Question

How many electrons are present in each of the following ions? a. \(\mathrm{Se}^{2-}\) b. \(\mathrm{Br}^{-}\) c. \(\mathrm{Cr}^{3+}\) d. \(\mathrm{Rb}^{+}\) e. \(\mathrm{Bi}^{3+}\) f. \(\mathrm{Cu}^{2+}\)

Short answer

The number of electrons in each ion is as follows: a. \( \mathrm{Se}^{2-}\): 36 electrons b. \( \mathrm{Br}^{-}\): 36 electrons c. \( \mathrm{Cr}^{3+}\): 21 electrons d. \( \mathrm{Rb}^{+}\): 36 electrons e. \( \mathrm{Bi}^{3+}\): 80 electrons f. \( \mathrm{Cu}^{2+}\): 27 electrons

Step-by-step solution

Identify the atomic number of the element

Atomic number can be found in the periodic table. Look up the atomic number for each element given in the exercise.

Add or subtract the charge to find the number of electrons

If the ion has a negative charge, add the charge to the atomic number to find the number of electrons. If the ion has a positive charge, subtract the charge from the atomic number to find the number of electrons. a. \( \mathrm{Se}^{2-}\) Step 1: The atomic number of Se (Selenium) is 34. Step 2: Se has a 2- charge, so add 2 to the atomic number: \(34 + 2 = 36\) electrons. b. \( \mathrm{Br}^{-}\) Step 1: The atomic number of Br (Bromine) is 35. Step 2: Br has a 1- charge, so add 1 to the atomic number: \(35 + 1 = 36\) electrons. c. \( \mathrm{Cr}^{3+}\) Step 1: The atomic number of Cr (Chromium) is 24. Step 2: Cr has a 3+ charge, so subtract 3 from the atomic number: \(24 - 3 = 21\) electrons. d. \( \mathrm{Rb}^{+}\) Step 1: The atomic number of Rb (Rubidium) is 37. Step 2: Rb has a 1+ charge, so subtract 1 from the atomic number: \(37 - 1 = 36\) electrons. e. \( \mathrm{Bi}^{3+}\) Step 1: The atomic number of Bi (Bismuth) is 83. Step 2: Bi has a 3+ charge, so subtract 3 from the atomic number: \(83 - 3 = 80\) electrons. f. \( \mathrm{Cu}^{2+}\) Step 1: The atomic number of Cu (Copper) is 29. Step 2: Cu has a 2+ charge, so subtract 2 from the atomic number: \(29 - 2 = 27\) electrons.

Key concepts

Atomic Number

The atomic number is a core concept in understanding elements and their properties. It is the number of protons in the nucleus of an atom, which is unique to each element. This number not only defines the type of element but also its position on the periodic table. For instance, an element like Selenium has an atomic number of 34, meaning it has 34 protons in its nucleus. Similarly, Bromine with an atomic number of 35, has 35 protons.

Understanding the atomic number is crucial because it also tells us about the number of electrons in a neutral atom of the element. In a neutral state, the number of electrons equals the number of protons, ensuring the electrical neutrality of the atom. This balance shifts when an atom becomes an ion by gaining or losing electrons, which will alter its charge but not its atomic number. Therefore, the atomic number remains constant regardless of the ion's charge.

Charge of Ions

Ions are charged particles that form when atoms gain or lose electrons. The charge of an ion is directly related to the difference between the number of protons and electrons. If an atom loses electrons, it becomes positively charged, known as a cation. Conversely, if it gains electrons, it becomes negatively charged, known as an anion.

To calculate the charge of an ion, consider the balance between protons (positive) and electrons (negative). For example, if an atom with atomic number 24 loses 3 electrons, it forms a \( \mathrm{Cr}^{3+} \) ion, indicating a loss of three negatively charged electrons, resulting in a net positive charge. Conversely, the \( \mathrm{Se}^{2-} \) ion occurs when a Selenium atom gains two electrons, leading to a net negative charge of two.

This concept is pivotal in understanding chemical reactions, particularly those involving ionic compounds, where ions are crucial in forming stable structures due to their electrostatic interactions.

Periodic Table

The periodic table is an essential tool for chemists, organizing all known elements according to their atomic number, electron configurations, and recurring chemical properties. It helps predict the behavior of elements and their ions in chemical reactions because it visually represents relationships between different elements.

Elements in the same column, or group, typically share similar chemical properties and the same number of electrons in their outer shell, which influences their reactivity. For instance, the halogens, such as Bromine, are all one electron away from a full outer shell, making them highly reactive.

Using the periodic table, you can determine the atomic number needed to find the number of electrons in an ion. If you know the charge of an ion, you can predict its electron count by adjusting for the number of electrons gained or lost. Understanding where an element sits on the periodic table is fundamental to predicting its chemical behavior and the formation of ions.