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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 1087 Ahatmose 05-Feb-12 03:39
Re: Totally confused on The Periodic Table 371 JonnyMcA 05-Feb-12 09:55
Re: Totally confused on The Periodic Table 259 Ahatmose 05-Feb-12 14:57
Re: Totally confused on The Periodic Table 345 JonnyMcA 05-Feb-12 15:52
Re: Totally confused on The Periodic Table 328 JonnyMcA 05-Feb-12 16:01
Re: Totally confused on The Periodic Table 296 Ahatmose 06-Feb-12 03:10
Re: Totally confused on The Periodic Table 296 JonnyMcA 06-Feb-12 18:26
Re: Totally confused on The Periodic Table 315 Spiros 05-Feb-12 22:00
Re: Totally confused on The Periodic Table 314 JonnyMcA 05-Feb-12 22:47
Re: Totally confused on The Periodic Table 311 Spiros 07-Feb-12 22:40
Re: Totally confused on The Periodic Table 307 Susan Doris 06-Feb-12 08:43
Re: Jonny/Ahatmose 340 Thunderbird 08-Feb-12 03:02
Index of Refraction 365 Ahatmose 09-Feb-12 03:08
Re: Index of Refraction 341 Ahatmose 09-Feb-12 03:25
Re: Index of Refraction 1284 JonnyMcA 09-Feb-12 08:34
Re: Index of Refraction 335 Ahatmose 09-Feb-12 16:10
Index of Refraction: The solution ? 359 Ahatmose 09-Feb-12 17:41
Re: Index of Refraction: The solution ? 347 JonnyMcA 11-Feb-12 09:03
Re: Index of Refraction 338 JonnyMcA 11-Feb-12 08:59
Re: Index of Refraction 324 Ahatmose 11-Feb-12 12:29
Re: Index of Refraction 362 JonnyMcA 11-Feb-12 14:12
Re: Index of Refraction 333 Ahatmose 12-Feb-12 13:11
Addendum: Index of Refraction 324 Ahatmose 12-Feb-12 13:19
Second Addendum: Index of Refraction 265 Ahatmose 12-Feb-12 13:26
Third Addendum: Index of Refraction 296 Ahatmose 12-Feb-12 13:32
Re: Third Addendum: Index of Refraction 303 JonnyMcA 12-Feb-12 16:03
Re: Third Addendum: Index of Refraction 319 JonnyMcA 12-Feb-12 21:17
Re: Second Addendum: Index of Refraction 326 JonnyMcA 12-Feb-12 15:40
Re: Addendum: Index of Refraction 306 JonnyMcA 12-Feb-12 15:34
Re: Index of Refraction 366 JonnyMcA 12-Feb-12 13:30
Re: Index of Refraction 311 Ahatmose 12-Feb-12 13:48
Re: Index of Refraction 323 JonnyMcA 12-Feb-12 15:32
Re: Index of Refraction 325 Ahatmose 13-Feb-12 01:23
Re: Index of Refraction 365 JonnyMcA 13-Feb-12 08:23
Re: Index of Refraction 351 Ahatmose 13-Feb-12 16:01
Re: Index of Refraction 342 JonnyMcA 13-Feb-12 19:01
Re: Index of Refraction 320 Ahatmose 13-Feb-12 16:47
Re: Index of Refraction 307 JonnyMcA 13-Feb-12 19:07
Re: Index of Refraction 321 Ahatmose 13-Feb-12 17:37
Re: Index of Refraction 306 JonnyMcA 13-Feb-12 19:20
Re: Index of Refraction 438 Spiros 05-Apr-18 14:20
Talk about stretching credibility: Index of Refraction 329 Ahatmose 13-Feb-12 05:48


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