An atomic nucleus consists of protons and, typically, neutrons, which have roughly the same mass as protons. (An electron has so little mass as to be negligible in these calculations.) A proton is said to have a charge of +1, and an electron a charge of –1. When combined in an atom, the charges of electrons and protons cancel out (the charge of an atom with 26 protons and 28 electrons, for instance, would be –2). The atomic number of an element is determined by the number of protons in its nucleus. When an atom loses protons, its atomic number changes. Each element has a unique atomic number. While the atomic number of an element does not change, the number of neutrons in the nucleus can vary.

Atoms of the same element but with different numbers of neutrons are referred to as different isotopes of the same element. Isotopes are identified by their atomic weight, which is the sum of their protons and neutrons; lead-214, for example, has an atomic weight of 214.

A radioactive element is one that is naturally unstable and decays by radiation. Among the types of radiation possible for radioactive elements are alpha, beta, and gamma radiation. In alpha radiation, a particle containing two protons and two neutrons leaves the decaying atom. Beta radiation consists of high-speed electrons. During beta decay a neutron in the atom splits into a proton, which stays in the nucleus, and an electron, which is emitted. Gamma rays are high-energy electromagnetic waves. An atom that decays by any of these three forms of radiation is called an emitter of that type of radiation. Different isotopes often have different radioactive properties. The half-life of a radioactive isotope determines its rate of decay–during one half-life period, approximately 50% of the atoms in a given sample of the isotope will decay according to the radioactive properties of that isotope. The half-life for a given isotope does not change.

Table 1 shows the atomic number, atomic weight, radioactive qualities, and half-life for several elements. (The atomic weight for a given isotope is the number following the name of the element.) Note that nearly all naturally radioactive elements emit gamma radiation, so this is not listed is the “Radiation” column.

Figure shows a visual representation of the half-life curve.

(i) According to the table, thorium is-