The two elements are elements 43, Tc, technetium, and 61, Pm, promethium.
Technetium has a few "nearly stable" isotopes with half-lives of over a million years. Promethium's longest lived isotope has a half-life of about 20 years.
The existence of nonradioactive isotopes of lighter elements had been suspected in studies of neon as early as 1913, and proven by mass spectroscopy of light elements in 1920.
A form of heavy and light elements of matter can be seen in isotopes of elements. Isotopes are variants of a particular chemical element which have the same number of protons in their atomic nuclei but different numbers of neutrons. This results in variations in atomic mass, with some isotopes being heavier or lighter than others.
No, isotopes of light elements exist as well. For example, there are several isotopes of the lightest element, hydrogen.No, even the lightest element (hydrogen (H)) has isotopes. These are called Deuterium and Tritium.All elements have isotopes but some of them are very unstable and have disappeared in nature over time.
All the isotopes of a chemical element are identical; some differences exist for light elements (ex. H or D).
A difference of chemical properties is observed for light elements; for other elements these differences are extremely small.
A natural chemical element may be monoisotopic or has isotopes. Isotopes are atoms but they differ from other isotopes by the number of neutrons. This involve a different atomic mass and different physical properties or sometimes (for light isotopes) different chemical properties. Also, all chemical elements have radioactive, artificial isotopes.
The mass effect between deuterium and the relatively light protium affects the behavior of their respective chemical bonds, by means of changing the center of gravity (reduced mass) of the atomic systems. However, for heavier elements, which have more neutrons than lighter elements, the ratio of the nuclear mass to the collective electronic mass is far greater, and the relative mass difference between isotopes is much less. - See more at: http://www.chacha.com/question/what-cases-are-differences-between-isotopes-important#sthash.oOOVoNRU.dpuf
The separation of isotopes is relatively easy for light elements as hydrogen, lithium, nitrogen etc. For elements with higher atomic weight and a small difference between the atomic masses of the isotopes the process is long and expensive. The insignificant differences between these isotopes doesn't facilitate the separation.
Atoms are matter, and all matter has mass. In that light, any (and every) atom has mass. Different atoms of different elements have different masses, as you would expect. As atomic numbers increase, the Atomic Mass of an atom increases, but we also need to account for the different isotopes of different elements. Isotopes are different "configurations" of the same element where there are different numbers of neutrons in the nucleus. This gives rise to different masses for different elements, and also for different isotopes of the elements. There is more to this discussion, and there are other questions here that are already posted and answered. Some of them are linked below.
Light elements (such as hydrogen, helium, and lithium) are predicted to have stable nuclei when the ratio of neutrons to protons falls within a certain range. This range is known as the "valley of stability" on the nuclear binding energy curve. Nuclei that lie within this valley are less likely to undergo radioactive decay.
Radioactive elements such as radium or uranium emit radiation. There are three types of "radiation"; alpha particles (which are solitary electrons), beta particles (helium nuclei, consisting of two protons and two neutrons) and "gamma radiation", which is high-energy electromagnetic energy similar to light or X-rays.
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