818:
831:
46:
1374:(u). For C, the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885. The difference of the actual isotopic mass minus the mass number of an atom is known as the
1456:: chlorine-35 and chlorine-37. In any given sample of chlorine that has not been subjected to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/
1401:
The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on C with equal numbers of protons and neutrons.
1467:
has only two stable isotopes, Br and Br, naturally present in approximately equal fractions, which leads to the standard atomic mass of bromine close to 80 (79.904 g/mol), even though the
1339:. If possible, a nuclide will undergo beta decay to an adjacent isobar with lower mass. In the absence of other decay modes, a cascade of beta decays terminates at the
1358:
excited state of an atomic nucleus. Since all the protons and neutrons remain in the nucleus unchanged in this process, the mass number is also unchanged.
1060:
1463:
Moreover, the weighted average mass can be near-integer, but at the same time not corresponding to the mass of any natural isotope. For example,
1529:
1452:
This weighted average can be quite different from the near-integer values for individual isotopic masses. For instance, there are two main
1408:
varies between nuclei. A nucleus with greater binding energy has a lower total energy, and therefore a lower mass according to
Einstein's
1437:) of an element, which is the ratio of the average atomic mass of the different isotopes of that element (weighted by abundance) to the
862:
776:
1730:
125:
1584:
1610:
429:
1495:
817:
1509:"How many protons, electrons and neutrons are in an atom of krypton, carbon, oxygen, neon, silver, gold, etc...?"
621:
326:
855:
1760:
1409:
639:
609:
110:
1755:
686:
236:
572:
848:
835:
567:
271:
1533:
1020:, which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (
562:
459:
424:
120:
1576:
1430:
616:
266:
231:
1697:
741:
626:
518:
681:
1716:
1453:
1438:
751:
726:
543:
28:
1382:
which is the difference between the mass of an atom and its constituent particles (namely
8:
1468:
1340:
646:
525:
419:
362:
355:
345:
286:
281:
115:
1629:
1569:
1487:
589:
584:
399:
1726:
1580:
1052:
761:
756:
716:
594:
333:
321:
304:
276:
246:
87:
1677:
1648:
1625:
1371:
1332:
1024:) as a subscript to the left of the element symbol directly below the mass number:
954:
927:
781:
771:
701:
454:
372:
340:
160:
92:
1420:. For Cl, the isotopic mass is less than 35, so this must be the dominant factor.
1346:
Another type of radioactive decay without change in mass number is emission of a
766:
746:
721:
651:
538:
466:
412:
377:
37:
1508:
1405:
1351:
1072:
966:
908:
822:
676:
671:
550:
483:
291:
226:
203:
190:
177:
77:
55:
1749:
1434:
1367:
1336:
1056:
958:
939:
801:
796:
791:
786:
736:
394:
367:
211:
150:
103:
82:
20:
1682:
1653:
994:
to the left of an element's symbol. For example, the most common isotope of
1609:
Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017).
1206:
731:
706:
691:
436:
384:
241:
1201:, whereby one neutron is transmuted into a proton with the emission of an
1611:"The AME2016 atomic mass evaluation (II). Tables, graphs, and references"
1490:(2005). The Origins of the Symbols A and Z for Atomic Weight and Number.
1457:
1379:
1375:
1222:
1088:
1075:. Thus the atomic number and the number of neutrons each decrease by 2 (
1071:, where the nucleus loses two neutrons and two protons in the form of an
1068:
1064:
991:
912:
696:
389:
311:
24:
1355:
1198:
666:
656:
513:
493:
316:
186:
1347:
1194:
999:
711:
661:
488:
476:
471:
350:
1378:, which for Cl is –0.03115. Mass excess should not be confused with
1391:
1202:
1720:
45:
1464:
1387:
990:
The mass number is written either after the element name or as a
950:
904:
900:
173:
146:
138:
70:
60:
1383:
995:
931:
896:
65:
1055:
are characterized by their changes in mass number as well as
1698:"Atomic Weights and Isotopic Compositions for All Elements"
1046:
923:
946:
1717:"The Structure of Matter and Chemical Elements (ch. 3)"
1083:: 146 → 144), so that the mass number decreases by 4 (
1429:
The mass number should also not be confused with the
1424:
957:, and the difference between the mass number and the
1217:= 14), while the number of neutrons decreases by 1 (
1608:
1568:
1747:
1511:. Thomas Jefferson National Accelerator Facility
1361:
1061:radioactive displacement law of Fajans and Soddy
1213:: 6 → 7) and the mass number remains the same (
16:Number of heavy particles in the atomic nucleus
945:of the nucleus (and also of the whole atom or
856:
1674:The IUPAC Compendium of Chemical Terminology
1645:The IUPAC Compendium of Chemical Terminology
1335:have mass differences on the order of a few
1209:. Thus the atomic number increases by 1 (
863:
849:
1681:
1652:
1602:
1366:The mass number gives an estimate of the
1331:Beta decay is possible because different
1225:, with seven protons and seven neutrons:
949:). The mass number is different for each
1668:"Relative atomic mass (Atomic weight), A
1397:There are two reasons for mass excess:
1047:Mass number changes in radioactive decay
1547:
1087:= 238 → 234); the result is an atom of
1748:
1566:
930:. Since protons and neutrons are both
1593:
1532:. Science Help Online. Archived from
911:. It is approximately equal to the
13:
1714:
1708:
1445:ratio, while the mass number is a
1425:Relative atomic mass of an element
1341:isobar with the lowest atomic mass
14:
1772:
1725:. Chiral Publishing. p. 93.
1530:"Elemental Notation and Isotopes"
1221:: 8 → 7). The resulting atom is
887:, "atomic weight"), also called
1599:Atoms with the same mass number.
830:
829:
816:
44:
1690:
1661:
1636:
1560:
1522:
1501:
1481:
1:
1630:10.1088/1674-1137/41/3/030003
1474:
1449:number (and so an integer).
1362:Mass number and isotopic mass
1722:An Introduction to Chemistry
1471:with such mass is unstable.
7:
1229:
1114:
610:High-energy nuclear physics
10:
1777:
1624:(3): 030003-1–030003-442.
18:
1575:. Career Press. pp.
1441:. The atomic weight is a
895:, is the total number of
883:, from the German word:
19:Not to be confused with
1683:10.1351/goldbook.R05258
1654:10.1351/goldbook.M03719
1410:mass–energy equivalence
1091:and an alpha particle (
121:Interacting boson model
1431:standard atomic weight
938:is identical with the
1567:Curran, Greg (2004).
508:High-energy processes
206:– equal all the above
104:Models of the nucleus
1555:Conceptual Chemistry
1454:isotopes of chlorine
1439:atomic mass constant
544:nuclear astrophysics
29:Relative atomic mass
1761:Chemical quantities
1193:On the other hand,
1059:, according to the
1051:Different types of
903:(together known as
526:Photodisintegration
449:Capturing processes
363:Spontaneous fission
356:Internal conversion
287:Valley of stability
282:Island of stability
116:Nuclear shell model
1643:"Mass excess, Δ".
1488:Jensen, William B.
1067:usually decays by
973:) in the nucleus:
967:number of neutrons
934:, the mass number
889:atomic mass number
823:Physics portal
617:Quark–gluon plasma
400:Radiogenic nuclide
1756:Nuclear chemistry
1732:978-0-9778105-4-3
1618:Chinese Physics C
1557:, 2007. Page 119.
1372:atomic mass units
1327:
1326:
1189:
1188:
1053:radioactive decay
928:atomic mass units
873:
872:
559:
305:Radioactive decay
261:Nuclear stability
88:Nuclear structure
1768:
1742:
1740:
1739:
1702:
1701:
1694:
1688:
1687:
1685:
1665:
1659:
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1656:
1640:
1634:
1633:
1615:
1606:
1600:
1597:
1591:
1590:
1574:
1571:Homework Helpers
1564:
1558:
1553:Suchocki, John.
1551:
1545:
1544:
1542:
1541:
1526:
1520:
1519:
1517:
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1505:
1499:
1485:
1323:
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1314:
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1310:
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1230:
1183:
1182:
1181:
1174:
1173:
1159:
1158:
1157:
1150:
1149:
1135:
1134:
1133:
1126:
1125:
1115:
1109:
1108:
1107:
1100:
1099:
1063:. For example,
1042:
1041:
1040:
1033:
1032:
1019:
1018:
1017:
1010:
1009:
986:
955:chemical element
865:
858:
851:
838:
833:
832:
825:
821:
820:
697:Skłodowska-Curie
557:
373:Neutron emission
141:' classification
93:Nuclear reaction
48:
34:
33:
1776:
1775:
1771:
1770:
1769:
1767:
1766:
1765:
1746:
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1735:
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1711:
1709:Further reading
1706:
1705:
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1637:
1613:
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1523:
1514:
1512:
1507:
1506:
1502:
1486:
1482:
1477:
1427:
1364:
1337:electron masses
1320:
1317:
1316:
1315:
1312:
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1306:
1305:
1304:
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1128:
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1103:
1102:
1098:
1095:
1094:
1093:
1092:
1049:
1039:
1037:
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1028:
1027:
1026:
1025:
1016:
1014:
1013:
1012:
1008:
1006:
1005:
1004:
1003:
974:
916:(also known as
869:
828:
815:
814:
807:
806:
642:
632:
631:
612:
602:
601:
546:
542:
539:Nucleosynthesis
531:
530:
509:
501:
500:
450:
442:
441:
415:
413:Nuclear fission
405:
404:
378:Proton emission
307:
297:
296:
262:
254:
253:
155:
142:
131:
130:
106:
38:Nuclear physics
32:
17:
12:
11:
5:
1774:
1764:
1763:
1758:
1744:
1743:
1731:
1715:Bishop, Mark.
1710:
1707:
1704:
1703:
1689:
1669:
1660:
1635:
1601:
1592:
1585:
1559:
1546:
1521:
1500:
1492:J. Chem. Educ.
1479:
1478:
1476:
1473:
1426:
1423:
1422:
1421:
1406:binding energy
1402:
1363:
1360:
1352:nuclear isomer
1329:
1328:
1325:
1324:
1318:
1308:
1302:
1299:
1293:
1285:
1279:
1276:
1270:
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1255:
1252:
1246:
1237:
1191:
1190:
1187:
1186:
1184:
1179:
1170:
1164:
1161:
1155:
1146:
1140:
1137:
1131:
1122:
1105:
1096:
1073:alpha particle
1048:
1045:
1038:
1029:
1015:
1007:
909:atomic nucleus
893:nucleon number
871:
870:
868:
867:
860:
853:
845:
842:
841:
840:
839:
826:
809:
808:
805:
804:
799:
794:
789:
784:
779:
774:
769:
764:
759:
754:
749:
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739:
734:
729:
724:
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629:
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619:
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600:
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551:Nuclear fusion
547:
537:
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510:
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502:
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338:
337:
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314:
308:
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302:
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298:
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292:Stable nuclide
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279:
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269:
267:Binding energy
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118:
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78:Nuclear matter
75:
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68:
58:
50:
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41:
40:
15:
9:
6:
4:
3:
2:
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1596:
1588:
1586:1-56414-721-5
1582:
1578:
1573:
1572:
1563:
1556:
1550:
1536:on 2008-09-13
1535:
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1472:
1470:
1466:
1461:
1459:
1455:
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1448:
1444:
1440:
1436:
1435:atomic weight
1433:(also called
1432:
1419:
1415:
1411:
1407:
1403:
1400:
1399:
1398:
1395:
1393:
1389:
1385:
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1373:
1369:
1368:isotopic mass
1359:
1357:
1353:
1349:
1344:
1342:
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1334:
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1300:
1280:
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1113:
1112:
1111:
1090:
1086:
1082:
1078:
1074:
1070:
1066:
1062:
1058:
1057:atomic number
1054:
1044:
1023:
1001:
997:
993:
988:
985:
981:
977:
972:
968:
964:
960:
959:atomic number
956:
952:
948:
944:
941:
940:baryon number
937:
933:
929:
926:expressed in
925:
921:
919:
915:
910:
906:
902:
898:
894:
890:
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561:
560:
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534:
527:
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520:
519:by cosmic ray
517:
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512:
511:
505:
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492:
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430:pair breaking
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395:Decay product
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368:Cluster decay
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83:Nuclear force
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63:
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59:
57:
54:
53:
52:
51:
47:
43:
42:
39:
36:
35:
30:
26:
22:
21:Atomic number
1736:. Retrieved
1721:
1692:
1673:
1663:
1644:
1638:
1621:
1617:
1604:
1595:
1570:
1562:
1554:
1549:
1538:. Retrieved
1534:the original
1524:
1513:. Retrieved
1503:
1491:
1483:
1462:
1451:
1446:
1442:
1428:
1417:
1413:
1396:
1370:measured in
1365:
1345:
1330:
1218:
1214:
1210:
1207:antineutrino
1192:
1084:
1080:
1076:
1050:
1021:
989:
983:
979:
975:
970:
962:
942:
935:
917:
913:
892:
888:
884:
880:
876:
874:
437:Photofission
385:Decay energy
312:Alpha α
219:
215:
195:
191:
178:
165:
164:
151:
1380:mass defect
1376:mass excess
1223:nitrogen-14
1089:thorium-234
1079:: 92 → 90,
1069:alpha decay
1065:uranium-238
992:superscript
953:of a given
885:Atomgewicht
877:mass number
742:Oppenheimer
420:Spontaneous
390:Decay chain
341:K/L capture
317:Beta β
187:Isodiaphers
111:Liquid drop
25:Atomic mass
1750:Categories
1738:2008-07-08
1540:2008-08-27
1515:2008-08-27
1494:82: 1764.
1475:References
1469:isotope Br
1356:metastable
1199:beta decay
1197:decays by
965:gives the
772:Strassmann
762:Rutherford
640:Scientists
595:Artificial
590:Cosmogenic
585:Primordial
581:Nuclides:
558:Processes:
514:Spallation
1412:relation
1392:electrons
1348:gamma ray
1195:carbon-14
1000:carbon-12
777:Świątecki
692:Pi. Curie
687:Fr. Curie
682:Ir. Curie
677:Cockcroft
652:Becquerel
573:Supernova
277:Drip line
272:p–n ratio
247:Borromean
126:Ab initio
1676:. 2014.
1647:. 2014.
1404:Nuclear
1388:neutrons
1203:electron
918:isotopic
907:) in an
905:nucleons
901:neutrons
879:(symbol
836:Category
737:Oliphant
722:Lawrence
702:Davisson
672:Chadwick
568:Big Bang
455:electron
425:Products
346:Isomeric
237:Even/odd
214: –
189:– equal
176:– equal
174:Isotones
163:– equal
149:– equal
147:Isotopes
139:Nuclides
61:Nucleons
1700:. NIST.
1465:bromine
1447:counted
1384:protons
1350:from a
1333:isobars
1301:+
1278:+
1254:→
1205:and an
1163:+
1139:→
951:isotope
932:baryons
922:of the
897:protons
792:Thomson
782:Szilárd
752:Purcell
732:Meitner
667:N. Bohr
662:A. Bohr
647:Alvarez
563:Stellar
467:neutron
351:Gamma γ
204:Isomers
161:Isobars
56:Nucleus
1729:
1583:
1313:ν
1298:
1275:
1251:
1160:
1136:
996:carbon
961:
920:) mass
914:atomic
834:
802:Wigner
797:Walton
787:Teller
717:Jensen
484:proton
227:Stable
1614:(PDF)
1577:78–79
1002:, or
767:Soddy
747:Proca
727:Mayer
707:Fermi
657:Bethe
232:Magic
27:, or
1727:ISBN
1581:ISBN
1496:link
1443:mass
1390:and
924:atom
899:and
875:The
757:Rabi
712:Hahn
622:RHIC
242:Halo
1678:doi
1672:".
1649:doi
1626:doi
1460:).
1458:mol
1394:).
1354:or
1110:):
998:is
947:ion
891:or
627:LHC
541:and
1752::
1719:.
1622:41
1620:.
1616:.
1579:.
1418:mc
1416:=
1386:,
1343:.
1175:He
1151:Th
1147:90
1123:92
1101:He
1043:.
987:.
982:−
978:=
494:rp
460:2×
327:0v
322:2β
218:↔
23:,
1741:.
1686:.
1680::
1670:r
1657:.
1651::
1632:.
1628::
1589:.
1543:.
1518:.
1498:.
1414:E
1319:e
1289:e
1266:N
1262:7
1242:C
1238:6
1219:N
1215:A
1211:Z
1171:2
1127:U
1097:2
1085:A
1081:N
1077:Z
1034:C
1030:6
1022:Z
1011:C
984:Z
980:A
976:N
971:N
969:(
963:Z
943:B
936:A
881:A
864:e
857:t
850:v
489:p
477:r
472:s
334:β
220:N
216:Z
196:Z
192:N
179:N
166:A
152:Z
71:n
66:p
31:.
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