Skills to Develop
To become familiar with the organization of the periodic table.
Rutherford’s nuclear model of the atom helped explain why atoms of different elements exhibit different chemical behavior. The identity of an element is defined by its atomic number (Z), the number of protons in the nucleus of an atom of the element. The atomic number is therefore different for each element. The known elements are arranged in order of increasing Z in the periodic table (Figure 2.5.1). The rationale for the peculiar format of the periodic table is explained later. Each element is assigned a unique one-, two-, or three-letter symbol. The names of the elements are listed in the periodic table, along with their symbols, atomic numbers, and atomic masses. The chemistry of each element is determined by its number of protons and electrons. In a neutral atom, the number of electrons equals the number of protons.
Figure 2.5.1: The Periodic Table Showing the Elements in Order of Increasing Z. The metals are on the bottom left in the periodic table, and the nonmetals are at the top right. The semimetals lie along a diagonal line separating the metals and nonmetals.
The elements are arranged in a periodic table, which is probably the single most important learning aid in chemistry. It summarizes huge amounts of information about the elements in a way that facilitates the prediction of many of their properties and chemical reactions. The elements are arranged in seven horizontal rows, in order of increasing atomic number from left to right and top to bottom. The rows are called periods, and they are numbered from 1 to 7. The elements are stacked in such a way that elements with similar chemical properties form vertical columns, called groups, numbered from 1 to 18 (older periodic tables use a system based on roman numerals). Groups 1, 2, and 13–18 are the main group elements, listed as A in older tables. Groups 3–12 are in the middle of the periodic table and are the transition elements, listed as B in older tables. The two rows of 14 elements at the bottom of the periodic table are the lanthanides and the actinides, whose positions in the periodic table are indicated in group 3.
Metals, Nonmetals, and Semimetals
The heavy orange zigzag line running diagonally from the upper left to the lower right through groups 13–16 in Figure 2.5.1 divides the elements into metals (in blue, below and to the left of the line) and nonmetals (in bronze, above and to the right of the line). Gold-colored lements that lie along the diagonal line exhibit properties intermediate between metals and nonmetals; they are called semimetals.
The distinction between metals and nonmetals is one of the most fundamental in chemistry. Metals—such as copper or gold—are good conductors of electricity and heat; they can be pulled into wires because they are ductile; they can be hammered or pressed into thin sheets or foils because they are malleable; and most have a shiny appearance, so they are lustrous. The vast majority of the known elements are metals. Of the metals, only mercury is a liquid at room temperature and pressure; all the rest are solids.
Nonmetals, in contrast, are generally poor conductors of heat and electricity and are not lustrous. Nonmetals can be gases (such as chlorine), liquids (such as bromine), or solids (such as iodine) at room temperature and pressure. Most solid nonmetals are brittle, so they break into small pieces when hit with a hammer or pulled into a wire. As expected, semimetals exhibit properties intermediate between metals and nonmetals.
Based on its position in the periodic table, do you expect selenium to be a metal, a nonmetal, or a semimetal?
Asked for: classification
Find selenium in the periodic table shown in Figure 2.5.1 and then classify the element according to its location.
The atomic number of selenium is 34, which places it in period 4 and group 16. In Figure 2.5.1, selenium lies above and to the right of the diagonal line marking the boundary between metals and nonmetals, so it should be a nonmetal. Note, however, that because selenium is close to the metal-nonmetal dividing line, it would not be surprising if selenium were similar to a semimetal in some of its properties.
Based on its location in the periodic table, do you expect indium to be a nonmetal, a metal, or a semimetal?
As previously noted, the periodic table is arranged so that elements with similar chemical behaviors are in the same group. Chemists often make general statements about the properties of the elements in a group using descriptive names with historical origins. For example, the elements of Group 1 are known as the alkali metals, Group 2 are the alkaline earth metals, Group 17 are the halogens, and Group 18 are the noble gases.
Group 1: The Alkali Metals
The alkali metals are lithium, sodium, potassium, rubidium, cesium, and francium. Hydrogen is unique in that it is generally placed in Group 1, but it is not a metal.
The compounds of the alkali metals are common in nature and daily life. One example is table salt (sodium chloride); lithium compounds are used in greases, in batteries, and as drugs to treat patients who exhibit manic-depressive, or bipolar, behavior. Although lithium, rubidium, and cesium are relatively rare in nature, and francium is so unstable and highly radioactive that it exists in only trace amounts, sodium and potassium are the seventh and eighth most abundant elements in Earth’s crust, respectively.
Group 2: The Alkaline Earth Metals
The alkaline earth metals are beryllium, magnesium, calcium, strontium, barium, and radium. Beryllium, strontium, and barium are rare, and radium is unstable and highly radioactive. In contrast, calcium and magnesium are the fifth and sixth most abundant elements on Earth, respectively; they are found in huge deposits of limestone and other minerals.
Group 17: The Halogens
The halogens are fluorine, chlorine, bromine, iodine, and astatine. The name halogen is derived from the Greek words for “salt forming,” which reflects that all the halogens react readily with metals to form compounds, such as sodium chloride and calcium chloride (used in some areas as road salt).
Compounds that contain the fluoride ion are added to toothpaste and the water supply to prevent dental cavities. Fluorine is also found in Teflon coatings on kitchen utensils. Although chlorofluorocarbon propellants and refrigerants are believed to lead to the depletion of Earth’s ozone layer and contain both fluorine and chlorine, the latter is responsible for the adverse effect on the ozone layer. Bromine and iodine are less abundant than chlorine, and astatine is so radioactive that it exists in only negligible amounts in nature.
Group 18: The Noble Gases
The noble gases are helium, neon, argon, krypton, xenon, and radon. Because the noble gases are composed of only single atoms, they are called monatomic. At room temperature and pressure, they are unreactive gases. Because of their lack of reactivity, for many years they were called inert gases or rare gases. However, the first chemical compounds containing the noble gases were prepared in 1962. Although the noble gases are relatively minor constituents of the atmosphere, natural gas contains substantial amounts of helium. Because of its low reactivity, argon is often used as an unreactive (inert) atmosphere for welding and in light bulbs. The red light emitted by neon in a gas discharge tube is used in neon lights.
The noble gases are unreactive at room temperature and pressure.
- The periodic table is used as a predictive tool.
The periodic table is an arrangement of the elements in order of increasing atomic number. Elements that exhibit similar chemistry appear in vertical columns called groups (numbered 1–18 from left to right); the seven horizontal rows are called periods. Some of the groups have widely-used common names, including the alkali metals (Group 1) and the alkaline earth metals (Group 2) on the far left, and the halogens (Group 17) and the noble gases (Group 18) on the far right. The elements can be broadly divided into metals, nonmetals, and semimetals. Semimetals exhibit properties intermediate between those of metals and nonmetals. Metals are located on the left of the periodic table, and nonmetals are located on the upper right. They are separated by a diagonal band of semimetals. Metals are lustrous, good conductors of electricity, and readily shaped (they are ductile and malleable), whereas solid nonmetals are generally brittle and poor electrical conductors. Other important groupings of elements in the periodic table are the main group elements, the transition metals, the lanthanides, and the actinides.
Ujuzi wa Kukuza Ili kufahamu utaratibu wa meza ya vipindi. Mfano wa nyuklia wa Rutherford wa atomu ulisaidia kueleza kwa nini atomu za elementi tofauti-tofauti huonyesha tabia tofauti za kemikali. Kiwango cha msingi cha atomu hufafanuliwa na nambari yake ya atomu (Z), idadi ya protoni katika kiini cha atomu ya atomu. Kwa hiyo nambari ya atomu ni tofauti kwa kila elementi. Vitu vinavyojulikana vimepangwa kwa utaratibu wa kuongezeka kwa Z katika meza ya mara kwa mara (Sura ya 2.5.1). Sababu ya muundo wa kipekee wa meza ya vipindi inaelezwa baadaye. Kila kipengele hupewa alama ya pekee ya herufi moja, mbili, au tatu. Majina ya elementi zimeorodheshwa katika meza ya vipindi, pamoja na alama zao, nambari za atomu, na uzito wa atomu. Kemia ya kila elementi huamuliwa na idadi yake ya protoni na elektroni. Katika atomu isiyo na chembechembe, idadi ya elektroni ni sawa na idadi ya protoni. Picha ya pili: Meza ya Mzunguko inaonyesha vipengele katika utaratibu wa kuongezeka kwa Z. Chuma kiko chini kushoto katika meza ya vipindi, na visivyo chuma viko juu kulia. Semimetals uongo kando ya mstari diagonal kutenganisha metals na nonmetals. Vitu hivyo vimepangwa katika meza ya vipindi, ambayo labda ndiyo msaada muhimu zaidi wa kujifunza katika kemia. Kwa mfano, kuna habari nyingi kuhusu vitu vya asili, na inaweza kuwezesha utabiri wa sifa zao na athari zao za kemikali. Vitu hivyo vimepangwa katika safu saba za usawa, kwa utaratibu wa kuongezeka kwa nambari ya atomu kutoka kushoto kwenda kulia na kutoka juu hadi chini. Safu hizo zinaitwa vipindi, na zimepangwa namba kuanzia 1 hadi 7. Vitu hivyo vimejipanga kwa njia ya kwamba vitu vyenye sifa kama hizo hufanyiza safu za wima, zinazoitwa vikundi, vilivyoorodheshwa kutoka 1 hadi 18 (mashine za zamani za mzunguko hutumia mfumo unaotegemea tarakimu za Kiroma). Makundi ya 1, 2 na 13 <unk> 18 ni vipengele kuu vya kundi, vilivyoorodheshwa kama A katika meza za zamani. Kikundi cha 3-12 ni sehemu ya kati ya meza ya vipindi na ni vipengele vya mpito, vilivyoorodheshwa kama B katika meza za zamani. Katika safu ya kwanza ya elementi 14 za mfululizo wa vitu vya asili, kuna lanthanides na actinides, ambazo zinaonekana katika safu ya vitu vya asili. Chini ya mchoro wa rangi ya machungwa, mstari wa zigzag unaoelekea upande wa kulia kutoka juu kushoto hadi chini kulia kupitia makundi 13 <unk> 16 katika Mchoro 2.5.1 unagawanya elementi katika metali (kwa rangi ya bluu, chini na kushoto ya mstari) na nonmetals (kwa shaba, juu na kulia ya mstari). Lements za rangi ya dhahabu ambazo ziko kando ya mstari wa mstatili zinaonyesha mali za kati kati kati ya metali na zisizo metali; zinaitwa semimetals. Tofauti kati ya metali na zisizo metali ni mojawapo ya tofauti za msingi zaidi katika kemia. Chuma - kama vile shaba au dhahabu - ni conductors nzuri ya umeme na joto; wanaweza kuvutwa katika waya kwa sababu wao ni ductile; wanaweza kuwa hammered au kushinikizwa katika karatasi nyembamba au foils kwa sababu wao ni malleable; na wengi wana kuonekana glossy, hivyo wao ni lustrous. Sehemu kubwa ya elementi zinazojulikana ni metali. Kati ya metali hizo, ni zebaki tu ambayo ni kioevu katika joto na shinikizo la chumba; nyingine zote ni imara. Kwa kutofautisha, kwa ujumla si chuma kinachoongoza joto na umeme vizuri na hakina kung'aa. Nonmetals inaweza kuwa gesi (kama vile klorini), kioevu (kama vile bromini), au imara (kama vile iodini) katika joto la chumba na shinikizo. Vitu vingi visivyo vya chuma ni dhaifu, kwa hiyo huvunjika vipande vidogo-vidogo vinapopigwa kwa nyundo au kuvutwa kwenye waya. Kama inavyotarajiwa, semimetals kuonyesha mali kati kati ya metals na nonmetals. Kulingana na mahali pake katika meza ya vipindi, je, unatarajia kwamba seleniamu ni chuma, si chuma, au nusu chuma? "Kuchunguza ""Selenium"" katika meza ya mara kwa mara iliyoonyeshwa katika Picha 2.5.1 na kisha kuorodhesha kipengele kulingana na eneo lake." Nambari ya atomu ya seleniamu ni 34, ambayo inaweka katika kipindi cha 4 na kundi la 16. Katika Picha 2.5.1, seleniamu iko juu na kulia ya mstari wa mstatili unaoweka alama ya mpaka kati ya metali na zisizo metali, kwa hivyo inapaswa kuwa isiyo ya metali. Hata hivyo, kwa kuwa seleniamu iko karibu na mstari wa kutenganisha chuma na kitu kisicho chuma, haingeshangaza ikiwa seleniamu ingekuwa sawa na nusu-metali katika baadhi ya sifa zake. Kulingana na mahali inapo katika meza ya vipindi, je, unatarajia kwamba indiamu iwe si chuma, chuma, au nusu chuma? Kama ilivyotajwa hapo awali, meza ya vipindi imepangwa kwa njia ambayo elementi zenye tabia kama hizo za kemikali ziko katika kikundi kimoja. Mara nyingi wanakemia hutoa taarifa za jumla kuhusu sifa za elementi katika kikundi fulani kwa kutumia majina ya ufafanuzi yenye asili ya kihistoria. Kwa mfano, elementi za kundi la 1 zinaitwa alkali metals, zile za kundi la 2 zinaitwa alkali earth metals, zile za kundi la 17 zinaitwa halogens, na zile za kundi la 18 zinaitwa noble gases. Alkali ni chuma cha msingi ambacho kinajumuisha lithiamu, sodiamu, potasiamu, rubidiamu, cesiamu na frantiamu. Hydrogen ni ya kipekee kwa kuwa kwa ujumla imewekwa katika Kikundi cha 1, lakini si chuma. Misombo ya metali za alkali ni ya kawaida katika asili na maisha ya kila siku. Chumvi ya meza (kloridi ya sodiamu) ni mfano mmoja; misombo ya lithiamu hutumiwa katika mafuta, katika betri, na kama dawa za kutibu wagonjwa wanaonyesha tabia ya manic-depressive, au bipolar. Ingawa lithiamu, rubidiamu, na cesiamu ni nadra sana katika asili, na francium ni isiyo imara na yenye mnururisho mkubwa hivi kwamba inapatikana katika kiasi kidogo tu, sodiamu na potasiamu ni elementi za saba na za nane kwa wingi katika tabaka la dunia. Alkali ya Dunia: Alkali ya Dunia ni pamoja na: Beryllium, Magnesiamu, Kalsiamu, Strontiamu, Barium, na Radium. Beryllium, strontium, na barium ni nadra, na radium si thabiti na ina mnururisho mwingi. Kwa upande mwingine, kalsiamu na magnesiamu ni elementi za tano na sita kwa wingi duniani, na zinapatikana katika madini makubwa ya chokaa na madini mengine. Halogeni: Halogeni ni fluorini, klorini, bromu, iodini, na astatini. Halogeni ni neno la Kigiriki linalomaanisha 'kufanyiza chumvi' na linatokana na hali ya kuwa halogeni huungana na metali na kutokeza kemikali kama vile sodium chloride na calcium chloride (zinatumiwa kama chumvi ya barabarani). Viungo vyenye ion ya fluoridi huongezwa kwenye dawa ya meno na maji ya kunywa ili kuzuia kuharibika kwa meno. Fluorini hupatikana pia katika vitambaa vya Teflon kwenye vyombo vya jikoni. Ingawa kemikali za kutengeneza mafuta ya klorofluorokaboni (CFCs) na baridi zinaaminika kusababisha uharibifu wa tabaka la ozoni duniani na zina fluorini na klorini, mwishowe ni wajibu kwa athari mbaya juu ya tabaka la ozoni. Bromini na iodini ni duni kuliko klorini, na astatini ni yenye mnururisho sana hivi kwamba inapatikana kwa kiasi kidogo tu katika asili. Gesi za Noble: Helium, Neon, Argon, Krypton, Xenon, na Radon Kwa sababu gesi zenye thamani hufanyizwa na atomu moja tu, zinaitwa atomu moja. Katika joto na shinikizo la chumba, ni gesi zisizoweza kuathiriwa. Kwa sababu ya ukosefu wao wa mwitikio, kwa miaka mingi ziliitwa gesi zisizo na nguvu au gesi nadra. Hata hivyo, kemikali za kwanza zenye gesi hizo za hali ya juu zilitengenezwa mwaka wa 1962. Ingawa gesi za hali ya juu ni sehemu ndogo ya angahewa, gesi ya asili ina kiasi kikubwa cha heliamu. Kwa sababu ya mwitikio wake mdogo, argon hutumiwa mara nyingi kama hali isiyo na mwitikio (inert) kwa kulehemu na katika balbu za taa. Nuru nyekundu inayotokezwa na neon katika bomba la kutolea nje gesi hutumiwa katika taa za neon. Gesi za hali ya juu hazitendani wakati wa joto la chumba na shinikizo. - Jedwali la vipindi hutumiwa kama chombo cha kutabiri. Jedwali la vipindi ni mpangilio wa elementi kwa utaratibu wa kuongezeka kwa nambari ya atomu. Vitu vinavyoonyesha kemia sawa huonekana katika safu za wima zinazoitwa vikundi (nambari 1 hadi 18 kutoka kushoto hadi kulia); safu saba za usawa huitwa vipindi. Baadhi ya makundi hayo yana majina ya kawaida yanayotumiwa sana, ikiwa ni pamoja na metali za alkali (Kundi la 1) na metali za alkali-ardhi (Kundi la 2) upande wa kushoto, na halogeni (Kundi la 17) na gesi za kipekee (Kundi la 18) upande wa kulia. Vitu hivyo vinaweza kugawanywa katika metali, visivyo metali, na nusu metali. Semimetals kuonyesha mali kati kati ya wale wa metali na nonmetals. Chuma kiko upande wa kushoto wa meza ya vipindi, na visivyo vya chuma kiko upande wa juu kulia. Zimegawanywa na ukanda wa nusu-metali. Chuma ni mwangaza, conductors nzuri ya umeme, na kwa urahisi umbo (wanakuwa ductile na malleable), wakati imara nonmetals ni kwa ujumla brittle na conductors mbaya ya umeme. Makundi mengine muhimu ya elementi katika meza ya vipindi ni elementi za kikundi kikuu, metali za mpito, lanthanides, na actinides.