A period 1 element is an element in the first period (row) of the periodic table. The periodic table is arranged in rows to show repeating properties of the elements. When the atomic number increases, the element have different properties. A new row begins when chemical properties repeat. It means that elements in the same group have similar properties. The first period has less elements than any other periods in the periodic table. There are only two elements in the first period: hydrogen and helium. We can explain why "there is less elements in the first row" in modern theories of atomic structure. This is because in quantum physics, this period fills up the 1s orbital. Period 1 elements follows the duet rule, they only need two electrons to complete their valence shell. These elements can only hold two electrons, both in the 1s orbital. Therefore, period 1 can have only two elements.