\(2.5\) Give the number of valence clectrons for each of the following elements: (a) Ga; (b) Be; (c) Cr; (d) Sn.

Understanding the periodic table is fundamental for grasping the concept of valence electrons, which are the electrons located in the outermost shell of an atom. These electrons play a crucial role in chemical bonding and the properties of elements.

The periodic table is organized into 18 vertical columns known as groups or families. Each group typically shares common chemical properties and the same number of valence electrons. For instance, Group 1 elements have one valence electron, while Group 2 elements have two. These similarities among groups help predict the behavior of elements during chemical reactions.

• Group 1: Alkali metals with one valence electron
• Group 2: Alkaline earth metals with two valence electrons
• Group 13: Boron group elements with three valence electrons
• Group 14: Carbon group elements with four valence electrons
• Group 15: Pnictogens with five valence electrons
• Group 16: Chalcogens with six valence electrons
• Group 17: Halogens with seven valence electrons
• Group 18: Noble gases with eight valence electrons (except Helium, which has two)

Remember, while Group numbers can provide a quick reference to determine valence electrons, there are exceptions, particularly among the transition metals.

Transition metals are located in groups 3 through 12 on the periodic table and have unique electron configurations. Unlike elements in other groups, transition metals can exhibit various oxidation states and have a variable number of valence electrons. This is because they can involve their d-orbital electrons in bonding, in addition to their outermost s-orbital electrons.

For example, Chromium (Cr) from Group 6 does not strictly adhere to the expected 'group number minus 10' rule for determining valence electrons. Instead, it often has multiple valence electron configurations due to the energy proximity of its s and d orbitals. Therefore, Chromium may typically be treated as having 6 valence electrons.

It is important to note that the electron configuration of transition metals can have a profound impact on their chemical properties and reactivities. This is one of the many reasons why transition metals are so versatile and why they have such a wide range of applications in various industries.

Electron configuration is a term used to describe the arrangement of electrons around an atom's nucleus according to specific rules. The electron configuration determines how many valence electrons an element has. Knowing the electron configuration enables us to predict how an element will react chemically.

Electrons fill up available orbitals starting with the lowest energy level, progressing to higher ones in a manner described by the Aufbau principle. Electrons are placed into orbitals following Hund's rule, which states that every orbital in a subshell is singly occupied before any orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.

The noble gas shorthand is a common way to represent electron configurations for elements with a large number of electrons. This method uses the configuration of the nearest noble gas element (preceding the element in question), followed by the configuration of the remaining electrons. For example, the element Tin (Sn) would have an electron configuration expressed as [Kr]5s²4d¹⁰5p², indicating that beyond the electron configuration of Krypton, Sn has two electrons in the 5s orbital, ten in the 4d orbitals, and two in the 5p orbitals. This results in a total of four valence electrons.