Question

How many valence electrons does each of the following atoms possess? a. lithium, \(Z=3\) b. aluminum, \(Z=13\) c. argon, \(Z=18\) d. phosphorus, \(Z=15\)

Short answer

a. Lithium (Z=3) has 1 valence electron. b. Aluminum (Z=13) has 3 valence electrons. c. Argon (Z=18) has 8 valence electrons. d. Phosphorus (Z=15) has 5 valence electrons.

Step-by-step solution

a. Lithium, Z=3

The atomic number (\(Z\)) determines the number of protons and electrons in a neutral atom. Lithium has an atomic number \(Z=3\). Let's look at the electron configuration for lithium: 1s² 2s¹ Which means there are 2 electrons in the 1s subshell and 1 electron in the 2s subshell. The valence electrons are present in the outermost shell. Since lithium has one electron in its outermost shell (shell 2), it has 1 valence electron.

b. Aluminum, Z=13

The atomic number of aluminum is \(Z=13\). The electron configuration for aluminum is: 1s² 2s² 2p⁶ 3s² 3p¹ The valence electrons are present in the outermost shell (i.e., shell 3). Aluminum has a total of 2 (from 3s²) + 1 (from 3p¹) = 3 valence electrons.

c. Argon, Z=18

The atomic number of argon is \(Z=18\). The electron configuration for argon is: 1s² 2s² 2p⁶ 3s² 3p⁶ As we can see, the outermost shell (shell 3) has 8 electrons in total. Thus, argon has 8 valence electrons. This is also why argon is a noble gas because it has a filled outermost electron shell.

d. Phosphorus, Z=15

The atomic number of phosphorus is \(Z=15\). The electron configuration for phosphorus is: 1s² 2s² 2p⁶ 3s² 3p³ The outermost shell of phosphorus contains 5 electrons (2 from 3s² and 3 from 3p³). Therefore, phosphorus has 5 valence electrons.

Key concepts

Understanding Atomic Number

The atomic number, often denoted as Z, is a fundamental characteristic of an element that tells us the number of protons present in the nucleus of an atom. Since elements are electrically neutral in their most stable form, the atomic number also indicates the number of electrons orbiting the nucleus. For example, lithium has an atomic number of 3, which means it has 3 protons and, consequently, 3 electrons.

The atomic number is unique to each element and determines its position in the periodic table. It is essential for identifying an element and influences the chemical properties, including how atoms interact with one another. It is the increasing number of protons that differentiates one element from another. This knowledge is crucial when we delve into concepts like valence electrons and electron configuration, which are deeply interconnected with an element's chemical behavior.

Deciphering Electron Configuration

Electron configuration reveals the distribution of electrons among the various shells and subshells in an atom. The arrangement is determined by principles such as the Pauli exclusion principle, Hund's rule, and the Aufbau principle, which dictate that electrons fill the lower-energy orbitals before occupying higher-energy levels.

For instance, aluminum's electron configuration is expressed as 1s² 2s² 2p⁶ 3s² 3p¹. This notation indicates the number of electrons present in each subshell: two in the 1s, two in the 2s, six in the 2p, two in the 3s, and finally one in the 3p subshell. The most distant electrons from the nucleus, the valence electrons, play a vital role in chemical bonding and reactivity. Therefore, understanding electron configuration is essential for predicting how an element will react chemically. To facilitate learning, one should recognize patterns in the configurations across periods and groups in the periodic table.

The Stability of Noble Gases

Noble gases, located in group 18 of the periodic table, are renowned for their chemical inertness. These elements, including helium (He), neon (Ne), argon (Ar), and others, possess a complete valence shell, typically achieving the stable electronic configuration of a filled s and p subshell in their outermost energy level.

For example, argon has the electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁶, resulting in a full outer shell of eight valence electrons. This full valence shell is often referred to as the 'octet' and is associated with chemical stability, a characteristic that makes noble gases very unreactive. The desire of other elements to achieve this 'noble gas configuration' through reactions, such as gaining, losing, or sharing electrons, is a driving force behind many chemical processes. Students might find it helpful to explore how other elements interact to achieve electron configurations analogous to noble gases.