Lesson 3 Ions and Isotopes
Objectives- Be able to identify atoms as ions or isotopes
So far we have assumed that protons equal electrons. In a neutral atom, that is true. But not all
atoms are neutral. As we will explore further in the next chapter, atoms can gain or lose electrons.
When that happens, the atom is no longer neutral, but become charged. A charged atom is called an
ion. When an atom gains electrons, it becomes negatively charged. The charge equals the number of
electrons gained. If an atom gains 1 electron, the atom has a 1- charge, gains 2, a 2- charge etc.
Negatively charged ions are called anions. Nonmetals are most likely to gain electrons
When an atoms loses electrons, it becomes positively charged. A positively charged ion is called
a cation. Like anions, the charge on the cation equals the number of electrons lost. Metals are most
likely to form cations.
Electrons are not the only part of the atom that can be different, neutrons can also vary. Before
we go into this, there are a few things we need to review. First, the number of protons will not change.
If they did, then an elements atomic number would change. The atomic number is what an element
identified by. Second the atomic mass is made up of the number of protons and neutrons. Both of
these have a mass of 1 amu. Take a look at the atomic mass of most all the elements. Most all the
atomic masses end in decimals. If the protons and neutrons both have a mass of 1, how could the sum
of those two equal a decimal?
The atomic mass is an average mass of a sample of atoms from an element. The reason the
atomic mass is a decimal is due to some of the atoms having different masses. The different masses are
due to different number of neutrons. These are called isotopes. Since the most common isotope will
affect the average mass the most, its mass will be the one on the table.
A good example to show this is Boron (B). Boron has 5 protons. It can have either 5 or 6
neutrons. The atoms will have a mass of either 10 or 11 amu’s. Approximately 80% of the atoms have a
mass of 11 and 20% have a mass of 10. With a little math, you can see why the average mass for Boron
is 10.8. Carbon-14 is an isotope. You may be familiar with C-14 from determining the age of once living
organisms.
Questions
Define Ion.
How does losing an electron affect an atom? Gaining an electron?
Define Isotope
How does being an isotope affect the atomic mass?
If the atomic mass is the number of protons and neutrons added together, why is the atomic mass a
decimal?
How can an element be identified as an isotope?
How many isotopes are there for hydrogen? What are their names and how many neutrons does each
have? What is the most common one?
Carbon has several forms. Is carbon-14 the most common isotope? If not, which one is?