Why do atoms always contain the same number of electrons and protons?
Category: Chemistry Published: June 7, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Atoms do not always contain the same number of electrons and protons, although this state is common. When an atom has an equal number of electrons and protons, it has an equal number of negative electric charges (the electrons) and positive electric charges (the protons). The total electric charge of the atom is therefore zero and the atom is said to be neutral. In contrast, when an atom loses or gains an electron (or the rarer case of losing or gaining a proton, which requires a nuclear reaction), the total charges add up to something other than zero. The atom is then said to be electrically charged, or "ionized". There is a major difference between the neutral state and the ionized state. In the neutral state, an atom has little electromagnetic attraction to other atoms. Note that the electric field of a neutral atom is weak, but is not exactly zero because the atom is not a point particle. If another atom gets close enough to the atom, they may begin to share electrons. Chemically, we say that the atoms have formed bonds.
In contrast to neutral atoms, the field due to an ionized atom is strong, even at larger distances. The strong electric field of ions makes them strongly attracted to other atoms and molecules, to the point of being highly chemically reactive. Ionized atoms can be free radicals, which are atoms with a dangling bond that are highly reactive. In the human body, free radicals can react with DNA, leading to mutations and possibly cancer. Atoms become ionized when light with enough energy knocks off some of their electrons. Only light waves at the frequencies of X-rays and gamma rays have enough energy to ionize atoms and therefore lead to cancer. The cancer-causing power of only certain frequencies is why you can use your cell phone as much as you want, but you can only get an X-ray image taken on rare occasions. Free radicals occur naturally in your body. They only become dangerous when there are more free radicals than your body can handle.
But not all ions in the body are bad. Because of the charged nature of ions, the human body makes use of them to pass electric signals through nerves. The body also uses ions to control fluid levels and blood pressure. The most-used ions in the human body are sodium, potassium, calcium, magnesium and chloride. Ions are also created whenever you electrostatically charge an object, such as when you rub a balloon on your hair. For this reason, your clothes dryer machine can be thought of as an ion maker. As clothes rub together in the machine, electrons get knocked from one atom to another. The result is the all-too familiar static cling. Electricity and strong electric fields do a good job of creating ions (think lightning).
The neutral state of an atom is typically the most stable configuration (unless molecular bonds and the chemical environment complicates the picture), so ions tend to discharge and return to their neutral state over time. The reason for this is that, as an ion, the atom has a strong electric field that attracts the needed electron or the needed atom to take its extra electron. But once the atom becomes neutral, it has an equal number of electrons and protons, it does not have a very strong field, and therefore has little possibility of changing.