Question

How do we determine the total number of valence electrons for an ion? Provide an example of an anion and a cation, and explain your answer.

Short answer

To determine the total number of valence electrons for an ion, find the element's group number in the periodic table, which represents the number of valence electrons for a neutral atom. Then, add or subtract the ionic charge to calculate the total number of valence electrons for the ion. For example, for the sulfide ion (S^2-), a sulfur atom is in Group 16, so a neutral sulfur atom has 6 valence electrons. As an anion with a 2- charge, add the charge to the group number to get 8 valence electrons. For the calcium ion (Ca^2+), a calcium atom is in Group 2, so a neutral calcium atom has 2 valence electrons. As a cation with a 2+ charge, subtract the charge from the group number to get 0 valence electrons.

Step-by-step solution

Identify the element of the ion

Refer to the periodic table to know the element of the ion. Elements in the periodic table are arranged according to the number of protons in their nucleus; this number is known as the atomic number.

Determine the group of the element

Identify the group number of the element in the periodic table. The group number represents the number of valence electrons for a neutral atom (an atom with no ionic charge).

Determine the ionic charge

Know the ionic charge of the ion. Anions have a negative charge, indicating an increase in electrons; cations have a positive charge, indicating a decrease in electrons.

Calculate the total number of valence electrons for the ion

Add or subtract the ionic charge from the group number to determine the total number of valence electrons for the ion. If the ion is an anion, add the absolute value of its charge to the group number. If the ion is a cation, subtract the charge from the group number. Example:

Anion example: Sulfide ion (S^2-)

Step 1: Identify the element of the ion - Sulfur (S) Step 2: Determine the group of the element - Group 16 (6 valence electrons for a neutral sulfur atom) Step 3: Determine the ionic charge - 2- (anion) Step 4: Calculate the total number of valence electrons - 6 (group number) + 2 (ionic charge) = 8 valence electrons

Cation example: Calcium ion (Ca^2+)

Step 1: Identify the element of the ion - Calcium (Ca) Step 2: Determine the group of the element - Group 2 (2 valence electrons for a neutral calcium atom) Step 3: Determine the ionic charge - 2+ (cation) Step 4: Calculate the total number of valence electrons - 2 (group number) - 2 (ionic charge) = 0 valence electrons

Key concepts

Understanding the Periodic Table and Valence Electrons

The periodic table isn't just a colorful wall chart found in science classrooms; it's a treasure trove of information about the elements that make up our world. Arranged by increasing atomic number, the table groups elements with similar properties. One particularly valuable piece of information is the group number for each element, which corresponds to the number of valence electrons in a neutral atom of that element. Valence electrons play a pivotal role in chemical bonding and reactions, as they are the outermost electrons and, thus, the most reactive.

For instance, oxygen, which is in group 16 of the periodic table, has six valence electrons when neutral. These valence electrons determine the chemical behavior of the atom and its ability to form bonds. By understanding the arrangement of the periodic table, you can predict an element’s reactivity and the types of bonds it can form, which is fundamental in fields such as chemistry and materials science.

Ion Charge and Valence Electrons

An ion is an atom or molecule that has lost or gained one or more of its valence electrons, giving it a net positive or negative electrical charge. Gaining electrons creates an anion (negative charge), while losing electrons forms a cation (positive charge). This charge has profound effects on the properties of the ion compared to its neutral state.

For example, a neutral sodium atom has one valence electron, but as an ion (Na⁺), it has lost that one electron, resulting in no valence electrons remaining. This loss or gain of electrons transforms an atom's ability to engage in chemical interactions, which is crucial for processes like the conduction of electricity in our cells, or the formation of salts. The ion charge provides important information for predicting the chemical formulas of compounds formed by ionic bonds.

Anions and Cations Examples

Anions and cations are the two categories of ions, differentiated by whether they carry a net negative or positive charge, respectively. The sulfide ion (S^2-), for example, is an anion because it has two extra electrons, giving sulfur a total of eight valence electrons instead of its neutral state of six. This additional electron count makes it more stable, as it completes sulfur's valence shell, similar to the noble gases known for their low reactivity.

Conversely, the calcium ion (Ca²⁺) is a cation, having lost two electrons, which leads to it having zero remaining valence electrons. This change in electron count affects how calcium ions interact with other ions and molecules and is a key player in biological processes such as muscle contractions and neurotransmission. Whether dealing with an anion or cation, the change in the number of valence electrons is central to the concept of ionic bonds forming between oppositely charged ions. The ability to predict the charge of ions formed by elements is a fundamental skill in understanding chemical reactions.