Sodium (Na, atomic number 11) is generally more reactive than lithium (Li, atomic number 3). Both elements are classified as alkali metals, a group defined by having a single valence electron in their outermost shell. This shared characteristic explains why both are highly reactive metals that readily seek to lose that single electron to form a stable positive ion. The difference in their reactivity is a direct consequence of their placement in Group 1 of the periodic table, where reactivity increases as you move down the group.
Defining Chemical Reactivity in Alkali Metals
Chemical reactivity in alkali metals is primarily determined by how easily the atom can lose its single valence electron to achieve a stable electron configuration. The loss of this electron converts the neutral metal atom into a cation with a charge of +1 (Li⁺ or Na⁺). This process is known as oxidation, and the metal’s tendency to undergo this change is a measure of its reactivity. The easier it is for the atom to shed its outermost electron, the more reactive the metal is considered to be. Sodium’s electron is lost more readily than lithium’s, which is the underlying reason for sodium’s greater reactivity.
Comparing the Atomic Structures of Lithium and Sodium
The difference in reactivity stems from the atomic structure of the two elements. Lithium (atomic number 3) has its electrons arranged in two energy shells, with its single valence electron residing in the second shell. Sodium (atomic number 11) distributes its electrons across three energy shells, making the sodium atom significantly larger and increasing its atomic radius. Sodium’s single valence electron is located in the third, outermost shell, physically further away from the positively charged nucleus. This greater distance is amplified by a full inner shell of electrons, which effectively shields the outermost electron from the nuclear pull.
Ionization Energy and the Explanation for Reactivity
The energy required to remove the outermost electron is quantified by the first ionization energy. Because the valence electron in sodium is in a shell further from the nucleus, it experiences a weaker attractive force from the positive protons in the nucleus. This weakness is a result of the increased distance and the shielding effect from the ten inner-shell electrons. Consequently, the first ionization energy of sodium is lower than that of lithium.
Lithium holds its valence electron more tightly due to its smaller size, requiring more energy to remove that electron. The measured first ionization energy for lithium is approximately 520 kilojoules per mole (kJ/mol), while for sodium it is lower, at about 495 kJ/mol. This lower energy requirement confirms that sodium is more chemically reactive than lithium, as it takes less energy to initiate the reaction.
Practical Differences in Reaction with Water
The difference in ionization energy translates to an observable difference in the reaction of the two metals with water (H₂O). Both metals react with water to produce the metal hydroxide and hydrogen gas (H₂). However, the reaction involving sodium is noticeably more vigorous than the reaction with lithium.
When lithium is placed in water, it floats and fizzes gently as hydrogen gas is released, slowly forming lithium hydroxide (LiOH). The heat generated is released slowly, and lithium’s high melting point prevents it from melting.
In contrast, sodium reacts so quickly that enough heat is generated to melt the metal instantly. The molten sodium forms a small, silvery ball that darts rapidly across the water’s surface, creating a trail of sodium hydroxide (NaOH). This rapid reaction has the potential to ignite the hydrogen gas, requiring safety precautions when handling any alkali metal.