Neutrons play a critical role in the stability of an atom. They act as a buffer between protons in the nucleus, reducing the electrostatic repulsion between these positively charged particles. Without neutrons, the repulsive forces between protons would be too strong, leading to an unstable nucleus. For lighter elements, the number of neutrons is often equal to or slightly more than the number of protons for stability. In heavier elements, a larger proportion of neutrons is required to maintain stability. If neutrons were absent, atoms would not be able to exist in their current form, leading to a fundamentally different universe. In the absence of neutrons, elements as we know them would not be able to form, and complex molecules necessary for life, like DNA, would not exist.

The electron cloud model represents a significant advancement over earlier atomic models, like the Bohr model or the plum pudding model. Earlier models depicted electrons in fixed orbits or randomly scattered within a positively charged sphere. The electron cloud model, however, introduces the concept of probability regions where electrons are likely to be found, instead of fixed paths. This model aligns with the principles of quantum mechanics, acknowledging that the exact position of an electron cannot be determined. It depicts electrons in terms of probabilities and energy levels, rather than precise locations. This approach offers a more accurate and nuanced understanding of atomic structure, accommodating the complex, wave-like behaviour of electrons as described by quantum physics.

Yes, an atom can have more neutrons than protons. This is common in heavier elements, where a greater number of neutrons is necessary to counteract the repulsive forces between the large number of protons in the nucleus. Atoms with different numbers of neutrons compared to protons are known as isotopes. These isotopes have the same chemical properties (since they have the same number of protons and electrons) but different physical properties, such as mass and stability. Some isotopes are unstable and undergo radioactive decay, emitting radiation as they transform into more stable forms. This phenomenon has practical applications in fields like medicine (radiotherapy), archaeology (carbon dating), and energy (nuclear reactors).

The number of energy levels or shells that electrons occupy in an atom is primarily determined by the atom's electron configuration, which is governed by the atomic number. The atomic number dictates the total number of electrons in a neutral atom. These electrons fill the energy levels following specific rules: the lowest energy levels fill up first, and each level has a limited capacity. For example, the first energy level can hold up to 2 electrons, the second up to 8, and so on. As the atomic number increases, more electrons are added, necessitating more energy levels. The arrangement of these electrons in various shells shapes the chemical properties of the element, influencing how it interacts with other atoms.

Atoms form ions to achieve a more stable electronic configuration, often resembling the nearest noble gas which has a full outer electron shell. This drive towards stability is a key factor in chemical reactivity. When an atom gains or loses electrons, becoming an ion, it alters its chemical properties significantly. For instance, a neutral sodium atom is a soft, reactive metal. However, when it loses an electron to become a Na⁺ ion, it forms stable, non-reactive compounds like sodium chloride. Similarly, chlorine, a toxic gas, forms the chloride ion (Cl⁻) by gaining an electron, becoming part of common salts. This change in reactivity due to ionisation is fundamental to many chemical reactions, including the formation of salts, the conduction of electricity in solutions, and various biological processes.