The size of an atom or ion is determined by the arrangement and distribution of its electrons. In the case of cations, which are positively charged ions, they are generally smaller in size than their parent atoms. This decrease in size can be attributed to the loss of one or more electrons during the formation of the cation.
When an atom loses an electron to become a cation, it becomes positively charged because the number of protons in the nucleus remains the same while the number of electrons decreases. This loss of electrons results in a decrease in the electron-electron repulsion within the ion, as there are fewer negatively charged particles to repel each other. Consequently, the remaining electrons are pulled closer to the nucleus, leading to a smaller size of the cation compared to the parent atom.
To understand this concept further, let’s consider an example. Take the alkali metal sodium (Na) with an atomic number of 11. In its neutral state, sodium has 11 electrons, each occupying a specific energy level or electron shell. These electrons are distributed in different energy levels, with two in the first shell and eight in the second shell. The outermost shell, also known as the valence shell, contains only one electron.
When sodium loses this valence electron to form a cation, it becomes Na+ with a positive charge of +1. Now, the cation has only 10 electrons, but the number of protons in the nucleus remains 11, as the loss of an electron does not affect the number of protons. With one less electron, there is a reduction in the electron-electron repulsion, allowing the remaining electrons to be more strongly attracted to the nucleus.
Consequently, the electrons are pulled closer to the nucleus, resulting in a decrease in the atomic radius of the cation compared to the neutral atom. This decrease in size is more pronounced in cations with higher positive charges, as the electron-electron repulsion is further reduced.
It is important to note that the size difference between cations and their parent atoms is not solely due to the loss of electrons. Other factors such as electron configuration, effective nuclear charge, and shielding effects also contribute to the overall size of an ion. However, the loss of electrons and the resulting change in electron distribution play a significant role in determining the size of cations.
Cations are smaller than their parent atoms because they have fewer electrons. The loss of electrons reduces the electron-electron repulsion within the ion, causing the remaining electrons to be more strongly attracted to the nucleus. This results in a decrease in the atomic radius of the cation compared to the neutral atom.