You know that atoms are made of protons, neutrons, and electrons. The protons and neutrons belong in the middle of the atom: the nucleus. The electrons whip around the nucleus at various distances. These distances are strictly prescribed:
Electrons can only have whole-number multiples of a certain basic amount of energy called the quantum energy. They can be found in shells around the nucleus that have limits on how many electrons can fit in each shell. Each shell is known as an energy level and each shell is farther away from the nucleus than the last. The farther away from the nucleus an electron shell is, the higher in energy it is. The chart shows the maximum number of electrons in each shell.
|Shell No.||No. of Electrons|
|This table is only completely accurate for atoms from hydrogen (H) through calcium (Ca). For atoms with more than 20 protons the electron structure gets more complicated.|
A chart showing the protons, neutrons and electrons present in each of the first ten elements appears below. Notice that for He and Ne the chart indicates that they haved filled shells. Helium and neon are two members of that group of elements called the Noble Gases. These elements almost never react with other elements to form compounds.
The outermost shell of electrons in the electron structure of an atom is called the valence shell. The valence electrons which are found in the valence shell are very important because they determine the properties and reactivity of an element. When atoms react chemically they typically react in such a way that when they are done, they have filled their valence shells. This can be accomplished by losing electrons to become an cation, by gaining electrons to become an anion, or by sharing electrons in what is called a covalent bond. Losing or gaining electrons, or forming a covalent bond are ways in which atoms minimize energy. Energy minimization is a key principle in our understanding of the physical world. For example, objects roll downhill, dissipating their gravitational potential energy as kinetic energy. Bits of metals move toward magnets, losing their magnetic potential energy and turning it into kinetic energy. When fuel burns, it releases chemical energy as heat and light: ashes have less energy in them than wood.
A cation (CAT-eye-on) is an atom or molecule with a positive charge and comes from losing electrons. An anion (AN-eye-on) is an atom or molecule with a negative charge and comes from gaining electrons. A covalent bond is when two atoms are bound together by the fact that they are sharing their valence electrons so that each atom in the bond has a full valence shell. Atoms tend to lose or gain the proper number of electrons so that their valence shell is filled. You will use this information to predict the probable charges of atoms.
One of the major reasons the periodic table is such a valuable tool is that it shows how the various elements are related to each other. If you have not already done so, ask your teacher to show you a periodic table colored by the groups of related elements. Color your own periodic table to match. The groups come about because elements in the same column have the same number of valence electrons. Even though Br has 18 more electrons than Cl the two elements have similar properties because in each element there are seven electrons in the outermost, valence, shell of electrons. All halogens have seven valence electrons.
Fill in the following table with the number of electrons in each shell. Identify the valence shell of each element by circling the number of electrons you write down in the valence shell. Also, place a star in the boxes where the outermost shell is a filled shell.
Atoms of an element that are electrically neutral have an overall charge of zero. That means that for an electrically neutral atom, the number of electrons equals the number of protons. For example, a neutral atom of carbon (6 p+) has six electrons.
|Element||No. of p+||Total
No. of e-
|Shell 1||Shell 2||Shell 3||Shell 4|
Atoms are not always neutral and when they are not, they are called ions. Atoms can become ions only by gaining or losing electrons. In chemical reactions atoms never gain or lose protons or neutrons. If an atom gains an electron it becomes an anion. If it loses an electron it becomes a cation. You symbolize this by using the atomic symbol followed by a superscript showing the charge. Some cations: Na+, Mg2+. Some anions: O2–, F–. The number is shown first, then the charge.
So if a neutral carbon atom loses one electron this equation would read 6 – 5 = +1 and its symbol becomes C+. If a C atom gains four electrons the equation would read 6 – 10 = –4 and its symbol becomes C4–. The key to understanding how to find the charge on an ion is to always remember that an atom of any element always has the same number of protons.
Write the ionic symbol and calculate the charge on the following atoms. Write whether the ion is a cation or an anion.
For the following elements, predict the charge they will have based on what you know about the number of valence electrons in a neutral atom of that element. Remember, atoms gain or lose electrons so that they end up with a full valence shell. This tendency is sometimes called the octet rule because full shells have eight electrons. (Except for Shell 1).
Lewis Dot Diagrams