Modern Atomic Theory states that atoms are composed of:
1) protons - positively charged particles located within the nucleus.
2) neutrons - uncharged particles located within the nucleus.
3) electrons - negatively charged particles which orbit the nucleus.
Mass of sub-atomic particles:
|Particle||Actual mass||Mass in amu|
|proton||1.673 x 10-24 g||1 amu|
|neutron||1.675 x 10-24 g||1 amu|
|electron||9.11 x 10-28 g||0.005 amu|
Protons and neutrons are roughly the same mass. The actual masses of protons and neutrons are so small and the numbers are so awkward to use, that chemists made up a new unit. The atomic mass unit (amu) is the mass of a proton. Since a neutron is about the same mass, we consider the mass of the neutron to be 1 amu as well. Electrons, on the other hand, are considered to have virtually no mass, because the electronÕs actual mass is only 0.005 amu, which does not figure in the mass of the whole atom.
Relation to the periodic table
The periodic table holds a vast amount of information in its setup and the numbers it contains. Two of the numbers that are on almost all periodic tables are the atomic number and the atomic mass.
Atomic Number - tells the number of protons in an atom. In a neutral atom, the number of protons must equal the number of electrons. On the example below it is the whole number above the symbol for the element.
|Diagram courtesy of ChemCom, 4th Ed.|
Atomic Mass (Atomic Weight)- tells the total mass of the atom (protons and neutrons). Electrons are not included because they have virtually no mass. On the example above, it is the decimal below the symbol for the element.
In a given element, the number of protons must remain the same to keep it the same element. If the number of protons changes, the atom changes into a new atom. For example, if an atom has 6 protons, it is carbon atom, regardless of how many electrons or neutrons it has. If someone were to add a proton to the carbon, it would now have 7 protons, and be the new element, nitrogen.
Unlike protons, a given element can change the number of neutrons and electrons that it retains. Atoms of the same element with different numbers of electrons are called ions. Atoms of the same element with different numbers of neutrons are called isotopes.
When a neutral atom gains or loses one or more electrons, it acquires an electrical charge, and becomes an ion. If the atom loses electrons, it becomes more positive, and this is called a cation. (positive charge) If it gains electrons, it becomes more negative, and this is called an anion. (negative charge)
Charge of sub-atomic particles:
|Particle||Actual charge (Coulombs)||Easier Charge|
|proton||+1.602 x 10-19 C||+1|
|electron||1.602 x 10-19 C||-1|
Similar to the masses of subatomic particles, because the size of the charge is cumbersome to use when speaking of the charge of an ion, chemists commonly say that a proton has a charge of +1. Electrons commonly have a charge of Š1. When the charges are balanced (when the number of protons and electrons are equal), the atom is neutral, or has a charge of zero. When the charges are unbalanced, the atom is charged, or becomes an ion.
An ionÕs charge can be determined by the following formula:
Charge of ion = # of protons - # of electrons
Every atom of a given element has the same number of protons. However, like the electrons, the amount of neutrons in any atom of that element can be different. Two atoms that are the same element, but have different numbers of neutrons (and different masses) are called isotopes of that element. At right, you can see three isotopes of hydrogen. These three isotopes (labeled hydrogen-1, hydrogen-2 and hydrogen-3) all have 1 proton, making it hydrogen. Hydrogen-2 has one more neutron than hydrogen-1, giving it a larger mass. The chemical properties of each isotope are identical. The only difference is the physical property of mass caused by the different number of neutrons within the nucleus.
Isotopes of an element are typically listed by the name of the element followed by a mass number. The mass number tells the total mass of the element (protons + neutrons).
Mass Number = protons + neutrons
For example, carbon-14 has a total mass of 14. All carbon atoms have 6 protons, therefore the carbon isotope has 8 neutrons to bring the mass up to 14 (6 protons + 8 neutrons = 14).
Complete chemical symbols are a way to give all the information about an atomÕs protons, neutrons and electrons. A complete chemical symbol contains the chemical symbol, surrounded by the atomic number, the mass number of the isotope and the charge of the ion. A generic complete chemical symbol is as follows:
Example 1: If an atom were to contain 13 protons, 14 neutrons and 10 electrons, the complete chemical symbol would look as follows:
1) The atomic number, the lower left hand number, is equal to the number of protons. From the protons, the symbol can be determined. An atom with 13 protons is Aluminum, symbol Al.
2) The mass number, the upper left hand number of the symbol, is equal to the protons + neutrons. Mass number = 13 + 14, or 27.
3) The charge, the upper right hand number of the symbol, is equal to protons - electrons. Charge = 13 - 10, or +3.
Example 2: If the following complete chemical symbol were given:
it would possible to determine the protons, neutrons and electrons as follows:
1) The protons are always equal to the atomic number, which is the number listed in the lower left hand corner. The protons = 35.
2) The neutrons can be determined from the mass number, in the upper left hand corner of the symbol. The mass number = protons + neutrons, so 82 = 35 + n, therefore neutrons = 47.
3) The electrons can be determined from the charge, in the upper right hand corner of the symbol. The charge = protons - electrons, so -1 = 35 - e, so the electrons = 36.
Complete the following worksheet: Protons, Neutrons and Electrons
Links to more information/examples on Subatomic Particles and the Periodic Table
Bishop's Isotopic Notation
Science Help Online
Notation and Isotopes
Science Help Online - Elemental Notation and Isotopes