CASE II: In nature, a number of atoms of some elements have been identified, which have the same atomic number hut different mass numbers. For example, take the case of hydrogen atom, it has three atomic species, namely protium (1 H), deuterium (1 HP or D) and tritium ( H or T). The atomic number of each one is 1 but the mass number is 1, 2 and 3, respectively. Other such examples are (1) carbon, 6C!2 and 6C14, (ii) chlorine . C1 and 21"etc.
Q1. What are these elements with same atomic number but different mass numbers are known as?
Q2. Why their chemical properties are same? Q3. Write two uses of these elements.
Q4. An element X bas 7 electrons in its outermost M shell. What is its atomic number and valen
Answers
Learning Objective
Discuss the chemical properties of hydrogen’s naturally occurring isotopes.
Key Points
Protium is the most prevalent hydrogen isotope, with an abundance of 99.98%. It consists of one proton and one electron. It is typically not found in its monoatomic form, but bonded with itself (H2) or other elements.
Deuterium is a hydrogen isotope consisting of one proton, one neutron and one electron. It has major applications in nuclear magnetic resonance studies.
Tritium is a hydrogen isotope consisting of one proton, two neutrons and one electron. It is radioactive, with a half-life of 12.32 years.
Terms
diatomicConsisting of two atoms.
isotopeForms of an element where the atoms have a different number of neutrons within their nuclei. As a consequence, atoms of the same isotope will have the same atomic number, but a different mass number.
Properties of Isotopes of Hydrogen
Hydrogen has three naturally occurring isotopes: 1H (protium), 2H (deuterium), and 3H (tritium). Other highly unstable nuclei (4H to 7H) have been synthesized in the laboratory, but do not occur in nature. The most stable radioisotope of hydrogen is tritium, with a half-life of 12.32 years. All heavier isotopes are synthetic and have a half-life less than a zeptosecond (10-21 sec). Of these, 5H is the most stable, and the least stable isotope is 7H .
ProtiumProtium, the most common isotope of hydrogen, consists of one proton and one electron. Unique among all stable isotopes, it has no neutrons.
Protium
1H is the most common hydrogen isotope with an abundance of more than 99.98%. The nucleus of this isotope consists of only a single proton (atomic number = mass number = 1) and its mass is 1.007825 amu. Hydrogen is generally found as diatomic hydrogen gas H2, or it combines with other atoms in compounds—monoatomic hydrogen is rare. The H–H bond is one of the strongest bonds in nature, with a bond dissociation enthalpy of 435.88 kJ/mol at 298 K. As a consequence, H2 dissociates to only a minor extent until higher temperatures are reached. At 3000K, the degree of dissociation is only 7.85%. Hydrogen atoms are so reactive that they combine with almost all elements.
Deuterium
2H, or deuterium (D), is the other stable isotope of hydrogen. It has a natural abundance of ~156.25 ppm in the oceans, and accounts for approximately 0.0156% of all hydrogen found on earth. The nucleus of deuterium, called a deuteron, contains one proton and one neutron (mass number = 2), whereas the far more common hydrogen isotope, protium, has no neutrons in the nucleus. Because of the extra neutron present in the nucleus, deuterium is roughly twice the mass of protium (deuterium has a mass of 2.014102 amu, compared to the mean hydrogen atomic mass of 1.007947 amu). Deuterium occurs in trace amounts naturally as deuterium gas, written 2H2 or D2, but is most commonly found in the universe bonded with a protium 1H atom, forming a gas called hydrogen deuteride (HD or 1H2H).
Chemically, deuterium behaves similarly to ordinary hydrogen (protium), but there are differences in bond energy and length for compounds of heavy hydrogen isotopes, which are larger than the isotopic differences in any other element. Bonds involving deuterium and tritium are somewhat stronger than the corresponding bonds in protium, and these differences are enough to make significant changes in biological reactions. Deuterium can replace the normal hydrogen in water molecules to form heavy water (D2O), which is about 10.6% denser than normal water. Heavy water is slightly toxic in eukaryotic animals, with 25% substitution of the body water causing cell division problems and sterility, and 50% substitution causing death by cytotoxic syndrome (bone marrow failure and gastrointestinal lining failure). Consumption of heavy water does not pose a health threat to humans. It is estimated that a 70 kg person might drink 4.8 liters of heavy water without serious consequences.