Some Background on The Alkali Metals
The alkali metals are probably most commonly remembered by people as 'that fun bit from chemistry'. The experiments involving the first three of the alkali metals (Consisting of cutting a sliver of one of the soft metals and dropping it into a small body of water.) are likely the most visually impressive seen in high school science lessons. I feel it is only right to honour these inspiring reactants with some information on each of them.
Firstly, when we say 'alkali metals', we mean all of the elements (Excluding hydrogen sitting at the top.) on the far left of the periodic table. In order from lowest atomic mass to highest, these are: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr). If you chose not to pursue the field of Chemistry further than high school, you likely only recognise the first three, lithium, sodium and potassium. If you can cast your mind back to the reaction of some water and an alkali metal, you may remember that lithium wasn't very reactive, sodium was a bit more reactive, and potassium was very reactive. You will notice as you travel down the alkali metals, the reactivity of each respective metal increases. This is to do with the fact that when an alkali metal reacts with something else (In this case, water.) it loses it's outermost electron. As you travel down the periodic table, the amount of electrons of an atom increases, and thus the force of attraction exerted on the outermost electron by the nucleus decreases, making it easier to lose. For those interested, an opposite effect can be seen with halogens, as they must gain an electron.
Not to get sidetracked by the halogens, let's look at the first alkali metal; lithium. Lithium has an atomic number of 3, it has 4 neutrons and 3 protons and 3 electrons. It was discovered in 1817 by Johan August Arfwedson in Sweden. As remarked before, it is the least reactive of the alkali metals, and can even be cut with a knife! If you were to get a sample of lithium and cut it with a knife (Taking appropriate safety precautions, of course.) you would notice that the newly exposed face reacts quickly with the air, quickly turning dull. It's reaction with water is a lot more exciting, it fizzes and hisses while popping about the container. If you were to drop some universal indicator in beforehand, you'll notice the lithium leaves behind a deep purple colouring. This is actually where the alkali metals earn their name, as when they react with water they leave behind a very alkaline solution. Without a doubt lithium's most helpful use is its role in lithium-ion batteries, which hold the power of the modern world.
Next is sodium, which keeps with the alkali metal tradition in being more reactive than lithium. It has atomic number 11, with 11 protons, 11 electrons and 12 neutrons. It will turn dull faster when in contact with the air and will react more violently with water. Sodium is used just as much as lithium in daily life, in the form of salt (Or sodium chloride.), or as a coolant for some nuclear reactors when in liquid form (Between 98 and 883 degrees celsius; which admittedly seems hot for a coolant, but nuclear reactors can run a lot hotter than 93 degrees.).
Potassium is the next alkali metal, with 19 protons, 19 electrons and 20 neutrons. Potassium's reaction with water is leagues more visually awe-inspiring than the first two, reacting violently enough to ignite the hydrogen it releases during the reaction. I have a particular memory from one chemistry lesson wherein a classmate likened it to an 'atomic goldfish'. Potassium ions are essential to human life, most importantly in the transmission of signals across cells. Potassium is also a vitally important fertiliser, with crop production even doubling with the use of potassium and phosphorous based fertilisers! Like most resources on Earth, potassium is in limited concentrated supply; with some experts estimating that we may have already hit or be about to hit peak potassium/phosphorous.
Rubidium follows potassium, with 37 protons and electrons respectively and with 48 neutrons. Rubidium is so reactive that it must be stored in a petroleum jelly or else it will ignite in air. Rubidium does not play a biological role like potassium or sodium do, but it is used in the function of ultra-accurate atomic clocks. This is because a second is exactly equal to 6,834,682,610.9 (To 1 decimal place.) oscillation of the outermost electron of a rubidium atom. By counting these oscillations an atomic clock can provide extreme accuracy.
Getting into the extremely reactive range now with caesium. Caesium also has to be stored in jelly or oil and will even explode in water, in fact it has been shown with a high-speed camera to become transparent during its aggressive reaction with water.It has 55 protons, 55 electrons and 78 neutrons. Like the element above, it is also used in atomic clocks for precision time-keeping. Caesium is incredibly rare, with only around 18,000 kilograms being produced annually.
Following tradition to the extreme is francium, with an extremely large atom of 87 protons, 87 electrons and 136 neutrons. The explosion francium would produce upon contact with water would be magnitudes larger than caesium. The properties of this alkali metal aren't particularly well-documented as it is much more difficult to study than the other alkali metals, due to it being so radioactive that within at least 22 minutes half of it would have decayed. It cannot be found naturally occurring on Earth and can only be produced with incredibly expensive machinery for international research collaboration projects.