What is an Unstable Word™?

Question

_{In the spirit of the What is a Word™/Phrase™ series started by JLee, a special brand of Phrase™ and Word™ puzzles. Wanted to try making one of these for myself.}

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If a word conforms to a special rule, I call it an Unstable Word™.
Use the examples below to find the rule.

Unstable Words™	Not Unstable Words™
CHLORINE	BROMINE
HORSES	CHICKENS
INITIATE	COMMENCE
MASSIVE	MIGHTY
MOUNTAIN	VALLEY
POISON	VENOM
PROMPT	SWIFT
RESOLUTE	DECISIVE
RICKETY	DERELICT
SNOOKER	SOCCER
UMBRELLA	PARASOL
UNSTABLE	WOBBLY
VISION	REVERIE

And, if you want to analyze, here is a CSV version:

Unstable Words™,Not Unstable Words™
CHLORINE,BROMINE
HORSES,CHICKENS
INITIATE,COMMENCE
MASSIVE,MIGHTY
MOUNTAIN,VALLEY
POISON,VENOM
PROMPT,SWIFT
RESOLUTE,DECISIVE
RICKETY,DERELICT
SNOOKER,SOCCER
UMBRELLA,PARASOL
UNSTABLE,WOBBLY
VISION,REVERIE

The puzzle satisfies the series' inbuilt assumption, that each word can be tested for whether it is an Unstable Word™ without relying on the other words.
These are not the only examples of Unstable Words™; many more exist.

What is the special rule these words conform to?

Answer 1

The rule:

The sum of all the letters (A = 1; Z = 26) should be the atomic number of a radioactive element in order for it to be an Unstable word.

Example:

Take HORSES. When you sum up all the letters using the rule stated above, you get 8+15+18+19+5+19 = 84. Element 84 is Polonium, which is a radioactive element. Hence HORSES is an Unstable word. Here is a Python script that I came up with to calculate the sum of all the letters.

Counter-example:

The sum for BROMINE is 76. This corresponds to Element 76: Osmium, which is a stable element. Hence BROMINE is not an Unstable word.