Lithium (Li, atomic number 3) is the lightest alkali metal and exists in two primary states: a neutral atom and an ion. Whether lithium is negative or positive depends entirely on which form is being discussed. The charged form, the lithium ion, is the state most frequently utilized in modern technology, particularly in rechargeable batteries.
The Neutral Lithium Atom
The neutral lithium atom represents the element in its uncharged state. It contains three positively charged particles, called protons, in the nucleus and an equal number of negatively charged electrons orbiting it. This balance means the total positive charge perfectly balances the total negative charge. This results in a net electrical charge of zero, defining the atom as neutral. This configuration is the starting point for any chemical reaction involving lithium metal.
The Lithium Ion (\(\text{Li}^+\))
The lithium ion (\(\text{Li}^+\)) is the charged form of the element and is consistently positive. It forms when the neutral atom loses one of its three electrons. The number of protons never changes, meaning the nucleus retains its three positive charges.
The loss of one electron reduces the count of negative charges to two. The imbalance between the three positive protons and the two negative electrons results in an overall charge of positive one (\(1+\)). This positively charged particle is classified as a cation. The lithium ion is the mobile species that moves within a battery to store and release energy.
Why Lithium Forms a Positive Ion
The formation of the positive lithium ion is driven by the chemical quest for stability. A neutral lithium atom has a single electron in its outermost energy shell, which is an unstable arrangement. Atoms react to achieve a full outer shell, mimicking the stable configuration of noble gases like helium.
For lithium, the most energetically favorable path is to lose this solitary outermost electron. Gaining seven electrons to fill the outer shell requires significantly more energy. By losing just the one valence electron, the lithium ion is left with a full, stable inner shell of two electrons. This tendency to easily give up its electron is reflected in its low first ionization energy.
Practical Application: Lithium in Batteries
The ability of lithium to readily convert into a stable, positive ion is the core principle behind lithium-ion batteries. These batteries store energy by moving the \(\text{Li}^+\) ions back and forth between two electrodes, the anode and the cathode. The ion’s positive charge allows it to be efficiently attracted and repelled by the charged electrodes.
During the discharge cycle, the positive lithium ions travel through an electrolyte from the anode to the cathode. This movement is paired with the flow of electrons through the external circuit, which generates the electric current. When charging, the process reverses, and the positive lithium ions move back from the cathode to the anode.