An electron is a negatively charged subatomic particle that can be either bound to an atom or free (not bound). An electron that is bound to an atom is one of the three primary types of particles within the atom -- the other two are protons and neutrons. Together, electrons, protons and neutrons form an atom's nucleus. A proton has a positive charge that counters the electron's negative charge. When an atom has the same number of protons and electrons, it is in a neutral state. Electrons are unique from the other particles in multiple ways. They exist outside of the nucleus, are significantly smaller in mass and exhibit both wave-like and particle-like characteristics. An electron is also an elementary particle, which means that it is not made up of smaller components. Protons and neutrons are thought to be made up of quarks, so they are not elementary particles. In the early days of atomic study, scientists believed that an atom's electrons circled the nucleus in spherical orbits at specific distances, much like planets circle a sun. In this model -- referred to as the Bohr model -- the orbits furthest from the nucleus contain the greatest amount of energy. When an electron jumps from a higher energy orbit to a lower energy orbit, the atom releases electromagnetic radiation. The Bohr model is no longer thought to be accurate, particularly as it pertains to how the electrons orbit the nucleus. While the model can still be useful in understanding the basics of electron distribution and different energy levels, it fails to consider the complexity of that distribution and how electrons inhabit the space around the nucleus, according to current quantum theory. Electron movement is determined by calculating the probability of finding electrons in specific regions within the space that surrounds the atom's nucleus -- rather than by assuming fixed trajectories. The mathematically defined regions are based on three structural patterns: An atom's shells are numbered consecutively, starting at the nucleus and working out. A shell's number is often referred to as its n value. For example, the third shell might be referred to as n=3 or 3n. Letters are also sometimes used to refer to the shells. These include K, L, M, N, O, P and Q, again starting from the nucleus and working out. For instance, the third shell might be referred to as the M shell or 3m.
Each shell contains one or more specific types of subshells, which determine the maximum number of electrons that the shell can contain. For example, the first shell (K) contains a single s subshell that includes only one s orbital. As a result, the maximum number electrons that the shell can contain is two. This means that an atom that has only a K shell is limited to two electrons. Only two elements, hydrogen and helium, have a single shell. Hydrogen contains only one electron and helium contains two.
The subshell/orbital configuration varies from one shell to the next, growing more complex until the fifth shell, at which point the complexity starts to taper off. For instance, the second shell (L) includes an s subshell and a p subshell. The s subshell contains one s orbital, and the p subshell contains three p orbitals. This means the shell can support up to eight electrons.
However, an atom with an L shell also contains a K shell. In fact, the L shell will start filling up after the K shell is filled. This means that an atom with an L shell can support up to 10 electrons because of the presence of both the K and L shells. For example, lithium and neon contain both K and L shells. A lithium atom has only three electrons, two in the K shell and one in the L shell, but a neon atom has 10 electrons, two in the K shell and eight in the L shell.
In general, this same pattern continues for all seven shells, with the inner shells filling up with electrons before the outer shells. However, this is only a tendency. Electrons gravitate toward the most stable configuration, which is usually the inner shells, but it's also possible for an outer shell to start filling up with electrons before the lower shell is completely full.
Regardless of the order in which shells fill with electrons, the shells themselves determine the maximum number of electrons they can support based on their subshells and orbitals. All but the first shell includes a p subshell, only the third through sixth shells contain d subshells, and only the fourth and fifth contain f subshells. All seven shells include an s subshell.
In electrical conductors, current flows as a result of electrons jumping from atom to atom as they move from negative to positive electric poles. In semiconductor materials, current also results from electron movement, however, the movement is based on electron deficiencies in atoms. An electron-deficient atom in a semiconductor is called a hole. In this case, the current moves from positive to negative electric poles.
The charge of a single electron is referred to as the unit electrical charge. It carries a negative charge that is equal to but opposite the positive charge on a proton or hole. However, the amount of electrical charge is usually not measured on a single electron because that amount is so small.
Instead, the standard unit of electrical charge is the coulomb (symbolized by C). A coulomb contains about 6.24 x 1018 electrons. An electron's charge (symbolized by e) is about 1.60 x 10-19 C. The mass of an electron at rest (symbolized by me) is approximately 9.11 x 10-31 kilograms (kg). If electrons are accelerated to nearly the speed of light, as in a particle accelerator, they will have greater mass because of relativistic effects.
See also: Table of Physical Units, clean electricity, electrical pollution, electric grid, volt per meter, electron rest mass, ion, electric charge
So now you've become a sodium ion. You have ten electrons. That's the same number of electrons as neon (Ne). But you aren't neon. Since you're missing an electron, you aren't really a complete sodium atom either. As an ion you are now something completely new. Your whole goal as an atom was to become a "happy atom" with completely filled electron shells. Now you have those filled shells. You have a lower energy. You lost an electron and you are "happy." So what makes you interesting to other atoms? Now that you have given up the electron, you are quite electrically attractive. Other electrically charged atoms (ions) of the opposite charge (negative) are now looking at you and seeing a good partner to bond with. That's where the chlorine comes in. It's not only chlorine. Almost any ion with a negative charge will be interested in bonding with you.
Don't get worried about the big word. Electrovalence is just another word for something that has given up or taken electrons and become an ion. If you look at the periodic table, you might notice that elements on the left side usually become positively charged ions (cations) and elements on the right side get a negative charge (anions). That trend means that the left side has a positive valence and the right side has a negative valence. Valence is a measure of how much an atom wants to bond with other atoms. It is also a measure of how many electrons are excited about bonding with other atoms.
There are two main types of bonding, covalent and electrovalent. You may have heard of the term "ionic bonds." Ionic bonds are electrovalent bonds. They are just groups of charged ions held together by electric forces. Scientists call these groups "ionic agglomerates." When in the presence of other ions, the electrovalent bonds are weaker because of outside electrical forces and attractions. Sodium and chlorine ions alone have a very strong bond, but as soon as you put those ions in a solution with H+, OH-, F- or Mg++ ions, there are charged distractions that break the Na-Cl bond.
Look at sodium chloride (NaCl) one more time. Salt is a very strong bond when it is sitting on your table. It would be nearly impossible to break those ionic/electrovalent bonds. However, if you put that salt into some water (H2O), the bonds break very quickly. It happens easily because of the electrical attraction of the water. Now you have sodium (Na+) and chlorine (Cl-) ions floating around the solution. You should remember that ionic bonds are normally strong, but they are very weak in water.