f an atom is located in the orange areas of the table, the last electron is placed into an s subshell. f an atom is located in the blue area of the table, the last electron is placed into a d subshell, And so on. The elements in each block have related properties. The elements in the s-block are reactive metals. The elements in the d-block tend to form colorful compounds that are used as pigments. The elements in the p-block tend to form colorless compounds. Decoding the Table n order to write out an electron configuration for a specific element, you can simply "read" from the periodic table, moving across from left to right and then Lesson 24 Shell Game 119

5 mportant to Know down to the next row. For example, the sequence of subshells for argon, Ar, is s, 2s, 2p, 3s, 3p. The electron configuration for argon is s22s22p 6 3s 2 3pfi. Potassium. K L &' B- -T- - fi- 2p t: : Periodic Table-subshells r- Īs 35r- 3p 4sJ--3d' 4p j j \ S5t- 4d Sp j j 65!- sal r j, 6p, 6ill,;, -L 6 rw T r -T j j f TT 7 Sf T j f i, r V Argon.Ar Arsenic.As Everything runs smoothly until you reach the fourth row of the periodic table. After argon, you might expect the next electron to be in the 3d subshell. However, this does not happen. The next element is potassium, K. Like the other elements in Group la, potassium has one electron in the s subshell. Thus, you place an electron in 4s before 3d. The electron configuration for potassium is ls22s22p fi 3s 2 3p64sl.The electron configuration for arsenic is s22s22p63s23p64s23d104p3. You may have noticed that you only have to look at the ending of each electron configuration in order to figure out the identity of the element associated with it. The ending provides you with the exact spot on the periodic table where you can find the element. The s subshells fill with electrons before the d subshells from the previous shell. For example, the 4s subshell fills before the 3d subshell, the 5s subshell fills before the 4d subshell, and so on. Example 3 Electron Configuration of Cobalt Write the electron configuration for cobalt, Co. Solution Locate cobalt on the periodic table. t is element number 27 and is located in fhe fourth period of the periodic table. "_," Simply trace your finger across the periodic table of subshells, writing the subshells as you go. When you get to cobalt, stop writing. Every subshell up to the 4s subshell is completely filled. n addition, cobalt has seven electrons in the 3d subshell. The answer is s22s22p63s23p64s23d7. You can check that you have the correct electron configuration by adding the superscript numbers to make sure there are 27 electrons. 120 Unit Alchemy Section V Moving Electrons

6 o Noble Gas Shorthand Depending on the element, the electron configuration can be lengthy to write. Plus, each element just repeats the electron configuration of the previous element, but adds one more electron. Rather than repeat the same thing every time, chemists have devised a quicker way to write out electron configurations. They use the noble gas at the end of each period as a placeholder to symbolize all of the filled subshells before that place on the table. Using this "shorthand" method, the electron configuration of cobalt is [ArJ4s 2 3d 7. Shorthand notation allows you to make some interesting comparisons. Notice that the noble gas shorthand notation emphasizes the valence electrons. Using this method, it is easy to see that each element in Group 2A has two valence electrons, both located in an s subshell. Group 2A Ele ments Element Symbol Elec tron configuration beryllium Be [He]2s 2 magnesium Mg [Ne]3s 2 calcium ea [Ar]4s 2 strontium Sr [Kr]Ss2 barium Ba [Xe]6s 2 radium Ra [Rn]7s2 i Example 4 Electron Configuration of Selenium Find the element Selenium, Se, element 34, on the periodic table. a. What is the electron configuration of selenium? b. Write the electron configuration using noble gas shorthand. c. n what subshells are selenium's valence electrons? Solution a. Selenium is in the p-blcck, in Period 4. The electron configuration of selenium is s22s22p63s23p64s23dj04p4. b. The noble gas that comes before selenium is argon, Ar. Thus, the noble gas shorthand for this configuration is ta]4s23d104p4. c. Selenium's valence electrons are in subshells 4s and 4p. Key Term electron configuration Lesson Summary What does the periodic table indicate about the arrangements of electrons? Electrons in atoms are arranged into basic shells labeled n =, 2, 3, and so on. These shells are divided into subshells. The number of subshells in each shell is equal to n. Lesson 24 Shell Game 121 tr:\')(l1n VO\l r'jrri".,lllrn Dr-acC"

7 The subshells are referred to as s, p, d, and f subshells. The s, p, d, and f subshells can have a maximum of2, 6, 10, and 14 electrons, respectively. Chemists use electron configurations to specify the arrangements of electrons in subshells. The periodic table provides the information needed to write electron configurations. r EXERCSES 1 _ Reading Questions. What are electron subshells? 2. What is an electron configuration? 3. How is the arrangement of electrons in an atom related to the location of the atom on the periodic table? Reason and Apply 4. How many subshells are in each shell: n = 1, n = 2, n = 3, n = 4? 5. What is the total number of subshells for elements in Period 5 of the periodic table? 6. Draw a subshell model for each of the following elements, putting the electrons in their appropriate places. a. sodium, Na b. neon, Ne c. carbon, C d. vanadium, V 7. What is the outermost subshell for bromine, Br? 8. Name an element with electrons in the f subshell. 9. Consider the element with the atomic number 13. a. What is the electron configuration for the element with atomic number 13? b. How many valence electrons does element 13 have? How do you know? c. How many core electrons does element 13 have? How did you figure that out? 10. Explain why the chemical properties of argon, krypton, and xenon are similar, even though there are 18 elements between argon and krypton, 32 elements between krypton and xenon.. Write the electron configurations for each of these atoms. Then write it using the noble gas shorthand method. a. oxygen b. chlorine e. magnesmm f. silver g. silicon C. ron d. calcium h. mercury 12. You should be able to figure out the identity of an atom from its electron - configuration alone. Describe at least two ways you could do this. 13. Which elements are described by these electron configurations? a. s22s22p 6 3s 2 3p64s23d 4 b. s22s22p 6 3s 2 3p2 C. ls22s22p3 d. S22s22p63s23p64s23d104p6SS24dloSp66s1 e. s22s22p63s23p64s23d104p6ss24dlosp66s24fl4sdlo6p2f. [Kr]Ss24d 9 and 122 Unit Alchemy Section V Moving Electrons

8 V) V) +-' C (J) E (J) (J) '-.0 (]J...c..0 ::J (]J..0 U \::J oc (]J a.. CL CL CL CL CL 0J (Y) T LJ) '-.0 V) +-' C E_ (J) ' C E T V).s: u o CO.s: V)..0 V> +-' t t '-- v v v v (Y) T LJ) '-.0 t-t -[0. V) V) N (Y) T LJ) '-.0 r-, (Y) '-.0 0J - 0J T LJ) r-; POJd...::.. Y- Y- LJ) T '-.0 r-, 98 Unit Alchemy Lesson 24 Transparency Living By Chemistry Teaching and Classroom Masters: Units / Key Curriculum Press

9 r=; :tf-e-- Electron Configurations H ls1 He ls2 3 Li ls2 2S1 f Be ls2 2S2 s: B ls2 2S2 2p1 (, C ls2 2S2 2p2 7 N ls2 2S2 2p 3 0 ls2 2S2 2p4 er F ls2 2S2 2p S r'----"'" D Ne ls2 2S2 2p 6 tt Na ls2 2S2 2p 6 3s 1 t Mg ls2 2S2 2p 6 3s 2 /3 Al ls2 2S2 2p 6 3s 2 3p1 l L{ Si ls2 2S2 2p 6 3s 2 3p2 S P ls2 2S2 2p 6 3s 2 3p 3 /(; S ls2 2S2 2p 6 3s 2 3p4 7 C ls2 2S2 2p 6 3s 2 '3 p s -,.. t Ar ls2 2S2 2p 6 3s 2 3p 6 Living By Chemistry Teaching and Classroom Masters: Units / Key Curriculum Press Unit Alchemy 99 Lesson 24 Transparency