How many valence electrons are there in the compound HCO3-? How stable is a compound with so many valence electrons?
Answer
Bicarbonate is a polyatomic anion with one central carbon atom bonded to three oxygen atoms. One of the oxygen atoms is also bonded to a hydrogen atom. The entire ion has a single negative charge. Let's calculate how many valence electrons are present:
- Hydrogen (H): 1 valence electron x 1 atom = 1 valence electron
- Carbon (C): 4 valence electrons x 1 atom = 4 valence electrons
- Oxygen (O): 6 valence electrons x 3 atoms = 18 valence electrons
- Negative charge (-): The -1 charge indicates one extra electron = 1 valence electrons
Adding these up: 1+ 4 +18 + 1 = 24 valence electrons
The reactivity of a compound is not solely determined by the total number of valence electrons it possesses. Reactivity is primarily governed by the arrangement of these valence electrons; specifically how close the atoms are to achieving a stable electron configuration (usually an octet or a duet for hydrogen and lithium).
Factors influencing reactivity: the octet rule--atoms tend to gain, lose, or share electrons to achieve a full outer shell of 8 valence electrons (like the noble gases, which is a very stable configuration. Presence of lone pairs--lone pairs of electrons can make a molecule more reactive, as there are regions of high electron density. Formal charges--molecules with positive or negative charges on atoms tend to be more reactive as they seek to become neutral. Bond polarity--polar bonds are more susceptible to reactions with charged or polar species.
Other compounds with 24 valence electrons and varying reactivity: Nitrate ion (NO3-)--all atoms in the nitrate ion achieve an octet through resonance structures. It is a relatively stable ion. Formaldehyde (CH2O)-- all atoms achieve an octet. Formaldehyde is a reactive molecule due to the polar C=O double bond. Nitric acid (HNO3)--nitric acid is a strong acid and a powerful oxidizing agent, making it highly reactive.