Lithium's three electrons are arranged so that:

1. Lithium contains three electrons that occupy the first and second shells. 2. The 1s orbital fills first and holds a maximum of two electrons. 3. Two electrons fill the 1s orbital, and the third electron then fills the 2s orbital. 4. Putting all three in one orbital would breach the two-electron limit. 5. That single outer electron is why lithium loses it readily to form a positive ion. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_