Why are all naturally occurring radium isotopes radioactive?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkaline-earth metal Modern version of the periodic table of the elements (printable).

All naturally occurring isotopes of radium are radioactive because the element has an atomic number of 88. In nature, all elements with an atomic number of 84 or greater are radioactive, as their atomic nuclei are inherently unstable and undergo radioactive decay.2

Because all radium isotopes are radioactive and have relatively short half-lives on a geological time scale, any primeval radium that may have existed when Earth formed has long since disappeared. Consequently, the radium found in nature today exists only as a continuous disintegration product within the natural decay series of uranium, thorium, and actinium.1