GED Science Practice Test: Atom Structure

All atoms have the same basic structure, which includes three kinds of subatomic particles – protons, neutrons, and electrons. Protons, which have a positive charge, and neutrons, which have no charge, are located in the center of the atom, called the nucleus. The nucleus is surrounded by a cloud of negatively charged electrons which orbit the nucleus.  The electrons of an atom are bound to the nucleus by electromagnetic force that is caused by the attraction of the negatively charged electrons to the positively charged protons.


This information contained within each square in the periodic table can help you determine the number of protons, neutrons, and electrons that a particular element should have in its atoms:


The atomic number tells you the number of protons an element has.  The number of protons is what determines the actual element that you have.  For example, any atom with six protons will be carbon, while any atom with five protons will be boron.  In neutral, or uncharged atoms, the number of protons and electrons will be equal.  So a neutral atom of carbon has six protons, and thus, six electrons. The atomic mass in the periodic table describes the number of combined protons and neutrons.  So to determine the number of neutrons in an atom, you take the atomic mass and subtract the atomic number from it.  If it seems odd that the atomic mass of the atom does not consider the electrons, remember that the electrons are very small compared to the protons and neutrons.  Thus, 99.9% of the mass of an atom is contained within the nucleus, made of protons and neutrons.

In addition to knowing the number of protons, neutrons, and electrons an atom of an element has, it is very helpful to understand that the electrons of a particular element are located within different energy levels within an atom.  Some electrons are located at low energy levels, and some are located at high energy levels.  A commonly-used model to represent this idea is an atom with shells to represent energy levels that contain certain numbers of electrons.  Remember that models have limitations in how well they represent reality;  the following model shows these shells, but it is not as if electrons have fixed rigid locations:


What is important to understand about these shell diagrams, also called Bohr models to reflect Niels Bohrs’ work about the energy levels for electrons, is the number of electrons located in the outer shell, or valence shell, of the atom.  Notice in the above diagram, potassium has one valence electron, or one electron in its outermost shell.  Potassium is an example of an element found in the alkali metal group.  In the section describing the periodic table, we said that elements within a group share certain properties, and that the alkali metals were all highly chemically reactive.  The reason for this shared chemical property, and many chemical properties of elements, is due to the number of valence electrons in an atom.  Valence shells are considered unstable if they are not full, and most valence shells are full with eight electrons.  Potassium has one, so if it gets rid of one electron, its next shell in becomes full (with eight electrons), and potassium is more stable.  Getting rid of one electron is easy, so this is why the alkali metals are highly chemically reactive.

Ignoring the transition metals, the large group in the center of the periodic table, you can determine the number of valence electrons for a particular group of elements by counting across the columns.


Thus, the element nitrogen (N) has five valence electrons, while the element bromine (Br) has seven valence electrons.  Additionally, the period, or row, number, can tell you how many shells are in an atom of a particular element.  Using our same elements as examples:  nitrogen (N) has two shells , while bromine (Br) has four shells.  The number of shells also tells you something about the chemical reactivity; electrons in shells further away from the nucleus do not experience as much electromagnetic force, and can leave the atom more easily.  This means that the further down in the periodic table you go (the more shells), the more reactive an element is.  That pattern can be observed in the following diagram, again showing the alkali metals:


Chemical reactivity is just one property that can be predicted by patterns in the periodic table.  It is however, a very important one, as understanding valence electrons and chemical reactivity will help us to understand why certain compound are likely, or unlikely, to form from the combination of two or more elements.


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