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Electron configuration describes how electrons are distributed in an atom's atomic orbitals. Each element has a unique electron configuration that can be deduced from its atomic number. The atomic number tells us the number of protons and, in a neutral atom, also the number of electrons. For lithium (Li), the atomic number is 3, signifying it has three electrons.
The configuration for lithium is written as 1s² 2s¹. This means:

• The first two electrons fill the 1s orbital, the lowest energy level.
• The third electron resides in the 2s orbital, which is at a higher energy level than the 1s.

Understanding electron configurations is essential for predicting how atoms will interact in chemical reactions.

Atomic orbitals are regions within an atom where electrons are likely to be found. These orbitals are not physical objects but rather probability distributions derived from quantum mechanics. Each orbital can hold a specific maximum number of electrons.
The most common orbitals are designated as s, p, d, and f:

• s-orbitals can hold up to 2 electrons. They are spherical in shape.
• p-orbitals can hold a maximum of 6 electrons.
• d-orbitals accommodate up to 10 electrons.
• f-orbitals can fit a maximum of 14 electrons.

In lithium's case, the electrons are found in the 1s and 2s orbitals. Because s-orbitals can hold 2 electrons, the 1s orbital is filled first, followed by one electron in the 2s orbital.

Energy levels are specific, quantized energies that electrons in an atom can have. The electrons reside in orbitals corresponding to these energy levels, moving to higher or lower levels when energy is absorbed or released.
For lithium, the electrons occupy different energy levels. The 1s electrons are in a lower energy level as they are closer to the nucleus, minimizing potential energy. Meanwhile, the electron in the 2s orbital is at higher energy due to its greater average distance from the nucleus.
In photoelectron spectroscopy (PES), these energy levels are visualized as peaks on a graph. Each peak represents the energy required to remove an electron from its orbital. Because the 1s peak is higher, it reflects the presence of two electrons within that energy level. The 2s peak is lower, indicative of just one electron in a higher energy state.