Complete the following nuclear equation: ?? → 
Rn + 
He
Definitions:
Equitable Estoppel
A legal defense tool that prevents a party from taking back a promise or from arguing against facts they previously admitted to, when another party has relied on that promise or those facts.
Legal Obligation
A duty enforced by law, requiring an individual or entity to conform to certain conduct or to refrain from certain actions.
Gratuitous Promise
A promise made without expecting anything in return, which is generally not enforceable in contract law unless it is made in a deed.
Existing Duty
An obligation that is currently in effect under the law or within the terms of a contract.
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