8.6:
Alkali Metals
Alkali metals are group 1 elements, including lithium, sodium, potassium, rubidium, cesium, and francium. Francium is radioactive and has no stable isotopes.
All alkali metals are unusually soft and share metallic properties, such as high electrical and thermal conductivity. Moving down the group, the mass, density, and atomic radii increase, while the melting points and ionization energies decrease.
The electron configuration of these elements implies that the valence shell has only one electron.
Their low ionization energies indicate that alkali metals readily lose electrons to become cations with a noble gas configuration. This also explains why alkali metals are excellent reducing agents, with lithium being the least reactive and cesium the most.
Alkali metals react vigorously with nonmetals, such as halogens, to form salts. The reaction of sodium metal with chlorine gas is highly exothermic and releases sparks and heat. The intensity of the reaction increases down the group as the ionization energies of the alkali metals decrease.
Alkali metals react vehemently with water to generate hydrogen gas and a solution of alkali metal hydroxide like potassium hydroxide, which is a base often used in soap making.
The reaction becomes more violent for the heavier alkali metals given their lower ionization energies. The heat produced during the reaction may even ignite the released hydrogen gas leading to fire or an explosion.
Alkali metals can also react with hydrogen to form alkali metal hydrides, often used as strong bases. In this reaction, the hydrogen atom gains an electron and is present as the hydride ion.
Due to their high reactivity, alkali metals oxidize easily upon exposure to air and are therefore stored in mineral oils.
When alkali metal salts are placed in a flame and reduced to their gaseous metal atoms, they emit characteristic colors.
The heat excites the valence electron to a higher energy level. When the excited electrons return to the ground state, energy is emitted in the form of radiation in the visible region. The emission spectrum of each element is unique and used to identify the element qualitatively.
8.6:
Alkali Metals
Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the alkali metals are unusually low for metals and decrease going down the group, while the density increases going down the group with the exception of potassium (Table 1).
Table 1: Properties of the alkali metals
| Element | Electron Configuration | Atomic Radius (pm) | IE1 (kJ/mol) | Melting Point (°C) | Density at 25 °C (g/cm3) |
| Li | [He] 2s1 | 152 | 520 | 181 | 0.53 |
| Na | [Ne] 3s1 | 186 | 496 | 98 | 0.97 |
| K | [Ar] 4s1 | 227 | 420 | 64 | 0.86 |
| Rb | [Kr] 5s1 | 248 | 400 | 39 | 1.53 |
| Cs | [Xe] 6s1 | 265 | 380 | 29 | 1.93 |
Moving down a group, the principal quantum number, n, increases by one for each element. Thus, the outer electrons get farther from the nucleus, and the atomic radius increases from lithium to cesium.
The electron configuration of these elements shows that the valence shell has only one electron (Table 1). The loss of this electron produces a cation with the noble gas configuration. Thus, alkali metals have a great tendency to give away this electron and have low first ionization energies, which decrease down the group. Their ability to lose an electron readily makes them highly reactive and excellent reducing agents.
The alkali metals react vigorously with nonmetals, such as halogens. The reaction of sodium and chlorine to produce sodium chloride is exothermic. The alkali metals also react violently with water, also, and hydrogen gas and an alkali metal hydroxide are produced. The heat produced during the reaction can ignite the hydrogen gas released, leading to an explosion. Both of these reactions become more exothermic for the heavier alkali metals due to their lower ionization energies.
Exposure to air, as they react with oxygen to form oxides, reduces the metallic luster of the alkali metals. Lithium reacts with oxygen to give lithium oxide, while other alkali metals like sodium form oxides and peroxides. The peroxide ion has a single oxygen-oxygen covalent bond and is a powerful hydrogen ion acceptor, making the peroxides of the alkali metals strong bases. The alkali metals such as potassium, rubidium, and cesium also produce superoxides. Superoxides are characterized by the presence of O2−. Potassium superoxide is a yellow solid that decomposes at 560 °C.
This text is adapted from Openstax, Chemistry 2e, Section 6.5: Periodic Variations in Element Properties.