Which element has (a) two shells, both of which are completely filled with electrons? (b) the electronic configuration \(2,8,2 ?\) (c) a total of three shells, with four electrons in its valence shell? (d) a total of two shells, with three electrons in its valence shell? (e) twice as many electrons in its second shell as in its first shell?

Electrons in an atom are arranged in specific regions around the nucleus called electron shells. These shells, also known as energy levels, follow a simple rule for the maximum number of electrons they can hold: the first shell can contain up to 2 electrons, the second can hold up to 8, the third also accommodates up to 18, and so forth. This arrangement is based on the principal quantum number and helps determine the atom's reactivity and bonding behavior.

Understanding electron shells is key to finding the element's electronic configuration, which is simply the distribution of electrons among these shells. For example:

• An element with an electronic configuration of 2,8,2 has 2 electrons in its first shell, 8 in the second, and 2 in the third.
• If a configuration is 2,4, it means 2 electrons in the first shell and 4 in the second.

Recognizing these patterns helps identify elements and predict their chemical properties.

Valence electrons are the electrons in the outermost shell of an atom. They play a crucial role in determining an element's chemical properties and reactivity. When an element engages in chemical reactions, it is usually the valence electrons that are lost, gained, or shared.

The concept of valence electrons can be illustrated through electronic configurations:

• An element with the configuration 2,8,2 has 2 valence electrons in its outermost shell.
• A different element with 2,8,4 would have 4 valence electrons.

This number of valence electrons influences how elements form bonds with other atoms. Elements with a full outer shell, like noble gases, are typically very stable and unreactive, while those with only one or two valence electrons are more reactive, eager to achieve a stable electron configuration by interacting with other atoms.

To identify an element based on its electron configuration, understanding both electron shells and valence electrons is important. For example, let's identify the following element configurations:

(a) **Two shells completely filled with electrons:** This is 2 electrons in the first shell and 8 in the second (2,8), totaling 10 electrons. The element is Neon (Ne).
(b) **Configuration 2,8,2:** This indicates the element has three shells with 2 electrons in the first, 8 in the second, and 2 in the third. The element is Magnesium (Mg).
(c) **Three shells, four valence electrons:** The configuration is 2,8,4, which totals 14 electrons. The element is Silicon (Si).
(d) **Two shells, three valence electrons:** This pattern (2,3) reveals the element is Boron (B).
(e) **Twice as many electrons in the second shell as the first:** The configuration 2,4 corresponds to Carbon (C).

By matching the number of electrons in each shell to specific patterns, we can identify various elements effectively.