Question

Explain why the rows in the periodic table become progressively longer as we move down the table. For example, the first row contains 2 elements, the second and third rows each contain 8 elements, and the fourth and fifth rows each contain 18 elements.

Short answer

Rows in the periodic table become progressively longer as we move down because higher energy electron shells with additional subshells (s, p, d, and f) become available and are able to accommodate more electrons, leading to a greater number of elements per row.

Step-by-step solution

Understanding Electron Shells

Elements are arranged in the periodic table according to their atomic number which is the number of protons in their nucleus. Each row of the periodic table corresponds to the filling of a quantum shell of electrons. The first shell is filled with the first two elements, hydrogen and helium.

Progression to Higher Shells

As we move to the second and third rows, we are filling the second and third shells, respectively. These shells have subshells called 's' and 'p', and can hold more electrons than the first shell: the 's' subshell can hold 2 and the 'p' subshell can hold 6, for a total of 8 electrons per shell.

Increasing Subshell Capacity

Rows four and five not only fill 's' and 'p' subshells but also start filling the 'd' subshell which can hold a total of 10 electrons. This increases the total capacity for these rows to 18 electrons.

Adding f Orbitals

Further down the table, the periodicity includes filling of the 'f' subshells which can hold 14 electrons, in addition to the 's', 'p', and 'd' subshells. This contributes to even longer rows in the lower parts of the periodic table.

Key concepts

Electron Shells

Electron shells are the paths that electrons follow around an atom's nucleus. Each shell can hold a specific number of electrons, and they fill up in a methodical way. The first shell, closest to the nucleus, can hold up to 2 electrons. As we move outward to the second and third shells, each one can contain more electrons due to the addition of subshells.

The arrangement of the periodic table reflects this shell structure. The elements in the first row correspond to the filling of the first shell, which is why there are only two elements in that row. As you progress down the table, the increasing number of electrons that later shells can hold explains why rows become progressively longer.

Atomic Number

The atomic number of an element is a crucial concept in understanding the periodic table. It represents the number of protons in an atom's nucleus, and it also corresponds to the number of electrons in a neutral atom. This number increases by one from one element to the next, providing a clear and orderly progression.

The atomic number is what determines an element's position on the periodic table. As we ascend in atomic number, we fill the electron shells in what's known as the quantum shell filling order, leading to the observed pattern in the periodic table's structure.

Quantum Shell Filling

Quantum shell filling describes how electrons populate an atom's shells and subshells according to the rules of quantum mechanics. Electrons fill the lowest available energy levels before moving to higher ones. The first two electrons will fill the first shell (the '1s' subshell), the next eight electrons fill the second shell ('2s' and '2p' subshells), and so forth.

This filling order isn't just sequential but also considers subshell energies, which is why not all electron shells have the same capacity. As atomic numbers increase, electrons begin to fill the subsequent 's', 'p', 'd', and 'f' subshells following the quantum mechanical rules, resulting in the pattern we see in the periodic table.

Subshells Capacity

The capacity of subshells is a topic that explains why certain rows in the periodic table can accommodate more elements. Subshells are divisions of electron shells and come in four types: 's', 'p', 'd', and 'f'. Each type of subshell has a different capacity: 's' can hold 2 electrons, 'p' can hold 6, 'd' can hold 10, and 'f' can hold 14.

This increase in subshell capacity corresponds to the larger number of elements in the lower rows of the periodic table. As we move from the top of the table to the bottom, we begin filling the 'd' and 'f' subshells, which is why the fourth and fifth rows have 18 elements and further rows even more.

Periodicity in Chemistry

Periodicity in chemistry refers to the recurring trends that appear in the properties of elements as their atomic numbers increase. These periodic trends arise because of the shell and subshell structure of atoms. As elements are arranged by increasing atomic number, we notice patterns in their chemical behavior, reactivity, electron configurations, and other physical properties.

The lengthening of the rows on the periodic table is a prime example of periodic behavior relating to electron configuration. It shows how, as elements become larger and more complex, they require additional shells and subshells to house their increasing number of electrons. This underlying periodicity is what makes the periodic table such a valuable tool for predicting and understanding the behavior of elements.