The Atom

Bohr Diagram

Composition of the Atom

The atom is the smallest particle of an element that keeps the properties of that same element. Within the atom there are many different types of subatomic particles but since there are so many no theory of atomic structure can include all of them. In this case chemists focus on only three of them. They are electrons, protons, and neutrons. The electron is a negatively charged subatomic particle. It occupies most of the area outside the nucleus; the central most dense part of the nucleus which is made up of protons and neutrons. The proton is a positively charged subatomic particle. It is much heavier than the electron, 1840 times heavier to be exact. Also the neutron is much heavier than the electron so the nucleus will have a much greater mass than the electrons. The neutron is a subatomic particle with no charge. Even though it is neutral in charge, it weighs almost as much as the proton.

Atomic Number

The atomic number of an element indicates the number of protons and electrons the element contains within the nucleus of the atom. For example if you have the element carbon(C)it has an atomic number of 6. You can tell this because if you look at carbon on the periodic table you will see a large 6 in the top right hand corner of the square the element is in. This indicates the number of protons and electrons within the atom, thus giving you the atomic number.

Mass Number

The mass number of an element is the total number of protons and neutrons within the nucleus of an atom. Therefore if you have an atom of helium which has a mass number of 4 it would be four times as light as an atom of oxygen which has a mass number of 16. It can be calculated by rounding atomic mass of an element.

Atomic Mass

The atomic mass of an element is the weighted average of the masses of isotopes (atoms which contain the same number of protons but different number of neutrons, e.g. isotope hydrogen-2 would have a symbol of 21H, with the 2 indicating the mass number and the 1 indicating the atomic number) of that element.

Element Table

Element Symbol Atomic # Atomic Mass Mass # # Protons # Electrons # Neutrons
Potassium K 19 39.10 39 19 19 20
Sodium Na 11 22.99 23 11 11 12
Chlorine Cl 17 35.45 35 17 17 18
Mercury Hg 80 200.59 201 80 80 121

Main, Careers in Science, Perodic Table, MRHS Chemistry Faculty, Electron Configuration, The Atom,
Different Branches of Chemistry, Moles, Balancing Equations, Chemistry Definitions, Orbital Notation

Source: Text taken from the book, "Addison-Wesley Chemistry", Addison-Wesley Chemistry, SI Edition Copyright 1993 Addison-Wesley Chemistry Publisher Limited.