Question

List the number of elements in periods 1 and 2 of the periodic table. Why does each period have a different number of elements?

Short answer

Period 1 has 2 elements, and period 2 has 8 elements. This is because the number of available subshells increases with higher energy levels, allowing more elements per period.

Step-by-step solution

Identifying Period 1 Elements

To list the number of elements in period 1, locate the first row in the periodic table. This row includes hydrogen (H) and helium (He). Thus, there are two elements in period 1.

Identifying Period 2 Elements

To list the number of elements in period 2, locate the second row in the periodic table. This row starts with lithium (Li) and ends with neon (Ne), comprising a total of eight elements.

Understanding Period Length Differences

The difference in the number of elements in each period is due to the quantum mechanics of electron shells. Period 1 has two elements because it fills the 1s subshell. Period 2 has eight elements because it fills the 2s and 2p subshells. As the energy level of the period increases, the number of available subshells also increases, allowing for more elements.

Key concepts

Periods in Periodic Table

The periodic table is a remarkable tool that arranges all the known chemical elements in an informative array. The horizontal rows in this table are known as 'periods'. There are seven periods in the periodic table, each corresponding to the highest energy level of the electron shell that is occupied by electrons.

Period 1 contains only two elements, hydrogen and helium. These elements have their electrons in the first shell, known as the 1s shell. The second period features elements like lithium and neon, with a total of eight elements. As you move from left to right across a period, the number of protons and electrons increases, and the elements display a range of properties from metallic to non-metallic character.

Difference in Period Lengths

One might wonder why periods vary in length. The lengths of periods are determined by the number of orbitals available for electrons to occupy. Due to the quantum mechanical nature of atoms, these slots increase as the energy levels increase, allowing a greater number of elements within those periods.

Electron Shells

Electron shells are the layers of electrons that surround the nucleus of an atom. In the context of the periodic table, these shells are critical as they help to explain the arrangement of elements. An atom's electron configuration provides insight into its chemical behavior.

1s and 2s Orbitals

As highlighted in the periods of the periodic table, the 1s shell can hold up to two electrons, explaining why the first period has just two elements. In contrast, the second shell contains both the 2s and 2p orbitals, harboring up to eight electrons, which correlates with the eight elements found in the second period.

Electron shells fill according to the Madelung rule, where lower energy levels fill first. Thus, immediately after the 1s shell is filled, electrons begin to fill the 2s and then 2p orbitals in the second shell. This concept is crucial for understanding why elements in the same group of the periodic table have similar chemical properties.

Quantum Mechanics

Quantum mechanics plays an indispensable role in understanding the structure of the periodic table. It is the branch of physics that deals with the mathematical description of the motion and interaction of subatomic particles.

Subatomic Orbitals and Electron Configuration

Applying quantum mechanics to atoms reveals that electrons do not follow fixed orbits around the nucleus but rather exist in 'probabilistic' orbitals. These orbitals are defined by quantum numbers that describe their properties, such as energy level and shape. As a result, the notion of electron shells and subshells arises, explaining the distribution of electrons in an atom and the periodicity observed in the elements' properties.

Quantum mechanics also elucidates why the number of elements per period increases in higher periods. This increase is due to the presence of additional orbitals allowed by the underlying quantum mechanical principles, which affords the accommodation of more electrons, and by extension, increasing the number of possible elements as the energy level increases.