Formal Charge

Formal charge Formal charge is a concept used to determine the distribution of electron density within a molecule or ion. It helps us understand the charge distribution on individual atoms and evaluate their relative stability. The formal charge of an atom is calculated by comparing the number of valence electrons an atom should have (based on its position in the periodic table) with the number of valence electrons it actually possesses in a given molecule or ion. The formula to calculate the formal charge (FC) of an atom is: FC = (Number of valence electrons in the free atom) - (Number of lone pair electrons) (Number of bonding electrons) Example 1 Let's consider the molecule carbon dioxide (CO2): Step 1: Determine the Lewis structure of the molecule, which shows the arrangement of atoms and their bonding. Step 2: Count the valence electrons for each atom. Oxygen (O) has 6 valence electrons, and carbon (C) has 4 valence electrons. Step 3: Assign lone pairs and bonding pairs to each atom in the Lewis structure. In carbon dioxide (CO2), the oxygen atoms are each bonded to the central carbon atom by double bonds. Step 4: Calculate the formal charges for each atom. For the carbon atom: FC = (Number of valence electrons in the free atom) - (Number of lone pair electrons) -(Number of bonding electrons) = 4 - 0 - 4 (since carbon is involved in four bonding electrons) = 0 For each oxygen atom: FC = (Number of valence electrons in the free atom) - (Number of lone pair electrons) -(Number of bonding electrons) = 6-4-4 (since oxygen is involved in two lone pairs and two bonding electrons) =0 In the Lewis structure of carbon dioxide, each atom has a formal charge of zero. This distribution of formal charges indicates that the molecule is relatively stable. Example 2 Let's consider the molecule ammonia (NH3): Step 1: Determine the Lewis structure of ammonia, which shows the arrangement of atoms and their bonding. Step 2: Count the valence electrons for each atom. Nitrogen (N) has 5 valence electrons, and each hydrogen (H) atom has 1 valence electron. Step 3: Assign lone pairs and bonding pairs to each atom in the Lewis structure. In ammonia (NH3), the nitrogen atom forms three single bonds with the hydrogen atoms, resulting in a total of eight valence electrons. Step 4: Calculate the formal charges for each atom. For the nitrogen atom: FC = (Number of valence electrons in the free atom) - (Number of lone pair electrons) -(Number of bonding electrons) = 5-1-3 =1 For each hydrogen atom: FC = (Number of valence electrons in the free atom) - (Number of lone pair electrons) -(Number of bonding electrons) = 1-0-2 = -1 Therefore, the formal charge on each hydrogen atom in ammonia is -1. The hydrogen atoms have one more electron (formal charge of -1) compared to their neutral state, balancing the formal charge of the nitrogen atom (+1) to give the ammonia molecule an overall neutral charge. In the Lewis structure of ammonia, the ammonia atom has a formal charge of 0, indicating that it is neither positively nor negatively charged. It is important to note that formal charges do not necessarily represent the actual charge of an atom in a molecule or ion but provide a way to understand the electron distribution. By calculating formal charges, we can assess the relative stability of different resonance structures, predict reactivity patterns, and gain insights into molecular properties and behavior.