ISOTOPES

Unit Overview

In this unit we will explore isotopes, how they are used, and how they are written.

Isotopes

All matter is made of atoms. Until the late 1800’s scientists believed that atoms were the smallest particles to exist. Atoms are actually composed of subatomic particles called protons, neutrons and electrons. The nucleus, the centrally located part of the atom containing over 99% of the mass, consists of protons and neutrons. The area surrounding the nucleus is called the electron cloud. The electrons travel in three dimensional geometrically shaped orbits at such a high rate of speed that they effectively occupy the entire volume of the electron cloud.

The number of protons an atom has in its nucleus is known as the ATOMIC NUMBER. This number is unique to each different kind of atom and is the whole number found for each element on the periodic table (www.periodictable.com). You will want to download and print off a copy of the table now if you don’t have a copy.

Printable version of the Periodic Table

You should remember that atoms are neutral, meaning; they have no overall positive or negative charge. Therefore, the number of protons always equals the number of electrons. For instance, calcium’s atomic number is 20. This means it has 20 protons and 20 electrons. Iron’s atomic number is 25. Therefore it has 25 protons and 25 electrons.

Atomic Number (00:47)

You will also see an atomic mass or weight printed in each block for the elements. This will not be a whole number in most cases. The atomic masses shown in parentheses are estimates, indicating that the element is RADIOACTIVE and is spontaneously emitting particles thus changing its mass.

Radioactive Elements

All the atoms of a particular type of element do not necessarily have the same mass. Even though the number of protons and electrons always stays the same for a particular type of element, the number of neutrons may differ. Atoms of the same element having the same number of protons and electrons but different numbers of neutrons are known as ISOTOPES.

Isotopes (01:11)

The atoms of elements like aluminum, fluorine, gold and phosphorus exist in only one form. Tin, however, exists in ten different forms, each differing only in mass due to the different numbers of neutrons.

Most elements exist only in two or three different forms. There are three isotopes of hydrogen:

ISOTOPES	PROTONS	NEUTRONS	ELECTRONS
Protium: Hydrogen-1	1	0	1
Deuterium: Hydrogen-2	1	1	1
Tritium: Hydrogen-3	1	2	1

(Notice that the number of protons and electrons stays the same, only the number of neutrons varies.)

There are three different isotope notations. The first is the element’s name followed by a hyphen and it’s MASS NUMBER (Ex. carbon-14). The mass number is the sum of the protons plus neutrons in the nucleus. The second notation simply uses the element’s atomic symbol followed by its mass number (Ex. C-14). The third notation is known as the NUCLEAR NOTATION. It uses the symbol for the element with its mass number written to the upper left and its atomic number written to the lower left:

Any of the notations may be used to determine the number of protons, neutrons and electrons that the atoms of that isotope may contain. The first two notations require the use of the periodic table to obtain the element’s atomic number. This represents the number of protons as well as the number of electrons. Example: zinc-65. The atomic number of zinc is 30 so it has 30 protons and 30 electrons. The atomic number is then subtracted from the mass number to determine the number of neutrons.

	PROTONS	NEUTRONS	ELECTRONS
Zinc-65	30	65-30 = 35	30
Cl-35	17	35-17 = 18	17
P-31	15	31-15 = 1	15

What do you know about the Periodic Table?

The periodic table is not needed to determine particle counts when the nuclear notation is used. The lower number (atomic number) represents the protons as well as the electrons. The difference between the upper number (mass number) and lower number (atomic number) represents the number of neutrons.

Example:

	PROTONS	NEUTRONS	ELECTRONS
	6	12-6=6	6
	92	238-92=146	92
	11	23-11=12	11

Reading the Periodic Table

PRACTICE ACTIVITY

The following periodic table is missing a few parts. The cells are all present in the usual periodic table format. The group and period numbers are labelled and a nice solid line separates the metals from the metalloids and nonmetals. Test your knowledge of the elements by filling in the missing elements.

Printable Blank Periodic Table

Now answer question 1 through 34.