Is the significant atom-mass equivalent with the relative atom-weight?

No, usually both values are near with each other. The relative atom-weight calculates itself from the  procentage of all isotopes of one element. The significante atom-mass is declared as completely-number (mostly in the middle of the isotopes). It gives a reference number with the known isotopes [Naturally Abundant] and with the unknown elements a trend to the likely possible and stable elements.

You can see such isotopes  at the isotopes table. Attention, very long loading time!  The elements with significant atom-mass are marked as elements with blue or  red fields and white ink. [ see also: Moller Theoretical Nuclear Chat  ]   
  

Sn - 118     Tin

 h = 1

 EGT = 0

 ATM = 104

Print "significant atomic mass = neutrons"; ATM - Z; " + protons " Z; " ="; ATM  ATM = 118;      neutrons = 68;     protons = 50

Uuh - 297          Ununhexium

 h = 1

 EGT = 0

 ATM = 242

Print "significant atomic mass = neutrons"; ATM - Z; " + protons " Z; " ="; ATM ATM = 297;   neutrons = 181;    protons = 116

Z=127: EGT = -2: ATM = 0

 h = 1

 EGT = 0

 ATM=265

The identity of an element is the number of protons in its nucleus. Since protons repel each other, neutrons must be present in the nucleus to hold the protons together. For example, carbon has 6 protons in its nucleus by definition. To hold these protons together, it takes at least 6 neutrons. However, carbon atoms may also have 7 or 8 neutrons. Since carbon comes in more than one variety due to differing numbers of neutrons, carbon is said to come in more than one "isotope". All atoms are "isotopes". But each element has its most common isotope. Since many isotopes are unstable (radioactive), some people think the word "isotope" implies radioactivity, but really it doesn't.

Generally, the more protons in the atom, the higher the proportion of neutrons needed to hold it together.  This proportion is described by the aforementioned formula.   

2001-04-18

2006-11-26