5.    Which of the following characteristics is associated with electrons?
        a.  positive charge                        b.   located in the nucleus
        c.  mass equal to a neutron         d.  negative charge

This is a good example of the need to spend time memorizing the basic information.  You need to know the properties of the subatomic particles - protons, neutrons, and electrons. Make a table like the following and see if you can fill it out. You need to know everything in this table!

Subatomic Particle

Electrical Charge

Weight or Mass

Position in Atom

Number of Particles

proton

positive

1amu *

nucleus

Same for all atoms of a given element; each atom type has a specific number of protons; the number of protons an atom has is called its atomic number.

neutron

no charge

1amu

nucleus

Variable in atoms of the same element; thus, all atoms of a given element are not exactly the same; atoms of the same element with different numbers of neutrons in their nucleus are called isotopes.

electron

negative

0

energy levels or electron shells (2 electrons in the first shell, 8 in the second shell, and up to 18 in the third shell)

The number of electrons that an atom has is equal to the number of protons (atomic number) in its nucleus.  Thus an atom before it reacts is electrically neutral.

* amu = atomic mass units, a relative weight.  These particles have so little mass that the mass can't be evaluated in terms of grams or even micrograms. The amu is a relative weight.  Thus, protons and neutrons weigh about the same amount, and each proton and neutron is assigned a weight of one amu.  Compared to the weight of protons and neutrons, electrons don't have any weight.

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Emma Erdahl, Associate Professor of Biology
Northern Virginia Community College
Revised 8/23/00