What happens to the size of an atom when it becomes a negative ion? Why?
When an atom gains one or more electrons and becomes a negative ion, the size of the atom generally increases. This is primarily due to the increase in the number of electrons, which leads to an increased electron-electron repulsion and a larger size of the electron cloud.
To understand why this happens, it's helpful to know a bit about atomic structure. Atoms are composed of a central nucleus containing positively charged protons and uncharged neutrons, with negatively charged electrons orbiting around the nucleus in specific energy levels or shells.
In a neutral atom, the number of electrons equals the number of protons, resulting in a balanced positive charge from the protons and negative charge from the electrons. However, when an atom gains an electron (or multiple electrons), it becomes a negative ion. This makes the number of electrons greater than the number of protons, resulting in an overall negative charge.
The new electron(s) added to the atom will occupy the available energy levels in the outermost electron shell, which is called the valence shell. As the outermost shell becomes more populated with electrons, there is an increase in electron-electron repulsion. This repulsion will cause the electron cloud to expand, leading to an increase in the atomic size.
Additionally, the extra electron(s) may experience a weaker attractive force from the nucleus compared to the electrons in a neutral atom. As a result, the increased electron-electron repulsion combined with the weaker attractive force from the nucleus causes the negative ion to be larger than the neutral atom.