A lithium atom has three electrons around its nucleus. Two are in the K shell and one is in the L shell.
The single electron in the outermost L shell is the valence electron. Like sodium, lithium gives up this electron to become a singly charged positive ion, \mathrm{Li}^{+}.
Because a lithium atom has only two electron shells, it is the smallest of the alkali metal atoms, with a radius of about 152 pm.
The density of lithium is about 0.53 g/cm³, the lowest of all metals. A piece of lithium weighs only about half as much as the same volume of water.
Sodium and potassium are denser than kerosene, so they can be stored under it. Lithium floats even on kerosene (about 0.8 g/cm³), so it is stored with its surface coated in oil, or kept in argon.
Lithium also reacts with water, giving off hydrogen and forming lithium hydroxide. The solution is strongly basic, so water containing phenolphthalein turns pink.
2\mathrm{Li} + 2\mathrm{H_2O} \longrightarrow 2\mathrm{LiOH} + \mathrm{H_2}
The reaction is much gentler than that of sodium, however. The lithium stays afloat on the surface, giving off bubbles and slowly getting smaller.
Lithium melts at about 181 °C, which is high for an alkali metal, so unlike sodium it is not melted into a ball by the heat of the reaction.
When a platinum wire dipped in a solution containing lithium is held in the flame of a gas burner, the flame turns red. The heat of the flame raises the outer electron to a state of higher energy, and when the electron falls back, it gives out red light with a wavelength of about 671 nm.
Even a tiny amount of lithium colours the flame, so this flame test can show whether lithium is present.
Lithium is a key material in the lithium-ion batteries used in smartphones and electric vehicles.
During charging, lithium ions, \mathrm{Li}^{+}, move from the positive electrode to the negative electrode and enter the spaces between the layers of graphite in the negative electrode. During discharge, the lithium ions move back from the negative electrode to the positive electrode, while electrons flow through the external circuit from the negative electrode to the positive electrode.
Because lithium is light and readily forms positive ions, it makes batteries that are light and have a high voltage. In 2019, three researchers who developed the lithium-ion battery, including Akira Yoshino, were awarded the Nobel Prize in Chemistry.