<?xml version="1.0" encoding="utf-8"?><!DOCTYPE Zthes SYSTEM "http://zthes.z3950.org/xml/zthes-05.dtd">  <Zthes><term><termId>31344875</termId><termName>hydrogen isotopes</termName><termType>PT</termType><termNote><![CDATA[ Chemical elements of hydrogen having the same atomic number but different mass. Three of them are naturally occuring: 1H (protium), 2H (deuterium), and 3H (tritium). ]]></termNote><termCreatedDate>2026-03-30 20:33:01</termCreatedDate><relation><relationType>UF</relationType><termId>31344876</termId><termName>hydrogen isotope</termName><termType>ND</termType></relation><relation><relationType>UF</relationType><termId>31344877</termId><termName>isotopes of hydrogen</termName><termType>ND</termType></relation><relation><relationType>BT</relationType><termId>31320551</termId><termName>hydrogen and hydrogen isotopes</termName><termType>PT</termType></relation></term>  </Zthes>