Final answer:
The metal with atomic number 3 is Lithium (Li), which has three electrons and is part of the alkali metal group. Lithium has isotopes with mass numbers 6 and 7, known as Lithium-6 and Lithium-7 respectively. It holds significance in both chemical and cosmological studies.
Step-by-step explanation:
The metal with an atomic number of 3 is Lithium (Li). In its neutral state, lithium has three protons and, correspondingly, three electrons. The electronic configuration of lithium has the first two electrons filling the 1s orbital and the third electron occupying the 2s orbital. This arrangement reflects lithium's position as the third element on the periodic table, belonging to the alkali metal group.
Lithium has several isotopes, with Lithium-6 and Lithium-7 being the most common. Lithium-6 has three neutrons in addition to its three protons, giving it a mass number of 6. Conversely, Lithium-7 has four neutrons, resulting in a mass number of 7. These isotopes illustrate that while the atomic number of lithium is fixed at 3, indicating the number of protons, the number of neutrons can vary to produce different isotopes.
Furthermore, in terms of cosmology, lithium nuclei with three protons and four neutrons were among the elements produced during the early stages of the universe's expansion. This highlights lithium's significance not only in chemistry but also in understanding the origins of the universe.