5.9 Cations and Anions

• Neutral Atom:

• In a neutral atom, the number of positively charged protons in the nucleus is balanced by the same number of negatively charged electrons outside the nucleus, making the atom electrically neutral.

• Formation of Cations (Positive Ions):

• When one or more electrons are removed from the outermost energy level of a neutral atom, it loses its neutrality and becomes positively charged.

• A positively charged ion is called a cation.

• Metals on the left side of the periodic table typically lose electrons to form cations. This is because:

• Metals have fewer electrons (usually 1, 2, or 3) in their outermost energy level.

• Due to the larger size of metals, their outermost electrons are farther from the nucleus and are less strongly attracted to it, meaning they are easier to remove.

• This results in lower ionization energy (energy required to remove an electron), making it easier for metals to lose electrons.

• Example:

• Lithium (Li): Lithium has one electron in its outermost energy level. It donates this electron to achieve the electronic configuration of helium (He), becoming a lithium cation (Li⁺).

• Sodium (Na): Sodium also donates an electron from its outermost energy level to achieve the configuration of neon (Ne), forming a sodium cation (Na⁺).

• Why Metals Donate Electrons to Form Cations:

• As you move across a period in the periodic table from left to right, the metallic properties decrease, and non-metallic properties increase.

• Metals, which are on the left side of a period, have larger atoms with outer electrons farther from the nucleus, making it easier to lose electrons.

• This is the reason why metals donate electrons to form cations.

• Formation of Anions (Negative Ions):

• Non-metals, which are on the right side of the periodic table, do not form cations. Instead, they typically accept electrons to achieve a full outer energy level (octet).

• Non-metals usually have 5, 6, or 7 electrons in their outermost energy level and need to gain electrons to complete their octet, achieving the stable configuration of the nearest inert gas.

• Due to their smaller size, the outermost electrons of non-metals are closer to the nucleus and experience a stronger attractive force, requiring more energy to remove an electron. This makes them more likely to accept electrons rather than lose them.

• The acceptance of electrons from other atoms leads to an excess of electrons compared to protons, causing the atom to become negatively charged. These negatively charged atoms are called anions.

• Example:

• Chlorine (Cl): Chlorine has 7 electrons in its outermost energy level. It accepts one electron to achieve the electronic configuration of argon (Ar), forming a chloride anion (Cl⁻).

In summary:

• Cations are positively charged ions formed when metals lose electrons.

• Anions are negatively charged ions formed when non-metals gain electrons.