Ch.2 - Atoms, Molecules & Ions

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.2 - Atoms, Molecules & Ions

Problem 138

Chapter 2, Problem 138

How many protons and electrons are in each of the following ions?(a) Be2+b) Rb+c) Se2-(d) Au3+

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1. The number of protons in an atom or ion is equal to the atomic number of the element. You can find the atomic number of an element in the periodic table. For example, the atomic number of Be (Beryllium) is 4, so Be2+ has 4 protons.

2. The number of electrons in a neutral atom is equal to the number of protons. However, in an ion, the number of electrons is altered by the charge of the ion. A positive charge indicates that electrons have been lost, while a negative charge indicates that electrons have been gained. For example, Be2+ has lost 2 electrons, so it has 2 fewer electrons than protons. Therefore, Be2+ has 2 electrons.

3. Apply the same principles to find the number of protons and electrons in Rb+ (Rubidium ion), Se2- (Selenium ion), and Au3+ (Gold ion).

4. For Rb+, the atomic number of Rb is 37, so it has 37 protons. Since it has a +1 charge, it has lost 1 electron, so it has 36 electrons.

5. For Se2-, the atomic number of Se is 34, so it has 34 protons. Since it has a -2 charge, it has gained 2 electrons, so it has 36 electrons. For Au3+, the atomic number of Au is 79, so it has 79 protons. Since it has a +3 charge, it has lost 3 electrons, so it has 76 electrons.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Atomic Structure

Atoms consist of protons, neutrons, and electrons. Protons are positively charged particles found in the nucleus, while electrons are negatively charged and orbit the nucleus. The number of protons defines the element, and in a neutral atom, the number of electrons equals the number of protons.

Ions and Charge

Ions are atoms or molecules that have gained or lost electrons, resulting in a net charge. A positive charge indicates a loss of electrons (cations), while a negative charge indicates a gain of electrons (anions). The charge of an ion directly affects the balance between protons and electrons.

Determining Protons and Electrons in Ions

To find the number of protons and electrons in an ion, start with the atomic number, which indicates the number of protons. For cations, subtract the charge from the number of protons to find the number of electrons. For anions, add the charge to the number of protons to determine the number of electrons.

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Textbook Question

What is the identity of the element X in the following ions? (a) X2+, a cation that has 36 electrons (b) X-, an anion that has 36 electrons

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Use the data from the mass spectrum of a sample of an element to calculate the element's atomic weight. Identify the element.

Textbook Question

Refer to Figure 2.10 showing a schematic illustration of a mass spectrometer. (b) Compare two ions with a +1 charge traveling through the curved, evacuated tube in the mass spectrometer. Will a heavier ion or lighter ion be deflected to a greater degree by the magnetic field?

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Use the data from the mass spectrum of a sample of an element to calculate the element's atomic weight. Identify the element.