The numbers listed represent the number of electrons in the first, second, and third energy levels. respectively. On this basis, which of the following is an unstable or reactive atom?

A. Chloride ions: Chloride ions help stabilize the membrane potential by balancing charges, but they move passively along gradients set by other ions. Since chloride permeability is not the dominant factor at rest, they do not primarily determine the resting membrane potential.

B. Calcium ions: Calcium concentration is kept very low inside cells compared to outside. Its main role is in processes like muscle contraction and neurotransmitter release, rather than in establishing the electrical gradient at rest, so it contributes little to resting potential.

C. Potassium ions: Potassium ions are the major determinant of resting membrane potential because the membrane is most permeable to potassium at rest. Efflux of potassium through leak channels leaves the inside more negative, making it the primary driver of resting potential.

D. Sodium ions: Sodium ions are more concentrated outside the cell and tend to diffuse inward, but the membrane has low sodium permeability at rest. Although sodium influx is important during depolarization, it is not the chief factor in establishing resting membrane potential.

A. Ionic: Ionic bonds involve the complete transfer of electrons from one atom to another, creating oppositely charged ions that attract each other. In water, oxygen and hydrogen share electrons rather than transfer them.

B. Peptide: Peptide bonds form between amino acids during protein synthesis, linking the carboxyl group of one amino acid to the amino group of another. Since water is a simple molecule, not a protein, peptide bonds are not involved in its formation.

C. Polar covalent: Polar covalent bonds occur when atoms share electrons unequally due to differences in electronegativity. In water, oxygen pulls the shared electrons closer to itself, creating a partial negative charge on oxygen and partial positive charges on the hydrogens.

D. Nonpolar covalent: Nonpolar covalent bonds involve equal sharing of electrons between atoms with similar electronegativity. Because oxygen is more electronegative than hydrogen, the sharing in water is unequal.