Describe the shape and energy of an s orbital.

An s orbital is spherical in shape and has the lowest energy level among all orbitals.

The s orbital, named after the German word 'sphärisch' meaning spherical, is the first type of atomic orbital. It is characterised by its symmetrical, spherical shape around the nucleus of an atom. This shape is due to the equal probability of finding an electron at any given distance from the nucleus. The s orbital can hold a maximum of two electrons, each with opposite spins, which is a fundamental principle of quantum mechanics known as the Pauli Exclusion Principle.

In terms of energy, the s orbital has the lowest energy level among all types of orbitals (s, p, d, f). This is because it is closest to the nucleus and therefore experiences the greatest attractive force from the positively charged protons in the nucleus. The energy of an orbital is directly related to its distance from the nucleus; the closer an orbital is to the nucleus, the lower its energy level. This is why the 1s orbital (the s orbital in the first energy level) is always filled first during electron configuration.

The s orbital plays a crucial role in the structure and reactivity of atoms. For instance, the 1s orbital is the innermost orbital and is always filled in ground state atoms, contributing to the stability of the atom. Furthermore, the shape and energy of the s orbital influence the types of chemical bonds an atom can form. For example, hydrogen and helium, which only have s orbitals, can only form single bonds.

In summary, the s orbital is a spherical atomic orbital with the lowest energy level. Its shape and energy are fundamental to understanding the structure and reactivity of atoms in chemistry.