The determination of which electrons orbit an atom is based on the concept of electron configuration. Electron configuration describes the distribution of electrons into various energy levels and orbitals within an atom.
The electron configuration is determined by the filling order of orbitals, which follows certain rules such as the Aufbau principle, Pauli exclusion principle, and Hund's rule. These rules dictate how electrons occupy different energy levels and subshells within an atom.
The electron configuration of an atom provides information about the number of electrons in each energy level and the specific orbitals they occupy. For example, the electron configuration of carbon (C) is 1s² 2s² 2p², indicating that it has two electrons in the 1s orbital, two electrons in the 2s orbital, and two electrons in the 2p orbital.
The electron configuration determines the chemical properties of an atom, as it influences how the electrons interact with other atoms and participate in chemical bonding.
Regarding the second part of your question, the electron configuration of an atom can change over time. This can occur through various processes, including:
Excitation: Electrons can absorb energy and move to higher energy levels or orbitals. This temporary change in electron configuration occurs when the atom gains energy, such as through absorption of photons.
Ionization: Electrons can be completely removed from an atom, leading to the formation of ions. This occurs when an atom loses or gains electrons, resulting in a different electron configuration.
Chemical reactions: During chemical reactions, electrons can be shared, transferred, or redistributed between atoms, leading to changes in their electron configurations.
It's important to note that changes in electron configuration typically involve the outermost electrons, also known as valence electrons. The core electrons, which are tightly bound to the nucleus, usually remain unchanged unless a high-energy process such as nuclear decay occurs.
In summary, the determination of which electrons orbit an atom is based on its electron configuration, which follows specific rules. The electron configuration can change over time through processes such as excitation, ionization, and chemical reactions, primarily affecting the outermost electrons or valence electrons.