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The reason I believe is because they are artificial elements and so an atomic mass cannot be given. This is because the atomic mass is given by the
Quote

weighted average abundance of natural isotopes that occur in the Earth's crust and atmosphere. For synthetic elements, the isotope depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore, for synthetic elements the total nucleus count (protons plus neutrons) of the most stable isotope, i.e. the isotope with the longest half-lifeā€”is listed in brackets as the atomic mass.
That being said, this does not mean we do not have a clue what teh actual masses are since for the case of the 272Rg and 279Rg, you can see that the masses are known [en.wikipedia.org].

To find the measured or otherwise determined "real" mass of isotopes, it is often a case of looking up the nuclear physics literature, or always a good port of call is the CRC Handbook of Chemistry and Physics which is updated regularly (it is currently in its 92nd edition).

there is also the confusion of different forms of atomic masses, and I submit this for inspection [en.wikipedia.org]

As for calculating masses, no it is not that easy, especially for large nucleon numbers. The binding energies (and hence overall nuclear mass) is a quantum mechanical calculation, and depends upon the number of protons and neutrons in the nucleus, and how they fill their energy shells, as well as the balance of electrostatic and strong forces. It is not simple as my basic equation suggests, which was just used to illustrate how cobalt has a larger mass than Nickel, even though Nickel is higher in the periodic table than Cobalt. There is a semi-empirical method of calculation that uses what is known as the "liquid drop model" which you can read about here [en.wikipedia.org].

Jonny




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Subject Views Written By Posted
Totally confused on The Periodic Table 1116 Ahatmose 05-Feb-12 03:39
Re: Totally confused on The Periodic Table 379 JonnyMcA 05-Feb-12 09:55
Re: Totally confused on The Periodic Table 267 Ahatmose 05-Feb-12 14:57
Re: Totally confused on The Periodic Table 352 JonnyMcA 05-Feb-12 15:52
Re: Totally confused on The Periodic Table 338 JonnyMcA 05-Feb-12 16:01
Re: Totally confused on The Periodic Table 306 Ahatmose 06-Feb-12 03:10
Re: Totally confused on The Periodic Table 306 JonnyMcA 06-Feb-12 18:26
Re: Totally confused on The Periodic Table 324 Spiros 05-Feb-12 22:00
Re: Totally confused on The Periodic Table 326 JonnyMcA 05-Feb-12 22:47
Re: Totally confused on The Periodic Table 319 Spiros 07-Feb-12 22:40
Re: Totally confused on The Periodic Table 320 Susan Doris 06-Feb-12 08:43
Re: Jonny/Ahatmose 351 Thunderbird 08-Feb-12 03:02
Index of Refraction 373 Ahatmose 09-Feb-12 03:08
Re: Index of Refraction 349 Ahatmose 09-Feb-12 03:25
Re: Index of Refraction 1314 JonnyMcA 09-Feb-12 08:34
Re: Index of Refraction 343 Ahatmose 09-Feb-12 16:10
Index of Refraction: The solution ? 389 Ahatmose 09-Feb-12 17:41
Re: Index of Refraction: The solution ? 353 JonnyMcA 11-Feb-12 09:03
Re: Index of Refraction 344 JonnyMcA 11-Feb-12 08:59
Re: Index of Refraction 336 Ahatmose 11-Feb-12 12:29
Re: Index of Refraction 378 JonnyMcA 11-Feb-12 14:12
Re: Index of Refraction 341 Ahatmose 12-Feb-12 13:11
Addendum: Index of Refraction 331 Ahatmose 12-Feb-12 13:19
Second Addendum: Index of Refraction 277 Ahatmose 12-Feb-12 13:26
Third Addendum: Index of Refraction 305 Ahatmose 12-Feb-12 13:32
Re: Third Addendum: Index of Refraction 311 JonnyMcA 12-Feb-12 16:03
Re: Third Addendum: Index of Refraction 325 JonnyMcA 12-Feb-12 21:17
Re: Second Addendum: Index of Refraction 334 JonnyMcA 12-Feb-12 15:40
Re: Addendum: Index of Refraction 315 JonnyMcA 12-Feb-12 15:34
Re: Index of Refraction 375 JonnyMcA 12-Feb-12 13:30
Re: Index of Refraction 319 Ahatmose 12-Feb-12 13:48
Re: Index of Refraction 330 JonnyMcA 12-Feb-12 15:32
Re: Index of Refraction 333 Ahatmose 13-Feb-12 01:23
Re: Index of Refraction 377 JonnyMcA 13-Feb-12 08:23
Re: Index of Refraction 360 Ahatmose 13-Feb-12 16:01
Re: Index of Refraction 351 JonnyMcA 13-Feb-12 19:01
Re: Index of Refraction 328 Ahatmose 13-Feb-12 16:47
Re: Index of Refraction 315 JonnyMcA 13-Feb-12 19:07
Re: Index of Refraction 328 Ahatmose 13-Feb-12 17:37
Re: Index of Refraction 313 JonnyMcA 13-Feb-12 19:20
Re: Index of Refraction 448 Spiros 05-Apr-18 14:20
Talk about stretching credibility: Index of Refraction 337 Ahatmose 13-Feb-12 05:48


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