An atom has electronic configuration \(2,8,7\). (a) What is the atomic number of this element? (b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.) \(\begin{array}{llll}\text { N(7) } & \text { F(9) } & \text { P(15) } & \operatorname{Ar}(18)\end{array}\)

The atomic number of an element is fundamental to understanding its properties and behavior. It tells us the number of protons in the nucleus of an atom and, in a neutral atom, also equals the number of electrons. By knowing the atomic number, we can identify the element on the periodic table.

For the given exercise, the electronic configuration is 2,8,7. To find the atomic number, sum the electrons in each shell: \(2 + 8 + 7 = 17\). Thus, this element has an atomic number of 17, which corresponds to Chlorine (Cl) on the periodic table.

Electronic configuration describes how electrons are distributed in an atom's orbitals. It's important for predicting the chemical behavior of an element. The configuration given in the exercise is 2,8,7, which means:

• 2 electrons in the first shell (K-shell)
• 8 electrons in the second shell (L-shell)
• 7 electrons in the third shell (M-shell)

By following the electron distribution rules (Aufbau principle, Hund's rule, and Pauli exclusion principle), we can determine the arrangement of electrons.
In this case, the configuration leads us to an atomic number of 17, indicating it's Chlorine. This information is crucial for understanding the chemical properties and reactivity of the element.

Chemical similarity refers to elements behaving similarly due to similar electronic configurations, particularly in their outermost shells. Elements in the same group (column) of the periodic table often exhibit similar chemical properties because they have the same number of valence electrons.
In this exercise, we identified the element as Chlorine (Cl) with an atomic number of 17, placing it in Group 17 (halogens) of the periodic table.
To find an element chemically similar to Chlorine among the given options:

• Nitrogen (N) - Atomic number 7 - Group 15
• Fluorine (F) - Atomic number 9 - Group 17
• Phosphorus (P) - Atomic number 15 - Group 15
• Argon (Ar) - Atomic number 18 - Group 18

Fluorine (F) is in the same group as Chlorine, thus it is chemically similar due to both having seven valence electrons, influencing their similar reactivity and chemical characteristics.