What Are Rows In The Periodic Table Called

The horizontal rows in the periodic table are called periods. There are seven periods in total, each corresponding to the highest principal quantum number of the elements in that row. As you move from left to right across a period, the atomic number increases by one, meaning each element has one more proton than the previous one. This also means the number of electrons increases, and they fill up the available electron shells in a specific order.

The first period contains only two elements: hydrogen and helium. These are the simplest atoms, with just one and two electrons respectively. The second and third periods each have eight elements, filling up the second and third electron shells. From the fourth period onward, the pattern becomes more complex due to the introduction of d-orbitals, which are part of the transition metals. This is why the fourth and fifth periods each have 18 elements, and the sixth and seventh periods have even more due to the inclusion of the f-block, or lanthanides and actinides.

The reason periods exist in this way is rooted in the quantum mechanical model of the atom. Electrons occupy specific energy levels or shells around the nucleus, and each shell can hold a certain maximum number of electrons. The filling of these shells follows the Aufbau principle, which states that electrons fill the lowest available energy levels first before moving to higher ones. This orderly filling is what gives the periodic table its structure and explains the recurring chemical properties observed in elements of the same group (the vertical columns).

For example, all elements in the second period have their outermost electrons in the second shell. As you move across the period, electrons are added one by one to this shell until it is full at neon. The next element, sodium, starts a new period because its outermost electron enters the third shell. This pattern continues throughout the table, with the length of each period determined by the number of electrons needed to fill the available subshells.

Understanding periods is crucial for predicting the chemical behavior of elements. Elements at the beginning of a period, like the alkali metals, are highly reactive because they have one electron in their outer shell that they readily lose. In contrast, elements at the end of a period, like the noble gases, are very stable and unreactive because their outer shells are full. This trend in reactivity and other properties repeats with each new period, which is a key reason why the periodic table is such a powerful tool in chemistry.

The concept of periods also helps explain the existence of the lanthanides and actinides. These elements are placed separately at the bottom of the periodic table to keep it from becoming too wide, but they actually belong in periods 6 and 7. Their electrons are filling the f-orbitals, which can hold up to 14 electrons, leading to the extended length of these periods.

In summary, periods in the periodic table are the horizontal rows that group elements by the highest energy level of their electrons. The number of elements in each period reflects the way electrons fill available orbitals, following the rules of quantum mechanics. This structure not only organizes the elements but also reveals the underlying principles that govern their chemical properties and reactivity.