The periodic table is an essential tool for understanding the properties of elements and their relationship to one another. It is a visual representation of the chemical elements arranged in order of increasing atomic number, which is the number of protons in the nucleus of an atom. The table is divided into rows and columns, with the horizontal rows being called periods.
There are seven horizontal rows or periods on the periodic table, labeled 1 through 7. Each period corresponds to a principal quantum number, which represents the energy level of the valence electrons in an atom. The valence electrons are the outermost electrons of an atom that participate in chemical bonding with other atoms.
The first period of the periodic table consists of only two elements, hydrogen (H) and helium (He). Both of these elements have only one electron in their outermost shell, which is the first energy level. Helium, however, has a completely filled first energy level with two electrons, which makes it a noble gas.
The second period of the periodic table contains eight elements, from lithium (Li) to neon (Ne). These elements have two energy levels, with the valence electrons in the second energy level. The valence electrons of these elements are involved in chemical reactions, and they determine the chemical properties of these elements.
The third period of the periodic table contains eight elements, from sodium (Na) to argon (Ar). These elements have three energy levels, with the valence electrons in the third energy level. The valence electrons of these elements are also involved in chemical reactions, and they determine the chemical properties of these elements.
The fourth and fifth periods of the periodic table each contain eighteen elements. The fourth period runs from potassium (K) to krypton (Kr), while the fifth period runs from rubidium (Rb) to xenon (Xe). These elements have four and five energy levels, respectively, with the valence electrons in the fourth and fifth energy levels. The valence electrons of these elements are also involved in chemical reactions and determine the chemical properties of these elements.
The sixth period of the periodic table contains 32 elements, from cesium (Cs) to radon (Rn). These elements have six energy levels, with the valence electrons in the sixth energy level. The valence electrons of these elements are also involved in chemical reactions and determine the chemical properties of these elements.
The seventh period of the periodic table is still incomplete and contains only 26 elements, with the heaviest element being oganesson (Og). These elements have seven energy levels, with the valence electrons in the seventh energy level. Due to the high number of energy levels, the valence electrons of these elements are less reactive and do not participate in chemical reactions as readily as the valence electrons of the elements in the lower periods.
In conclusion, the horizontal rows on the periodic table are called periods, and there are seven of them. Each period corresponds to a principal quantum number and represents the energy level of the valence electrons in an atom. The valence electrons of the elements in each period are involved in chemical reactions and determine the chemical properties of these elements. Understanding the periodic table and the relationship between the elements is fundamental to chemistry and is essential to many fields of science and industry.