Which of the following is an alkali metal? A) Radium. B) Cerium. C) Cesium. D) Lanthanum. Or E) Calcium.
The first thing we need to do is define an alkali metal. An alkali metal is a member of group one on the periodic table. Alkali metals get their name from the fact they form alkaline solutions upon reaction with water. However, the modern definition relies on their presence in group one. Let’s bring up our periodic tables. Group one is the first column of the periodic table and can be found here. Hydrogen is the first member of group one. But hydrogen is not a metal and, therefore, is not an alkali metal.
Now that we have the periodic table, we can look for each element and see which one lies in group one. Radium has the symbol Ra and sits at the bottom of group two. Since radium is not in group one, it is not a correct answer. Meanwhile, cerium has the symbol Ce and sits in the second position in the lanthanides. So, cerium is not in group one, and cerium is an incorrect answer.
What about cesium with symbol Cs? Cesium is second from the bottom of group one. Since cesium is a metal in group one, it is an alkali metal and is, therefore, the correct answer. Going back to what we’ve said about alkali metals forming alkaline solutions upon reaction with water, cesium reacts with water to produce a solution of cesium hydroxide and hydrogen. Cesium is a base with the OH− group that produces alkaline solutions. So, we can be pretty confident that cesium is the correct answer.
But just to be safe, I’m going to go through the other two options. Lanthanum has the symbol La and is the first element in the lanthanides. Lanthanum is not in group one, so it’s not a correct answer. Finally, calcium has the symbol Ca. Calcium sits in group two of the periodic table. It’s not in group one. Therefore, it’s not an alkali metal, and calcium is an incorrect answer.
Another way of doing this would be to list all the alkali metals, lithium, sodium, potassium, rubidium, cesium, and francium, and finding which one overlapped with the options. Three of the options were elements with symbols that began with C. Be careful when looking at the periodic table and make sure the name as well as the symbol matches up. So, whichever route you took, of the five options given, the one that was an alkali metal was cesium.