Write the expected ground-state electron configuration for each of the following. a. the lightest halogen atom b. the alkali metal with only \(2 p\) and \(3 p\) electrons c. the Group \(3 \mathrm{~A}\) element in the same period as \(\mathrm{Sn}\) d. the nonmetallic elements in Group \(4 \mathrm{~A}\)

Electron configuration is a way to represent the distribution of electrons in the atomic orbitals of an atom. It follows a specific order known as the Aufbau Principle, which states that electrons fill lower-energy orbitals first before moving to higher-energy ones. This concept is vital because the arrangement of electrons determines the chemical properties of an element, including how it will react with other substances.
An easy way to remember this is to think of it like filling seats in a theater, where people (electrons) fill in from the front rows (lower energy levels) to the back (higher energy levels), one at a time. The resulting electron configuration for an atom will look something like this for Fluorine (F): 1s² 2s² 2p⁵, indicating that the first energy level (1s) is fully filled with 2 electrons, followed by the second level (2s) with another 2 electrons, and finally the 2p orbital has 5 electrons.

The periodic table is not just a table of elements; it's a map of the atomic landscape. This arrangement of all known elements is ordered by their atomic number — the number of protons in the nucleus — and grouped based on similar properties. Elements are placed into rows called periods and columns called groups.
Understanding the periodic table allows you to predict how elements will behave. For example, elements in the same group usually have similar chemical reactions, and you can see the pattern of their electron configurations. As for our exercise, when we know that Sodium (Na) sits in the alkali metals group, we can infer certain characteristics like its single valence electron, represented by the 3s¹ in its electron configuration.

Halogen atoms are located in Group 17 of the periodic table and have a distinct set of properties. They are one electron short of a full octet, meaning they are highly reactive and readily form compounds with alkali and alkaline earth metals. Halogens include elements like Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I).
Characteristic of Halogens:

• They exist in various physical states across the group (gas, liquid, and solid).
• Have high electronegativity.
• Form salts when reacting with metals.

In the exercise's context, the lightest halogen atom, Fluorine, has an electron configuration depicting its high reactivity: 1s² 2s² 2p⁵.

Alkali metals make up Group 1 of the periodic table and are known for being the most reactive group of metals. This group includes Lithium (Li), Sodium (Na), Potassium (K), etc., which all have one electron in their outermost shell, leading to their distinctive characteristics.
Characteristics of Alkali Metals

• Soft and easily cut.
• Have low melting points compared to most metals.
• Highly reactive, especially with water, leading to explosive reactions.
• Form strong alkali solutions when they react with water.

In our exercise, Sodium (Na) represents alkali metals with its configuration, 1s² 2s² 2p⁶ 3s¹, indicating a single valence electron ready to react.

Group A elements in the periodic table represent the 'main-group' elements, which include the most familiar metals, nonmetals, and metalloids. These groups are numbered from 1A to 8A (sometimes referred to simply as 1 to 18 without the letter 'A') and signify the number of valence electrons, which are the electrons available for bonding.
Group A elements are crucial for understanding chemical reactions because they often define the reactivity and bonding situations of the elements. Elements within the same group share similar properties because they have the same number of electrons in their outermost shell. For example, in our exercise, Gallium (Ga), a Group 3A element, shares its group with other elements that have three valence electrons, which is reflected in its electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p¹.