Izakhiwo ze-Lewis neshaji elisemthethweni: indlela ongayihlola

Ukubala ama-electron e-valence kumamolekyuli nakuma-ayoni anama-athomu amaningi

Isakhiwo ngasinye se-Lewis siqala ngenombolo eyodwa: inani eliphelele lama-electron e-valence atholakalayo ukubeka. Ezakhini zeqembu eliyinhloko, inani lama-electron e-valence lilingana nenombolo yokugcina yenombolo yeqembu ekubaleni kuka-1–18, ngakho i-carbon inikela ngo-4, i-nitrogen ngo-5, i-oxygen ngo-6, ne-halogen ngayinye ngo-7. I-hydrogen inikela ngo-1, futhi i-helium, ngo-2, yiyona kuphela ehlukile kumthetho wenombolo yeqembu.

Ishaji le-ayoni lishintsha isamba. Ishaji elingelihle lisho ama-electron ayengeziwe, ngakho wengeza i-electron eyodwa ngeyunithi ngayinye yeshaji elingelihle. Ishaji elihle lisho ama-electron angekho, ngakho uyakhipha. I-ayoni ye-ammonium, NH₄⁺, inama-electron e-valence angu-5 + 4(1) − 1 = 8, kanti i-ayoni ye-carbonate, CO₃²⁻, inangu-4 + 3(6) + 2 = 24.

Bhala lesi samba ngaphambi kokudweba noma yini; yinombolo ozohlola ngayo isakhiwo esiqediwe.

Ukwakha isiphenguli nokubeka amabhangqa ama-electron

Isiphenguli yiphethini yokuthi yiyiphi i-athomu exhumeka kweyiphi. I-hydrogen ne-fluorine cishe zihlala zakha isibopho esisodwa kuphela, ngakho ziyizingxenye zokugcina. I-athomu eyisisekelo ivame ukuba yi-athomu ene-electronegativity encane kakhulu ngaphandle kwe-hydrogen, evame ukuba yi-athomu evela kanye kufomula. Ku-oxyacid, ama-athomu e-hydrogen ngokujwayelekile abophelelwe ku-oxygen, hhayi e-athomu eyisisekelo.

Uma isiphenguli sesisethiwe, izinyathelo ezisele zihambisa ama-electron ukuze i-athomu ngalinye lifinyelele inombolo ezinzile. Ama-athomu esikhathi 2 (C, N, O, F) angaphatha ama-electron angu-8 kuphela ngoba anama-orbital e-valence amane kuphela, yingakho umthetho we-octet uyimikhawulo eqinile kuwo.

• Xhuma i-athomu ngayinye yokugcina ku-athomu eyisisekelo ngesibopho esisodwa (ama-electron angu-2 ngasinye).
• Qedela ama-octet ama-athomu okugcina ngamabhangqa amodwa (i-hydrogen idinga u-2 kuphela).
• Beka noma yiziphi izinhlayiya ze-electron ezisele ku-athomu eyisisekelo njengamabhangqa amodwa.
• Uma i-athomu eyisisekelo isenama-electron angaphansi kuka-8, guqula ibhangqa elilodwa ku-athomu eliseduze libe ibhangqa elabelwene ukwenza isibopho esiphindwe kabili noma kathathu.
• Hlola: ama-electron adwetshiwe alingana nesamba esibaliwe, futhi i-athomu ngayinye yesikhathi 2 ayinawo angaphezu kuka-8.

Ishaji elisemthethweni njengokuhlolwa phakathi kwezakhiwo ezimiswayo

Ishaji elisemthethweni libuza ukuthi i-athomu ledwetshiwe linawo mangaki ama-electron uma iqhathaniswa ne-athomu elikhululekile elingathathi hlangothi, uma ibhangqa ngalinye lokubopha lihlukaniswe ngokulinganayo. Ifomula ngu-FC = V − N − B/2, lapho u-V enenani lama-electron e-valence e-athomu, u-N enenani lama-electron angabophi (ebhangqa elilodwa) kuyo, no-B enenani lama-electron ezibophweni zayo. Amashaji asemthethweni esakhiweni ahlala ehlangana abe yishaji eliphelele lohlobo, okukunikeza ukuhlola kwezibalo okwakhelwe ngaphakathi.

Lapho izakhiwo eziningana zilandela inani lama-electron, khetha lapho amashaji asemthethweni eseduze kakhulu no-zero. Uma elinye ishaji elisemthethweni kufanele lisale, isakhiwo esingcono sibeka ishaji elingelihle ku-athomu ene-electronegativity enkulu futhi sigwema ukubeka amashaji osayini ofanayo emaqenjini aphelekezelwe.

Ishaji elisemthethweni liyincwadi yokubhala, hhayi ishaji elilinganisiwe. Ngoba lidlulisa i-electronegativity, i-athomu elinelishaji elisemthethweni elingu-zero lisengaphatha ishaji elingxenye elibalulekile kumolekyuli yangempela.

Isibonelo esisebenzile: i-phosgene ne-ayoni ye-cyanate

I-phosgene, COCl₂. Bala: u-C unikela ngo-4, u-O unikela ngo-6, no-Cl ngamunye unikela ngo-7, ngakho isamba siyi-4 + 6 + 2(7) = 24 electron. I-carbon iyi-athomu eyisisekelo. Izibopho ezintathu ezisodwa ezisuka ku-C ziye ku-O nakuma-Cl ngamunye zisebenzisa ama-electron angu-6. Ukuqedela ama-octet ka-O nama-Cl womabili kusebenzisa ama-electron angu-3 × 6 = 18 amanye, ukuze kube ngu-24 sekukonke, ngakho akusali ama-electron e-carbon. I-carbon inama-electron angu-6 kuphela ezungezile, ngakho guqula ibhangqa elilodwa libe yisibopho sesibili. Ukuhambisa ibhangqa elilodwa lika-O kunikeza u-C=O; ukuhambisa elika-Cl kunikeza u-C=Cl.

Ishaji elisemthethweni linquma phakathi kwazo. Nge-C=O, i-oxygen inama-electron angu-4 angabophi nama-electron angu-4 abophayo: FC = 6 − 4 − 4/2 = 0. U-Cl ngamunye unama-electron angu-6 angabophi no-2 abophayo: FC = 7 − 6 − 1 = 0. I-carbon inama-electron angu-8 abophayo: FC = 4 − 0 − 4 = 0. Nge-C=Cl esikhundleni, lowo-Cl uba ngu-7 − 4 − 2 = +1 futhi u-O obophelelwe ngesisodwa uba ngu-6 − 6 − 1 = −1. Isakhiwo esinakho konke ku-zero nesibopho esiphindwe kabili sika-C=O siyakhethwa.

I-ayoni ye-cyanate, OCN⁻. Bala: 6 + 4 + 5 + 1 = 16 electron, ne-carbon iyisisekelo. Ukuhlelwa okuthathu kunikeza i-carbon i-octet: O=C=N, O≡C–N, no-O–C≡N, ngakunye namabhangqa amodwa aqedela ama-octet ku-O no-N. Ku-O=C=N, O = 6 − 4 − 2 = 0 no-N = 5 − 4 − 2 = −1. Ku-O≡C–N, O = 6 − 2 − 3 = +1 no-N = 5 − 6 − 1 = −2. Ku-O–C≡N, O = 6 − 6 − 1 = −1 no-N = 5 − 2 − 3 = 0. I-carbon ingu-0 kuwo wonke amathathu, futhi isethi ngayinye ihlangana ibe ngu-−1 njengoba kufanele.

Uhlobo lwe-O≡C–N lunamashaji amakhulu kakhulu futhi lubeka ishaji elihle ku-oxygen, i-athomu ene-electronegativity enkulu kakhulu, ngakho lunikela kancane kakhulu. Uhlobo lwe-O–C≡N lubeka ishaji elingelihle ku-oxygen futhi lunikela kakhulu, no-O=C=N usondele ngemuva. I-ayoni yangempela iyingxube yalezi zakhiwo, njengoba isigaba esilandelayo sichaza.

I-resonance lapho umdweba owodwa ungakwazi ukuchaza izibopho

Ezinye izinhlobo zinezakhiwo ezimbili noma ngaphezulu ezivumelekile ze-Lewis ezihluka kuphela lapho ama-electron abekwe khona, nama-athomu ahlala endaweni efanayo. Lezi yizakhiwo ze-resonance, ezibhalwa ngomcibisholo onamakhanda amabili (↔) phakathi kwazo. Imolekyuli yangempela isakhiwo esisodwa, i-resonance hybrid, ezibopho zayo ziyisilinganiso sezinikelayo. Ayishintshi iye emuva naphambili phakathi kwazo.

I-ayoni ye-formate, HCOO⁻, isibonelo esicacile. Bala: 1 + 4 + 2(6) + 1 = 18 electron. Nge-H nama-athomu womabili e-O abophelelwe ku-C futhi ama-octet e-oxygen aqediwe, kusetshenziswa ama-electron angu-18 futhi i-carbon inawo angu-6 kuphela, ngakho ibhangqa elilodwa le-oxygen liba yisibopho se-C=O. U-O onesibopho esiphindwe kabili une-FC = 6 − 4 − 2 = 0 no-O onesibopho esisodwa une-FC = 6 − 6 − 1 = −1. Noma yiyiphi i-oxygen ingaba nesibopho esiphindwe kabili, ngakho kunezakhiwo ezimbili ezilinganayo.

Ngoba izinikelayo ezimbili zilingana, i-hybrid inezibopho ezimbili ze-carbon–oxygen ezifanayo, ngasinye siphakathi kwesisodwa nesiphindwe kabili, futhi i-oxygen ngayinye iphatha ingxenye yeshaji elingelihle ngesilinganiso. Ubude bezibopho obulinganisiwe kuma-ayoni e-carboxylate buyavumelana nalesi sithombe. Lapho izinikelayo zingalingani, njengase-cyanate, i-hybrid ifana kakhulu nezakhiwo ezinamashaji asemthethweni aphansi.

Lapho umthetho we-octet ugobhoza: amanani angajwayelekile, angaphelele nanwetshiwe

Izinhlobo ezintathu azifaneleli iphethini ye-octet. Amamolekyuli anenombolo engelona ngapheli yama-electron, njenge-nitrogen monoxide, NO, enama-electron e-valence angu-5 + 6 = 11, awakwazi ukunikeza i-athomu ngayinye i-octet; i-athomu eyodwa kufanele ibe ne-electron engabhangqiwe. Izinhlobo ezinjalo zibizwa ngama-radical futhi ngokuvamile ziyasabela.

Amanye amamolekyuli anama-octet angaphelele ku-athomu eyisisekelo. Ku-boron trifluoride, BF₃ (ama-electron angu-24), isakhiwo esinezibopho ezintathu ezisodwa ze-B–F sishiya i-boron nama-electron angu-6 kodwa sinikeza wonke ama-athomu ishaji elisemthethweni elingu-zero. Ukwakha isibopho esiphindwe kabili se-B=F kungaqedela i-octet ye-boron kuphela ngokunikeza i-fluorine ishaji elisemthethweni elingu-+1 ne-boron −1, ngakho isakhiwo esinezibopho ezisodwa siyincazelo evamile.

Ama-athomu esikhathi 3 nangaphansi angadwetshwa anama-electron angaphezu kwayisishiyagalombili azungezile. Ku-PCl₅ (ama-electron angu-40), izibopho ezinhlanu zinikeza i-phosphorus ama-electron angu-10. Ku-ayoni ye-triiodide, I₃⁻ (3 × 7 + 1 = 22 electron), i-iodine eyisisekelo iphethe izibopho ezimbili namabhangqa amathathu amodwa, futhi ishaji layo elisemthethweni ngu-7 − 6 − 2 = −1. Beka ama-electron asele ku-athomu eyisisekelo kuphela ngemva kokuba i-athomu ngayinye yokugcina ine-octet yayo.

Amaphutha avamile esakhiweni se-Lewis nendlela yokuwabamba

Amaphutha amaningi angamaphutha okubala, ngakho umkhuba othembekile kakhulu ukuqeda isakhiwo ngasinye ngokuhlola okubili: bala wonke ama-electron emdwebeni (2 ngesibopho ngasinye, 2 ngebhangqa elilodwa ngalinye) uwaqhathanise nesamba osibhalile, bese wengeza amashaji asemthethweni ukuqinisekisa ukuthi alingana neshaji eliphelele.

Uma isamba sakho sihluka kowesifundi esihlala nawe noma kokhiye ngokuphindwe kabili kweshaji le-ayoni, ishaji cishe lisetshenziswe ngophawu olungalungile. Uma umdweba uphethe inani elihlukile lama-electron kunesamba sakho, bheka izibopho ezibalwe njenge-electron eyodwa esikhundleni sezimbili, noma ama-electron asele angazange abekwe. Amaphutha athile angezansi abangela izakhiwo eziningi ezingalungile.

• Ukwengeza ama-electron e-cation noma ukuwakhipha e-anion. Ishaji elingelihle lisho ama-electron amaningi.
• Ukubala isibopho njenge-electron eyodwa. Umugqa ngamunye umele ibhangqa elabelwene, ngakho ama-electron angu-2.
• Ukunikeza i-hydrogen izibopho ezimbili noma ukuyenza i-athomu eyisisekelo. I-hydrogen iphatha ama-electron angu-2 kuphela.
• Ukubeka ama-electron angaphezu kuka-8 ku-C, N, O noma F. Amanani anwetshiwe ayenzeka kuphela esikhathini 3 nangaphansi.
• Ukuthatha ishaji elisemthethweni njengeshaji langempela le-athomu, noma ukulixuba nenombolo ye-oxidation, enikeza wonke ama-electron abophayo ku-athomu ene-electronegativity enkulu.
• Ukudweba izakhiwo ze-resonance ngokuhambisa ama-athomu. Ukuhambisa i-athomu kudala inhlanganisela ehlukile, hhayi isinikelayo se-resonance.