Editing Nuclide (section)

==Nuclear properties and stability ==
[[File:Isotopes and half-life.svg|thumb|right|320px|Stability of nuclides by {{nowrap|(''Z'', ''N'')}}, an example of a [[table of nuclides]]:<br/>
Black – stable (all are primordial)<br/>
Red – primordial radioactive<br/>
Other – radioactive, with decreasing stability from orange to white]]
{{See also|Stable nuclide}}
Atomic nuclei other than hydrogen {{nuclide|H|1}} have protons and neutrons bound together by the [[residual strong force]]. Because protons are positively charged, they repel each other. Neutrons, which are electrically neutral, stabilize the nucleus in two ways. Their copresence pushes protons slightly apart, reducing the electrostatic repulsion between the protons, and they exert the attractive nuclear force on each other and on protons. For this reason, one or more neutrons are necessary for two or more protons to be bound into a nucleus. As the number of protons increases, so does the ratio of neutrons to protons necessary to ensure a stable nucleus (see graph). For example, although the [[proton–neutron ratio|neutron–proton ratio]] of {{nuclide|He|3|link=yes}} is 1:2, the neutron–proton ratio of {{nuclide|U|238}} is greater than 3:2. A number of lighter elements have stable nuclides with the ratio 1:1 ({{nowrap|1=''Z'' = ''N''}}). The nuclide {{nuclide|Ca|40}} (calcium-40) is observationally the heaviest stable nuclide with the same number of neutrons and protons. All stable nuclides heavier than calcium-40 contain more neutrons than protons.

===Even and odd nucleon numbers===
{{Main|Even and odd atomic nuclei}}
{| class="wikitable" style="float:left; margin-right:1em"
|+'''Even/odd ''Z'', ''N'', and ''A'''''
| style="text-align:right;" | ''A''
! colspan=2 style="text-align:center;" |Even
! colspan=2 style="text-align:center;" |Odd
! rowspan=2 style="text-align:right;" |Total
|-
| style="text-align:right;" | ''Z'',''N''
!EE!!OO
!EO!!OE
|- style="text-align:right;"
! rowspan=2 |Stable
|145||5||53||48
| rowspan=2 |251
|- style="text-align:right;"
| colspan=2 style="text-align:center;" |150
| colspan=2 style="text-align:center;" |101
|- style="text-align:right;"
! rowspan=2 |Long-lived
|22||4||4||5
| rowspan=2 |35
|- style="text-align:right;"
| colspan=2 style="text-align:center;" |26
| colspan=2 style="text-align:center;" |9
|- style="text-align:right;"
! rowspan=2 |All primordial
|167||9||57||53
| rowspan=2 |286
|- style="text-align:right;"
| colspan=2 style="text-align:center;" |176
| colspan=2 style="text-align:center;" |110
|}
The proton–neutron ratio is not the only factor affecting nuclear stability. It depends also on even or odd [[parity (mathematics)|parity]] of its atomic number ''Z'', neutron number ''N'' and, consequently, of their sum, the mass number ''A''. Oddness of both ''Z'' and ''N'' tends to lower the [[nuclear binding energy]], making odd nuclei, generally, less stable. This remarkable difference of nuclear binding energy between neighbouring nuclei, especially of odd-''A'' [[isobar (nuclide)|isobars]], has important consequences: unstable isotopes with a nonoptimal number of neutrons or protons decay by [[beta decay]] (including positron decay), [[electron capture]] or more exotic means, such as [[spontaneous fission]] and [[cluster decay]].

The majority of stable nuclides are even-proton–even-neutron, where all numbers ''Z'', ''N'', and ''A'' are even. The odd-''A'' stable nuclides are divided (roughly evenly) into odd-proton–even-neutron, and even-proton–odd-neutron nuclides. Odd-proton–odd-neutron nuclides (and nuclei) are the least common.