IONS AND ISOTOPES

Unit Introduction

To build upon what you learned about atoms in the last unit, you will now investigate the result of atoms gaining or losing subatomic particles and how that affects the identity and characteristics of the atoms.

Atomic Number and Atomic Mass

Each element’s atoms has a different number of protons, as you may recall from the last unit. The number of protons in an atom is indicated on the periodic table by the element’s atomic number. The atomic number is also equal to the number of electrons in a neutral atom. The number of protons and neutrons in the nucleus of an atom is equal to the atom’s atomic mass. The atomic mass for each element is also indicated on the periodic table.

atomic number = # of protons OR # of electrons

atomic mass = # of protons PLUS # of neutrons

Here are some examples of periodic squares:

Element:	Lithium	Neon	Titanium
Periodic Square:
Atomic Number:	3	10	22
Atomic Mass:	6.941	20.1797	47.867

Isotopes

You may have noticed that the atomic numbers are whole numbers, but often the atomic masses of elements are decimal numbers. What does that mean, exactly? You know that the atomic mass is equal to the number of protons and neutrons in an atom. But there aren’t partial subatomic particles causing this number to be a decimal. Where is the decimal coming from?

Sometimes, different versions of an element can exist in nature (or in a lab-created setting). The different versions have different numbers of neutrons in the nucleus. These different versions of the element are called isotopes. An isotope is an atom of an element with a different number of neutrons. The atomic mass of an element is determined by averaging the isotopes according to their prevalence. For example, the element hydrogen has 3 isotopes, including protium, deuterium, and tritium. Hydrogen has the atomic number 1, meaning hydrogen has 1 proton and 1 electron. Protium has 0 neutrons, deuterium has 1 neutron and tritium has 2 neutrons.

Element	Isotope	Protons	Neutrons	Electrons	Mass
Hydrogen	Protium	1	0	1	1
Hydrogen	Deuterium	1	1	1	2
Hydrogen	Tritium	1	2	1	3

The atomic mass is calculated by averaging each isotope’s abundance and mass.

Hydrogen’s atomic number = 1

Average atomic mass = 1.00794

Based on hydrogen’s atomic mass, which isotope do you think is the most common? Since the atomic mass is 1.00794, which is very close to 1, the most common isotope must be protium, which has zero neutrons.

Here is what each hydrogen isotope’s atomic structure looks like:

Watch the following video clip to learn more about atomic number, atomic mass, and isotopes:

Ions

Atoms are electrically neutral when they have the same number of protons and electrons. For example, carbon’s atomic number is 6. This means that carbon has 6 protons. If a carbon atom is electrically neutral, it must also have 6 electrons. What do you think happens if a carbon atom has more or less than 6 electrons? If you think it changes the electrical charge of the atom, then you are correct!

Imagine that a carbon atom has 7 electrons. If that carbon atom has 6 protons and 7 electrons, there are more negatively charged particles than positively charged ones. Therefore, the charge of that carbon atom is -1. Now predict what the charge would be of a carbon atom which has 6 protons but only 4 electrons. This particular carbon atom would have more positive charge than negative charge. If you predicted that the charge of this carbon atom is +2, then you are right.

When an atom has a positive or negative electrical charge due to an imbalance of protons and electrons, the atom is known as an ion.

The following image shows a hydrogen ion with a charge of -1. H, the chemical symbol for hydrogen, is followed by a superscripted negative sign to show the charge.

There are two types of ions that are defined by their electrical charges. A cation is an ion with a positive (+) charge. Cations have less electrons than protons. An anion is an ion with a negative (-) charge. Anions have more electrons than protons.

Valence Electrons

Remember the Bohr models from the last unit? Also called the planetary model, the Bohr model of an atom has energy levels known as electron shells where the electrons are drawn. The electrons fill the shells closest to the nucleus first, and the electron shells are populated from the center outward. If an element has a complete outer shell of electrons, it is stable and doesn’t usually react with other elements. If the outer shell of electrons is not full, these “leftover” electrons are known as valence electrons. The valence electrons are only the electrons in the outer shell, furthest from the nucleus. The first electron shell only holds 2 electrons, and then the next shell holds eight. The third shell will also hold eight. Most elements need 8 electrons in the outer shell in order to be stable. If an element has less than 8 valence electrons, it is unstable and reactive with other elements. An element will take electrons from other elements or lose electrons to other elements in order to become more stable.

For example, sodium (Na) has one valence electron. In the Bohr model below, you can see two electrons in the first shell (full), eight electrons in the next shell (full), and one electron in the valence shell. This makes sodium extremely reactive and unstable.

Chlorine (Cl) has seven valence electrons. In the Bohr model below, you can see two electrons in the first shell (full), eight electrons in the second shell (full), and seven electrons in the valence shell. This makes chlorine extremely reactive and unstable.

Since sodium has one valence electron and chlorine has seven valence electrons, sodium has the tendency to give up its one electron to chlorine. This is the lowest energy way to make both elements more stable. That way, sodium has no valence electrons and chlorine now has 8 electrons in the outer shell, meaning it has no valence electrons either. Sodium becomes an ion with a positive charge, or a cation. Chlorine becomes an ion with a negative charge, or an anion. This “donation” of an electron creates a chemical bond between sodium and chlorine to make the compound known as sodium chloride.

PhET Simulation: Build an Atom

If you are having problems using the simulation in the unit open up the simulation at the following website and complete the activity. Submit your completed document as question #12 in the assessment portion of the unit.

https://phet.colorado.edu/sims/html/build-an-atom/latest/build-an-atom_en.html

Printable: PhET BUILD AN ATOM ACTIVITY

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