Problem 90 - Introductory Chemistry Atoms First

Question

How many valence electrons does each of these atoms have? (a) \(1 s^{2} 2 s^{2} 2 p^{3}\) (b) \(1 s^{2} 2 s^{1}\) (c) \(1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{7}\) (d) \(1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6}\) (e) \(1 s^{2}\)

Step-by-Step Solution

Answer

(a) 5 valence electrons (b) 1 valence electron (c) 2 valence electrons (d) 8 valence electrons (e) 2 valence electrons

Step 1: Identify the Outermost Energy Level for Each Atom

To determine the number of valence electrons, we first need to identify the outermost energy level (highest principal quantum number) for each atom. Look at the electron configurations given and identify the highest quantum number (indicated by the numbers before each letter, e.g., 1 in 1s²). (a) \(1 s^{2} 2 s^{2} 2 p^{3}\) - Outermost energy level: 2 (b) \(1 s^{2} 2 s^{1}\) - Outermost energy level: 2 (c) \(1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{7}\) - Outermost energy level: 4 (d) \(1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6}\) - Outermost energy level: 3 (e) \(1 s^{2}\) - Outermost energy level: 1

Step 2: Count the Number of Valence Electrons

Count the total number of electrons in the outermost energy level for each atom to find the number of valence electrons. (a) \(1 s^{2} 2 s^{2} 2 p^{3}\) - Valence electrons: 2 (from 2s²) + 3 (from 2p³) = 5 (b) \(1 s^{2} 2 s^{1}\) - Valence electrons: 1 (from 2s¹) = 1 (c) \(1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{7}\) - Valence electrons: 2 (from 4s²) = 2 (d) \(1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6}\) - Valence electrons: 2 (from 3s²) + 6 (from 3p⁶) = 8 (e) \(1 s^{2}\) - Valence electrons: 2 (from 1s²) = 2 So, the number of valence electrons for each atom is as follows: (a) 5 valence electrons (b) 1 valence electron (c) 2 valence electrons (d) 8 valence electrons (e) 2 valence electrons

Key Concepts

Electron Configuration

Understanding electron configuration is essential in chemistry, as it describes the distribution of electrons in the orbitals around the nucleus of an atom. The configuration follows a pattern known as the Aufbau principle, which states that electrons occupy the lowest energy orbitals first before moving to higher ones.

Let's consider an atom with the electron configuration of 1s^{2} 2s^{2} 2p^{3}. The notation here is divided into sections, with the principal quantum number (e.g., '1', '2') indicating the energy level and the letters 's' and 'p' representing the type of orbital. The superscript number tells us how many electrons are in that orbital. So, for 2p^{3}, it means three electrons in a 2p orbital. This pattern is crucial for determining the properties of elements, such as reactivity and bonding capability.

To improve comprehension of this subject, students should study the patterns of the periodic table which reflect how these configurations change across elements. By recognizing trends, you'll more quickly grasp where electrons reside in an atom's electron cloud.

Principal Quantum Number

The principal quantum number, symbolized by 'n', is a term used in quantum mechanics to indicate the energy level of an electron in an atom. Think of it as the 'floor' on which the electron 'lives', with the 'ground floor' being n=1. The higher the number, the further away from the nucleus and the higher the energy of the electrons.

For instance, in the configuration 4s^{2}, the number '4' before the 's' is the principal quantum number and it tells us those electrons are quite a distance from the nucleus, residing in the fourth energy level. Electrons in higher principal quantum levels have more energy and are more loosely bound to the nucleus, which often translates into more reactivity. For better retention, students may liken the principal quantum number to steps on a ladder, where each step represents a new energy level an electron can climb to.

Outermost Energy Level

The term outermost energy level refers to the energy level that contains the electron farthest from an atom's nucleus. Valence electrons, which are involved in chemical bonding and reactions, are the electrons in the outermost energy level. It is these electrons that largely determine an element's chemical properties.

By exploring the electron configuration, like 3s^{2} 3p^{6}, you recognize that the outermost energy level is indicated by the highest principal quantum number - in this case, '3'. Remember, the valence electrons are only those in the outermost level, not the sum of all electrons. To enhance understanding, one might imagine these outer electrons as the 'hands' of an atom, reaching out to interact with other atoms to create compounds. Drawing diagrams depicting electron shells can be particularly helpful in visualizing which electrons are in the outermost energy level.