Sodium was ‘discovered’ at least as a separate element, in 1807, though different properties of sodium and potassium salts were known long before.
The salt in seawater is mostly the chloride ion but about 1/3 of the mass of this salt is the sodium ion. Ions, by the way, are charged atoms; sodium has a charge of +1. This means the sodium ion, which begins its life having eleven negatively charged electrons, loses one of them thus becoming positive in charge. Its sister in seawater, chlorine, does the opposite, gaining an electron and becoming more negative (we call this the chloride ion now). If you took an empty two-liter soda bottle of and filled it with seawater and extracted the salts you would get 70 grams of salt, 20 grams of this would be sodium. If you’re metric-dyslexic one whole ibuprofen tablet - which has 300 mg or 0.3 g of drug – is about 4 g in total.
Not only is sodium a major component of seawater it is also a major component of our body fluid – which is very similar to seawater in many ways. The best-known connection of sodium and its friend chloride that most people know about is that of salt and high blood pressure. Table salt, sodium chloride (NaCl), you may have heard, is evil - linked to high blood pressure and thus heart attacks and stroke. Though we have heard this for years the evidence is actually much more tenuous than believed. Lowering salt intake seems to help some people, at least temporarily, but others ...not so much. However, considering how much salt the average North American has at hand watching your salt intake seems an appropriate measure just in case.
Amazing things animal do with sodium.
It turns out we need sodium – without it the neurons of our body would not be able to function. The nerves of our body are made up of many individual cells – the neurons. The number of neurons in a nerve varies greatly from thousands to tens of thousands or more. Our spinal cord is made up of estimated 1 billion neurons. All along each of these neurons are protein channels. There are many types of protein channels, some we call pumps because they use energy to transport materials ‘up’ from an area of low concentration to an area of high concentration. One of the first described and immensely important pumps is the sodium potassium pump. Jens Skou discovered the enzyme that runs this pump in the 1950s (and shared the 1997 Nobel Prize in Chemistry because of this). This pump takes 3 sodium ions and 2 potassium ions and exchanges them across the membrane of all our cells including the neurons. The sodium is pushed out and the potassium goes inside. Imagine this pump running for several minutes exchanging 3 for 2, now multiply that by, say, 100 pumps along a section of neuron. You will start to get the idea that with even a short amount of pump activity the outside of the neuron has many thousands of sodium ions compared to the inside, and conversely the inside has many more potassium ions. Now let us consider the electric charge; both sodium and potassium ions carry a charge of +1. The Na+/K+ pump is swamping these ions unequally, 3 out for 2 in. This leaves a neuron with a positive charge outside compared to inside, or a net negative charge inside - we have measured this inside charge and it turns out to be -70 millivolts. This is low compared to your nearest light-socket which carries 120 volts (or 120,000 millivolts) but we don’t need to light the room with our nerves. Something even more fascinating happens with this difference in charge across the neuron membrane than lighting a light bulb.
When a neuron gets a signal to “fire” the sodium allows the signal to be passed all along the neuron to the gap (called the synapse) between it and the next neuron. This happens via other proteins, not pumps this time but instead simply channels, channels that allow sodium to move into the neuron. Imaging putting your hand on a hot stove: The neurons in your hand need to get a signal to your spinal cord and the cord will interpret and send a signal back to the muscle of your arm yelling in effect ‘MOVE YOUR HAND’. So we need to get a signal up your arm and back down very fast, so fast that you don’t burn yourself too badly. All along your neurons in the pathway up to the spinal cord and back are neurons with millions of sodium ions outside just waiting for a chance to move into the cell. Now a channel opens that allows them to pass in – very quickly sodium will move into the cell, very quickly doesn’t really do justice to how fast this happens. The most interesting thing about this occurrence is that the first set of channels to open cause the next set of channels to open. The channels along the long part of a neuron, called the axon, are voltage regulated – that is they open in response to a change in the electric nature of the inside of the neuron. When sodium ions rush in they make the inside more positive – from -70mV to +30 mV in fact – the next set of channels down the line open and more sodium ions rush in. In this way the signal gets to the end of one neuron and gets set to pass on to the next neuron.
I left out an important part of this process that allows our neurons to speed this whole process up, its called saltatory conduction. Saltatory conduction in effect allows the voltage change from the sodium input in one area to jump – about 1 mm –down the neuron to the next gap (a node) in the coating of our neurons. The coatings of our neurons are called myelin. Thus the process of getting a signal from our hand to spinal cord and back can be as fast as 190 meters per second (390 feet per second); it would take about 0.006 seconds (6 thousandths of a second) for info to pass to our spinal cord and back to our hand.
If something gets ion the way of this process, we can be screwed. For example, some toxins disrupt our neurons (neurotoxins). The toxin from puffer fish for example. This toxin, called tetrodotoxin after this group of fishes (the order Tetraodontiformes) blocks the action of sodium channels. Don’t eat, or get stuck by, these fishes.
BTW: The symbol for sodium is Na because it comes from the latin natrium. Which, in turn, comes from a powder substance that has lots of sodium carbonate in it called natron because of the valley in which it was found, Wadi El Natrun in Egypt.
Read this stuff:
https://www.drugabuse.gov/publications/teaching-packets/power-science/section-i/3-how-neuron-works
Nelson, J.S. 1994. Fishes of the world. Third edition. John Wiley & Sons, Inc., New York. 600 p.
Weeks, M.E. 1932. The discovery of the elements. IX. Three alkali metals: Potassium, sodium, and lithium. Journal of Chemical Education. 9: 1035.