The force that helps maintain an atom's structure between negative electrons and positive protons is the electromagnetic force. This force is one of the four fundamental forces in nature, along with the strong force, the weak force, and the gravitational force.

The electromagnetic force is responsible for the interactions between charged particles, such as electrons and protons. In an atom, the electrons are negatively charged, while the protons are positively charged. The electromagnetic force attracts the electrons to the protons, holding them together and forming a stable atom.

The strength of the electromagnetic force between two charged particles depends on the magnitude of the charges and the distance between them. The greater the charges and the closer the particles are, the stronger the electromagnetic force will be.

In an atom, the electromagnetic force is strong enough to overcome the repulsive force between the positively charged protons. This allows the atom to maintain its structure and remain stable.