READ: Isotopes and Ions

READ: Isotopes and Ions

Isotopes

As stated earlier, not all atoms of a given element are identical. Specifically, the number of neutrons in the nucleus can vary for many elements. As an example, naturally occurring carbon exists in three forms, which are illustrated in the picture below.

Each carbon atom has the same number of protons (6), which is equal to its atomic number. Each carbon atom also contains six electrons, allowing the atom to remain electrically neutral. However the number of neutrons varies from six to eight. Isotopes are atoms that have the same atomic number but different mass numbers due to a change in the number of neutrons. The three isotopes of carbon can be referred to as carbon-12 (  C ), carbon-13 (

 C ), and carbon-14 (  C ).     As you can see, another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen.

Atomic Mass

Have you noticed that the atomic mass of an element is rarely a whole number on the periodic table?  This is because of isotopes. The atomic mass of an element is the weighted average of the atomic masses of all the naturally occurring isotopes of that element.  Atomic masses are calculated by figuring out how many atoms of each type of isotope are out there in the universe. For carbon, there are a lot of C-12, a couple C-13, and a few C-14 atoms. When you average out all of the masses, you get a number that is a little bit higher than 12. Therefore, the atomic mass for carbon is actually 12.011. You are not required to calculate the atomic mass of an element for this course. In chemistry, you will be required to calculate the atomic mass. 

Ions

For a neutral atom the number of electrons (-) is the same as the number of protons (+), since the charge on the atom must balance. But what happens if an atom gains or loses electrons? Does it mean that the atom will still be part of the same element? A change in the number of electrons of an atom does not change the type of atom that it is. Remember that only changing the number of protons changes what element it is.  However, changing the number of electrons does change the charge of the atom. The neutrality of the atom has changed. If electrons are added, then the atom will become more negative. This is because electrons are negatively charged. Since you have more negative charges with more electrons the overall charge of the atom is negative. If electrons are taken away then the atom will become more positive. Again, this is because electrons are negative. If you get rid of negative charges then the overall charge of the atom is positive. The atom that is formed in either of these two cases is called an ion. An ion is a charged atom. For example: a neutral sodium atom can lose one electron to become a positively charged sodium atom (Na ⁺). A neutral chlorine atom can gain one electron to become a negatively charged chlorine ion (Cl ^-).

Another example is Li ⁺, Lithium ion. Lithium atoms have 3 protons and 3 electrons.  When Lithium loses 1 electron it now has only 2 electrons and 3 protons. How does it have a positive charge?  As you can see the Lithium ion now has more protons, which are positive, than electrons, which are negative.  Continuing, consider a Fluoride ion, F ^-. Fluorine atoms start with 9 protons and 9 electrons.  When Fluorine gains 1 electrons it now has 9 protons and 10 electrons.  How does it have a positive charge? As you can see the Fluoride ion now has more electrons, which are negative, than protons, which are positive.

Understanding how many electrons an ion has is going to be important for the next couple of modules in this course. Look at the following table for more examples about how to determine how many electrons an ion has.

Your Turn

Georgia Virtual, Atomic Theory and Periodic Table, CC BY-NC-SA 3.0

After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete