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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 1152 Ahatmose 05-Feb-12 03:39
Re: Totally confused on The Periodic Table 384 JonnyMcA 05-Feb-12 09:55
Re: Totally confused on The Periodic Table 270 Ahatmose 05-Feb-12 14:57
Re: Totally confused on The Periodic Table 357 JonnyMcA 05-Feb-12 15:52
Re: Totally confused on The Periodic Table 347 JonnyMcA 05-Feb-12 16:01
Re: Totally confused on The Periodic Table 310 Ahatmose 06-Feb-12 03:10
Re: Totally confused on The Periodic Table 312 JonnyMcA 06-Feb-12 18:26
Re: Totally confused on The Periodic Table 330 Spiros 05-Feb-12 22:00
Re: Totally confused on The Periodic Table 334 JonnyMcA 05-Feb-12 22:47
Re: Totally confused on The Periodic Table 328 Spiros 07-Feb-12 22:40
Re: Totally confused on The Periodic Table 327 Susan Doris 06-Feb-12 08:43
Re: Jonny/Ahatmose 361 Thunderbird 08-Feb-12 03:02
Index of Refraction 386 Ahatmose 09-Feb-12 03:08
Re: Index of Refraction 353 Ahatmose 09-Feb-12 03:25
Re: Index of Refraction 1334 JonnyMcA 09-Feb-12 08:34
Re: Index of Refraction 358 Ahatmose 09-Feb-12 16:10
Index of Refraction: The solution ? 404 Ahatmose 09-Feb-12 17:41
Re: Index of Refraction: The solution ? 363 JonnyMcA 11-Feb-12 09:03
Re: Index of Refraction 355 JonnyMcA 11-Feb-12 08:59
Re: Index of Refraction 342 Ahatmose 11-Feb-12 12:29
Re: Index of Refraction 390 JonnyMcA 11-Feb-12 14:12
Re: Index of Refraction 350 Ahatmose 12-Feb-12 13:11
Addendum: Index of Refraction 335 Ahatmose 12-Feb-12 13:19
Second Addendum: Index of Refraction 286 Ahatmose 12-Feb-12 13:26
Third Addendum: Index of Refraction 315 Ahatmose 12-Feb-12 13:32
Re: Third Addendum: Index of Refraction 321 JonnyMcA 12-Feb-12 16:03
Re: Third Addendum: Index of Refraction 337 JonnyMcA 12-Feb-12 21:17
Re: Second Addendum: Index of Refraction 342 JonnyMcA 12-Feb-12 15:40
Re: Addendum: Index of Refraction 323 JonnyMcA 12-Feb-12 15:34
Re: Index of Refraction 386 JonnyMcA 12-Feb-12 13:30
Re: Index of Refraction 323 Ahatmose 12-Feb-12 13:48
Re: Index of Refraction 337 JonnyMcA 12-Feb-12 15:32
Re: Index of Refraction 340 Ahatmose 13-Feb-12 01:23
Re: Index of Refraction 384 JonnyMcA 13-Feb-12 08:23
Re: Index of Refraction 366 Ahatmose 13-Feb-12 16:01
Re: Index of Refraction 362 JonnyMcA 13-Feb-12 19:01
Re: Index of Refraction 336 Ahatmose 13-Feb-12 16:47
Re: Index of Refraction 322 JonnyMcA 13-Feb-12 19:07
Re: Index of Refraction 335 Ahatmose 13-Feb-12 17:37
Re: Index of Refraction 323 JonnyMcA 13-Feb-12 19:20
Re: Index of Refraction 458 Spiros 05-Apr-18 14:20
Talk about stretching credibility: Index of Refraction 350 Ahatmose 13-Feb-12 05:48


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