Basic Chemistry: Atomic Number and Atomic Mass, Isotopes and Radio Isotopes

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Environmental Chemistry: Atoms and Elements (Chemistry)

Table of Contents

  • Atomic number and atomic mass

  • Periodic Table and Chemical Bonds

  • Isotopes

  • Radioisotopes

Learning Outcomes

After studying this lesson, you shall be able to:

  • Atomic number and atomic mass

  • Isotopes

  • Radio isotopes

Atoms and Elements

Atomic Number and Atomic Mass

  • The atomic number of an atom is the number of protons in the atom

  • and if the atom is neutral it equals the number of electrons.

  • The atomic mass is the total mass of the protons and neutrons in the atom.

Atoms and Elements

Atoms and Elements

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  • The concept of chemical element is related to that of chemical substance.

  • A chemical element is specifically a substance which is composed of a single type of atom.

  • For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium.

  • Although all the nuclei of all atoms belonging to one element will have the same number of protons, they may not necessarily have the same number of neutrons; such atoms are termed isotopes.

  • In fact, several isotopes of an element may exist.

  • Ninety-four different chemical elements or types of atoms based on the number of protons are observed on earth naturally, having at least one isotope that is stable or has a very long half-life.

  • Isotopes are atoms that have the same atomic number (number of protons) but differ in atomic mass.

number of protons

Number of Protons

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  • For instance, most carbon has six protons and six neutrons and thus atomic mass is approximately 12.

  • Some carbon atoms have an atomic mass of 13 and thus have six protons and seven neutrons.

  • All isotopes of the same element have about the same chemical properties because the number of electrons is the same

carbon atoms

Carbon Atoms

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  • Many elements have multiple isotopes, some of which may be radioactive.

  • Radioactive isotopes are called radioisotopes and are characterized by a constant rate of decay into other elements or isotopes.

  • Some common isotopes in biology and their half-lives are here.



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Some Commonly used Isotopes

Some Commonly Used Isotopes

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  • The half-life of a radioisotope is the time in which it takes half of a starting amount of radioisotope to decay into something else.

  • This decay is accompanied by the release of various forms of radiation.

  • This is important for biologists because it allows them to use radioisotopes to trace the fate of elements in biological systems and to date fossils by measuring the ratios of different radioisotopes in the fossil.


1. Atoms that have the same atomic number but differ in atomic mass are called?

  1. Isotones

  2. Neutrons

  3. Element

  4. Isotopes

Answer: D. Isotopes

2. Which among the following denotes atomic number?

  1. Electron

  2. Nucleus

  3. Neutron

  4. Proton

Answer: D. Proton

#Atomic number and atomic mass


#Radio isotopes