While atoms are typically electrically neutral, they can gain or lose electrons, transforming into charged particles known as ions. When an atom acquires one or more additional electrons, it develops a net negative charge, becoming a specific type of ion called an anion.
How Anions Form
Atoms often gain electrons to achieve a more stable electron configuration, typically by filling their outermost electron shell. This stable state often mimics the electron arrangement of noble gases, which are known for their inertness and complete outer shells. For instance, a chlorine atom, which naturally has seven electrons in its outermost shell, readily gains one electron to complete its shell and become a chloride anion. This acquisition of negatively charged electrons results in the atom having more electrons than protons, thereby creating a net negative charge.
The Effect on Atomic Size
When an atom undergoes the transformation into an anion by gaining electrons, its overall size, specifically its radius, expands. The atomic radius represents the distance from the atom’s nucleus to the outermost boundary of its electron cloud. This increase in size means the anion is larger than its original, neutral atom. This change is a consistent phenomenon observed across various elements that form anions.
Understanding the Size Change
The expansion in an atom’s radius upon forming an anion is primarily due to two interconnected factors: increased electron-electron repulsion and a reduced effective nuclear charge. When extra electrons are added to an atom, they occupy the existing electron shells, increasing the total number of negative charges within the electron cloud. These newly introduced electrons, being negatively charged, repel each other and the electrons already present in the atom. This heightened repulsion pushes the electrons further apart from each other, causing the entire electron cloud to spread out and expand, much like inflating a balloon.
Concurrently, the addition of electrons, while not altering the number of positively charged protons in the nucleus, effectively dilutes the nucleus’s attractive pull on each individual electron. The nucleus’s positive charge remains constant, but it now has to attract a greater number of negatively charged electrons. This results in a phenomenon known as decreased effective nuclear charge. The added electrons also contribute to an increased shielding effect, where inner electrons block some of the nuclear attraction from reaching the outermost electrons. Consequently, the outer electrons experience a weaker net attractive force from the nucleus, allowing them to drift further away and contributing to the overall increase in the anion’s radius.