Mendelevium (₁₀₁Md) is a synthetic element, and thus a

standard atomic weight The standard atomic weight of a chemical element (symbol ''A''r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, is ...

cannot be given. Like all artificial elements, it has no stable isotopes. The first

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

to be synthesized was ²⁵⁶Md (which was also the first isotope of any element produced one

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

at a time) in 1955. There are 17 known radioisotopes, ranging in

atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nbs ...

from ²⁴⁴Md to ²⁶⁰Md, and 5

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

s. The longest-lived isotope is ²⁵⁸Md with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of 51.3 days, and the longest-lived isomer is ^258mMd with a half-life of 57 minutes.

List of isotopes

, - , ²⁴⁴Md , style="text-align:right" , 101 , style="text-align:right" , 143 , , , α , ²⁴⁰Es , - , rowspan=2, ²⁴⁵Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 144 , rowspan=2, 245.08081(33)# , rowspan=2, 0.90(25) ms , SF , (various) , rowspan=2, (1/2−)# , - , α (rare) , ²⁴¹Es , - , rowspan=2 style="text-indent:1em" , ^245mMd , rowspan=2 colspan="3" style="text-indent:2em" , 200(100)# keV , rowspan=2, 400(200) ms
.35(+23−16) s, α , ²⁴¹Es , rowspan=2, (7/2+) , - , β⁺ (rare) , ²⁴⁵Fm , - , rowspan=2, ²⁴⁶Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 145 , rowspan=2, 246.08171(28)# , rowspan=2, 1.0(4) s , α , ²⁴²Es , rowspan=2, , - , β⁺ (rare) , ²⁴⁶Fm , - , rowspan=2, ²⁴⁷Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 146 , rowspan=2, 247.08152(22)# , rowspan=2, 1.12(22) s , SF , (various) , rowspan=2, 1/2−# , - , α (rare) , ²⁴³Es , - , rowspan=2 style="text-indent:1em" , ^247mMd , rowspan=2 colspan="3" style="text-indent:2em" , , rowspan=2, ~0.2 s , α (99.99%) , ²⁴³Es , rowspan=2, , - , SF (10⁻⁴%) , (various) , - , rowspan=3, ²⁴⁸Md , rowspan=3 style="text-align:right" , 101 , rowspan=3 style="text-align:right" , 147 , rowspan=3, 248.08282(26)# , rowspan=3, 7(3) s , β⁺ (80%) , ²⁴⁸Fm , rowspan=3, , - , α (20%) , ²⁴⁴Es , - , β⁺, SF (.05%) , (various) , - , rowspan=2, ²⁴⁹Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 148 , rowspan=2, 249.08291(22)# , rowspan=2, 24(4) s , α (60%) , ²⁴⁵Es , rowspan=2, (7/2−) , - , β⁺ (40%) , ²⁴⁹Fm , - , style="text-indent:1em" , ^249mMd , colspan="3" style="text-indent:2em" , 100(100)# keV , 1.9(9) s , , , (1/2−) , - , rowspan=3, ²⁵⁰Md , rowspan=3 style="text-align:right" , 101 , rowspan=3 style="text-align:right" , 149 , rowspan=3, 250.08442(32)# , rowspan=3, 52(6) s , β⁺ (93%) , ²⁵⁰Fm , rowspan=3, , - , α (7%) , ²⁴⁶Es , - , β⁺, SF (.02%) , (various) , - , rowspan=2, ²⁵¹Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 150 , rowspan=2, 251.084774(20) , rowspan=2, 4.0(5) min , β⁺ (90%) , ²⁵¹Fm , rowspan=2, 7/2−# , - , α (10%) , ²⁴⁷Es , - , rowspan=2, ²⁵²Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 151 , rowspan=2, 252.08643(14)# , rowspan=2, 2.3(8) min , β⁺ (50%) , ²⁵²Fm , rowspan=2, , - , α (50%) , ²⁴⁸Es , - , rowspan=2, ²⁵³Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 152 , rowspan=2, 253.08714(3)# , rowspan=2, 12(8) min
(+12−3) min, β⁺ , ²⁵³Fm , rowspan=2, 7/2−# , - , α , ²⁴⁹Es , - , rowspan=2, ²⁵⁴Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 153 , rowspan=2, 254.08959(11)# , rowspan=2, 10(3) min , β⁺ , ²⁵⁴Fm , rowspan=2, (0−) , - , α (rare) , ²⁵⁰Es , - , rowspan=2 style="text-indent:1em" , ^254mMd , rowspan=2 colspan="3" style="text-indent:2em" , 50(100)# keV , rowspan=2, 28(8) min , β⁺ , ²⁵⁴Fm , rowspan=2, (3−) , - , α (rare) , ²⁵⁰Es , - , rowspan=3, ²⁵⁵Md , rowspan=3 style="text-align:right" , 101 , rowspan=3 style="text-align:right" , 154 , rowspan=3, 255.091084(7) , rowspan=3, 27(2) min , β⁺ (92%) , ²⁵⁵Fm , rowspan=3, (7/2−) , - , α (8%) , ²⁵¹Es , - , SF (.15%) , (various) , - , rowspan=2, ²⁵⁶Md , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 155 , rowspan=2, 256.09389(13)# , rowspan=2, 77(2) min , β⁺ (89%) , ²⁵⁶Fm , rowspan=2, (1−) , - , α (11%) , ²⁵²Es , - , rowspan=3, ²⁵⁷Md , rowspan=3 style="text-align:right" , 101 , rowspan=3 style="text-align:right" , 156 , rowspan=3, 257.0955424(29) , rowspan=3, 5.52(5) h , EC (84.8%) , ²⁵⁷Fm , rowspan=3, (7/2−) , - , α (15.2%) , ²⁵³Es , - , SF (1%) , (various) , - , rowspan=3, ²⁵⁸Md , rowspan=3 style="text-align:right" , 101 , rowspan=3 style="text-align:right" , 157 , rowspan=3, 258.098431(5) , rowspan=3, 51.5(3) d , α (99.99%) , ²⁵⁴Es , rowspan=3, (8−)# , - , β⁻ (.0015%) , ²⁵⁸No , - , β⁺ (.0015%) , ²⁵⁸Fm , - , rowspan=4 style="text-indent:1em" , ^258mMd , rowspan=4 colspan="3" style="text-indent:2em" , 0(200)# keV , rowspan=4, 57.0(9) min , EC (70%) , ²⁵⁸Fm , rowspan=4, 1−# , - , SF (20%) , (various) , - , β⁻ (10%) , ²⁵⁸No , - , α (1.2%) , ²⁵⁴Es , - , rowspan=2, ²⁵⁹MdNot directly synthesized, occurs as decay product of ²⁵⁹No , rowspan=2 style="text-align:right" , 101 , rowspan=2 style="text-align:right" , 158 , rowspan=2, 259.10051(22)# , rowspan=2, 1.60(6) h , SF (98.7%) , (various) , rowspan=2, 7/2−# , - , α (1.3%) , ²⁵⁵Es , - , rowspan=4, ²⁶⁰Md , rowspan=4 style="text-align:right" , 101 , rowspan=4 style="text-align:right" , 159 , rowspan=4, 260.10365(34)# , rowspan=4, 27.8(8) d , SF (85%) , (various) , rowspan=4, , - , α (5%) , ²⁵⁶Es , - , EC (5%) , ²⁶⁰Fm , - , β⁻ (3.5%) , ²⁶⁰No

Chronology of isotope discovery

References

* Isotope masses from: ** ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** ** {{Navbox element isotopes Mendelevium Mendelevium