LithiumElectron Configuration, Bohr Model, Valence Electrons & Orbital Diagram

The electron configuration of Lithium is written as 1s² 2s¹. Applying the Aufbau principle — filling orbitals from lowest to highest energy — plus the Pauli Exclusion Principle and Hund's Rule, we systematically place all 3 electrons: 1s² 2s¹. In the s-block, valence electrons fill spherical s-orbitals (maximum 2 electrons each). Lithium's first shell is completely filled, forming a helium-like inert core of 2 electrons.

Lithium follows the standard Aufbau filling order without exception. The noble gas shorthand [He] 2s¹ replaces the inner-shell electrons with the symbol of the preceding noble gas, highlighting that only the outer electrons — 2s¹ — are chemically active.

Shell-by-shell, Lithium's 3 electrons are distributed as: K-shell (n=1): 2 electrons; L-shell (n=2): 1 electron. The L-shell (n=2) is the valence shell, containing 1 electron.

Chemically, this configuration places Lithium in Group 1 with oxidation states of 1. One lone electron in the highest s-orbital, barely held by the nucleus through layers of shielding, explains Lithium's notoriously low ionization energy and explosive reactivity.

The SPDF orbital model describes Lithium's electrons not as planetary orbits but as three-dimensional probability clouds — each orbital a region of space where an electron is most likely to be found. Lithium's 3 electrons occupy 2 distinct subshells: 1s² 2s¹, governed by three quantum mechanical rules.

The Pauli Exclusion Principle ensures no two electrons in Lithium share the same four quantum numbers (n, l, m_l, m_s). This is why the 1s orbital holds only 2 electrons, the full p-subshell holds 6, d holds 10, and f holds 14. Without this rule, all 3 electrons would collapse into the 1s orbital. For Lithium's s-electrons, only two quantum states exist per subshell (spin up ↑ and spin down ↓), so Hund's Rule has minimal impact — both electrons in an s-orbital must pair with opposite spins per the Pauli Exclusion Principle.

Following standard orbital filling, Lithium fills orbitals in the sequence: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p. The final electron enters the 2s¹ subshell, making Lithium a s-block element with 1 valence electrons in Group 1.

The outermost electrons — 2s¹ — are Lithium's chemical agents. The single ns¹ electron occupies the top of the energy ladder, barely tethered to the nucleus, responsible for the entire chemical life of the alkali metal.

Lithium's chemical reactivity is shaped by three interlocking properties: electronegativity (0.98 Pauling), first ionization energy (5.392 eV), and electron affinity (0.618 eV). Its electronegativity is very low (0.98) — strongly electropositive, a natural electron donor. Lithium donates electrons to partners rather than accepting them — the hallmark of electropositive metals.

The first ionization energy of 5.392 eV is relatively low, confirming Lithium's readiness to lose electrons — a quintessentially metallic trait. The electron affinity of 0.618 eV represents the energy released when Lithium gains one electron, indicating a meaningful but moderate acceptance of electrons.

Lithium is among the most reactive metals on Earth. Contact with water releases H₂ exothermically; contact with halogens is immediate and often violent. Every reaction is driven by the energetic incentive of achieving noble gas configuration.

Placing Lithium between Helium (Z=2) and Beryllium (Z=4) reveals the incremental property changes that make the periodic table a predictive tool.

Helium → Lithium: adding one proton and one electron increases nuclear charge by 1. Valence electrons shift from 2 to 1 (Group 18 → Group 1). | Ionization energy: 24.587 → 5.392 eV. Atomic radius increases from 31 pm to 167 pm, consistent with descending a group with additional shells.

Lithium → Beryllium: the additional proton and electron in Beryllium changes the valence electron count from 1 to 2, crossing from Group 1 to Group 2. This boundary also marks a categorical transition from Alkali Metal to Alkaline Earth Metal. These comparisons confirm that Lithium sits at a well-defined chemical inflection point in the periodic table.