Alkali metals are a distinct family of elements located in Group 1 of the Periodic Table, including lithium (Li), sodium (Na), potassium (K), rubidium, cesium, and francium. They share unique physical and chemical characteristics that set them apart from most other metals. Alkali metals are among the softest known metallic elements. Their physical properties are a direct result of their atomic structure, which also dictates their extreme chemical tendencies.
Defining the Physical Property of Alkali Metals
The characteristic softness of these metals is immediately apparent. Alkali metals are so soft they can be easily cut with a simple knife, often compared to the consistency of hard butter or wax at room temperature. This unusual malleability is a distinguishing physical feature.
Softness is also linked to their low melting points, a property that decreases moving down the group. Cesium, for example, melts at just 28.4 degrees Celsius, meaning it can liquefy from the warmth of a human hand.
Lithium, sodium, and potassium also exhibit low densities. These three elements are less dense than water, causing them to float.
The Chemical Basis for Softness
The physical softness stems from the fundamental nature of the metallic bond within their structure. All alkali metals possess only one valence electron in their outermost shell. This single electron forms the “sea of electrons” that holds the positive metal ions together in the metallic lattice.
Alkali metal atoms are relatively large compared to other elements in the same row of the periodic table. Since the single valence electron is located far from the positively charged nucleus, the electrostatic force of attraction is relatively weak. This weak attraction results in a weak metallic bond holding the atoms together in the solid state.
Because the metallic bond is weak, alkali metals lack the structural rigidity of typical metals like iron or copper. This weakness requires very little energy to disrupt the crystal lattice, explaining both their low melting points and physical softness. The increasing atomic radius moving down the group further weakens this bond, which is why cesium is softer and has a lower melting point than lithium.
Extreme Chemical Behavior
The single, loosely held valence electron that causes physical softness also dictates the extreme chemical behavior of these elements. The low energy required to remove this electron means alkali metals readily lose it to form a stable cation with a +1 charge. This inherent tendency makes them highly reactive, particularly with substances that can easily accept one, such as water and oxygen.
Alkali metals react vigorously with water to produce a strong alkaline solution and flammable hydrogen gas. Due to this high reactivity with moisture and oxygen in the air, these metals cannot be left exposed. They must be stored under an inert, non-reactive medium, typically a hydrocarbon liquid like mineral oil or kerosene, to prevent spontaneous reactions.