Question

Moseley's experiments on \(X\) rays emitted from atoms led to the concept of atomic numbers. (a) If arranged in order of increasing atomic mass, which element would come after chlorine? (b) Describe two ways in which the properties of this element differ from the other elements in group 8A.

Short answer

The element that comes after chlorine in the periodic table is argon, with an atomic number of 18. Argon's properties differ from other elements in group 8A in two ways: its atomic radius (71 pm) is smaller than krypton, xenon, and radon but larger than helium and neon; and its ionization energy (1521 kJ/mol) is higher compared to most other noble gases like krypton, xenon, and radon but lower than helium and neon.

Step-by-step solution

Determine the atomic number of chlorine and find the following element

Using the periodic table, look up the atomic number of chlorine (Cl). Chlorine has an atomic number of 17. To find the element that follows chlorine, simply look for the element with an atomic number of 18.

Identifying the element with atomic number 18 and its group in the periodic table

The element after chlorine in the periodic table, with an atomic number of 18, is argon (Ar). Argon is a member of group 8A, along with other noble gases such as helium, neon, krypton, xenon, and radon.

Compare the properties of argon with other elements in group 8A

Two ways in which argon's properties differ from other elements in group 8A are: 1. Atomic radius: Argon's atomic radius (71 pm) is smaller than krypton, xenon, and radon, but larger than helium and neon. This difference affects its chemical reactivity and interactions with other elements. 2. Ionization energy: Argon has a higher ionization energy (1521 kJ/mol) compared to most other noble gases like krypton, xenon, and radon but lower than helium and neon. This means it takes more energy to remove an electron from argon compared to krypton, xenon, and radon, making it less reactive with other elements. In conclusion, the element that comes after chlorine in the periodic table is argon. Two ways in which argon differs from other elements in group 8A include its atomic radius and ionization energy.

Key concepts

Understanding the Periodic Table

The periodic table is a fundamental tool in chemistry, organizing all known elements in an informative array based on their atomic numbers, which represent the number of protons in an atom's nucleus. This ingenious chart classifies elements into groups and periods, revealing patterns in their chemical behavior.

As you move from left to right across a period, the atomic numbers increase. The modern periodic table arranges elements in order of increasing atomic number rather than atomic mass, a concept stemming from the pioneering work of Henry Moseley and his experiments with X-rays. This ordering not only reflects the elements' physical and chemical properties but also their electron configurations, leading to the periodic nature of the table.

For instance, if we consider chlorine with the atomic number 17, the next element in order is argon, with the atomic number 18. Students often find this arrangement helpful for predicting how an element will react chemically because elements that belong to the same group (vertical columns on the table) typically exhibit similar chemical behavior.

Chemical Properties and Their Periodic Trends

Chemical properties of elements are deeply interconnected with their position on the periodic table. Two such properties worth noting are atomic radius and ionization energy, which can vastly differ even among elements within the same group.

Atomic radius refers to the size of an atom, generally measured from the nucleus to the outer edge of the electron cloud. This size can affect an element's reactivity and bonding with others. As elements progress down a group, the radius typically increases due to additional electron shells.

Another critical property is ionization energy—the energy required to remove an electron from an atom in its gaseous state. Elements with higher ionization energies tend to be less reactive, as they are more reluctant to lose electrons. Across a period, ionization energy generally increases due to the rising nuclear charge, pulling electrons closer and making their removal more energy-intensive.

By understanding these properties, students can predict or explain the reactivity and other chemical behaviors of elements, enhancing their grasp of chemistry beyond rote memorization.

The Uniqueness of Noble Gases

Noble gases, located in group 8A—or group 18, using the IUPAC nomenclature—of the periodic table, are characterized by their remarkably low reactivity. This inert nature is due to their full valence electron shells, which make noble gases quite stable and generally resistant to forming bonds with other elements.

Argon, the noble gas that follows chlorine on the periodic table, serves as an exemplary case. Despite sharing the same group with other noble gases such as helium, neon, krypton, xenon, and radon, argon exhibits distinct chemical properties. Its atomic radius and ionization energy are pivotal in determining its stand within the group. Compared to its heavier counterparts, argon's relatively smaller atomic radius and higher ionization energy suggest that while still inert, its behaviors in certain situations can differ from other noble gases.

Understanding the nuances of noble gases, including argon's unique properties, not only sheds light on the incredible order of the periodic table but also assists students in grasping broader chemical concepts. It enables them to see the relative changes between each noble gas, rather than viewing the group as a monolith of inertness.