An atom is considered stable when the forces within its nucleus are balanced and it has enough binding energy to hold the nucleus together permanently. Stability depends largely on the strong nuclear force overcoming the repulsion between positively charged protons in the nucleus. Additionally, for atoms in terms of chemical stability, having a full outer electron shell (based on the octet rule or full valence shell) makes the atom chemically stable. Unstable atoms, often radioactive, have imbalanced forces in the nucleus due to too many or too few neutrons compared to protons and tend to emit radiation to reach a stable state.

Nuclear Stability

• Stability in the nucleus arises from the strong nuclear force that holds protons and neutrons together, overcoming their electromagnetic repulsion.
• The nucleus must have an appropriate neutron-to-proton ratio; too many or too few neutrons cause instability, leading to radioactive decay.
• Binding energy, the energy required to disassemble the nucleus, indicates stability; higher binding energy per nucleon means a more stable nucleus.

Electron Shell Stability

• Chemically, atoms are stable when their outermost electron shell is full, usually 8 electrons for many atoms (octet rule), but 2 electrons for lighter atoms like helium.
• Atoms achieve this full shell by gaining, losing, or sharing electrons, forming ionic or covalent bonds.

Summary

An atom's overall stability depends on both nuclear factors (balanced forces and neutron-proton ratio) and electronic configuration (full outer electron shell). Unstable atoms tend to undergo radioactive decay or chemical reactions to achieve a more stable configuration.