(a) The diagram shows the relationship between the visible lines in the hydrogen spectrum and the corresponding energy levels in a hydrogen atom - Leaving Cert Chemistry - Question 5 - 2016

An electron in a hydrogen atom can become excited when it absorbs energy from external sources, such as heat or electromagnetic radiation. This energy enables the electron to move from a lower energy level to a higher energy level.

The series of visible lines in the emission spectrum of hydrogen is produced when electrons transition between energy levels. When an electron falls from a higher excited state to a lower energy level, it emits a photon of light. The wavelength (and corresponding color) of light emitted depends on the energy difference between these levels, described by the Rydberg formula.

This series of lines is known as the Balmer series, which corresponds to transitions that end at the second energy level ($n = 2$).

There is no yellow line in the hydrogen emission spectrum because no electronic transitions correspond to the energy difference that would produce yellow light. The spectral lines are determined by specific allowed transitions between energy levels, and yellow light does not result from any of those transitions.

To perform a flame test for lithium, first clean a platinum (nichrome) wire by dipping it in concentrated hydrochloric acid and then rinsing it with distilled water. Next, dip the cleaned wire into the salt sample and place it in the flame of a Bunsen burner. A bright red (crimson) flame indicates the presence of lithium.

An atomic orbital is a region around the nucleus of an atom where there is a high probability of finding an electron. Each orbital has a specific shape and energy associated with it.

A 2p orbital refers to one of the orbitals (there are three: 2px, 2py, 2pz) within the 2p sublevel. The 2p sublevel contains these three orbitals, indicating that there can be a total of six electrons (2 per orbital) that occupy this sublevel.

The electron configuration for a calcium atom, which has 20 electrons, is:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s².

The arrangement of electrons in a calcium atom follows the principle that electrons fill the lowest energy levels first. The 4s sublevel is at a lower energy level than the 3d sublevel, even though the 3d sublevel can hold more electrons. Thus, for calcium, electrons fill the 4s sublevel (2 electrons) before filling the 3d sublevel, maintaining the electron configuration of 2, 8, 8, 2 instead of 2, 8, 10.