The ease of moving an electron out of a valence ring depends on the energy required to remove it, which is measured by the ionization energy. When comparing elements with 1 or 2 electrons in the outermost valence shell (such as alkali metals and alkaline earth metals) to elements with 7 or 8 electrons in the outer shell (such as halogens and noble gases), there are a few key factors that contribute to the observed difference in ease:
Electron-electron repulsion: In elements with only 1 or 2 electrons in the valence shell, there is relatively less electron-electron repulsion between the valence electrons. Since the positive charge of the nucleus is spread over fewer electrons, each valence electron experiences a weaker electrostatic repulsion from other electrons. This makes it easier to remove an electron, as less energy is required to overcome this repulsion.
Effective nuclear charge: The effective nuclear charge experienced by the valence electrons increases from left to right across a period in the periodic table. Elements with 7 or 8 electrons in the valence shell experience a greater effective nuclear charge compared to elements with 1 or 2 electrons. The increased positive charge of the nucleus exerts a stronger pull on the valence electrons, making it more difficult to remove them.
Stability of the octet: Elements with 7 or 8 valence electrons tend to have a high affinity to gain electrons in order to achieve a stable octet (except for the noble gases). These elements have a strong tendency to hold onto their electrons, resulting in higher ionization energies. Removing an electron from such elements would require overcoming the electrostatic attraction between the nucleus and the tightly held valence electrons.
It's important to note that these factors contribute to general trends, but there can be exceptions and variations based on specific elements and their electronic configurations. The ease of removing an electron is also influenced by factors such as atomic size, electron shielding, and orbital stability, which can further affect the ionization energy of an element.