Hydrogen is the first element on the periodic table of elements. The periodic table is designed so that elements with like properties are in the same column. What makes the elements similar is the fact that all of the ones in the same column have an equal number of valence electrons. Since hydrogen is the very first element in the table, the four elements with the most similar properties to hydrogen will be the next four down the column.

Lithium is the lightest metal in the world. It is often used in applications for aircraft for this reason. It is also a common element in batteries. Lithium also has the highest specific heat of any known element. This makes it valuable for heat transfer situations.

The element sodium is most well-known for its relation to table salt, which humans use abundantly in food preparation and preservation. The combination of sodium and chloride, which is table salt, is the most abundant element on the Earth. True, pure sodium is not found naturally. It has a violent reaction with water and can actually ignite under some circumstances. Vapors from sodium is used to create yellow lights. In its liquid form, sodium can actually act as a high-grade coolant. It is so effective at this that it is used to cool down nuclear reactors.

Potassium is another element that is present all over the Earth as a combination but is not found in its pure state in nature. Potassium is a metal that the human body uses as a vitamin. In humans, it counteracts the effects of too much salt in the diet. When combined with hydrogen, it creates a gas that is highly flammable. It is so flammable that it has to be stored in mineral oil to prevent ignition.

Rubidium is an element that begins as a side effect of creating the element lithium. Rubidium is a metal that is easily ionized. This makes it a practical choice for developing photocells. Rubidium may eventually be used to propel spacecraft through ionization. Despite being a known element since 1861, rubidium is still not completely understood.