Question

(a) True or false: An element's number of valence electrons is the same as its atomic number. (b) How many valence electrons does a nitrogen atom possess? (c) An atom has the electron configuration 1\(s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{2}\) . How many valence electrons does the atom have?

Short answer

(a) False. An element's number of valence electrons is not always the same as its atomic number. (b) A nitrogen atom has 5 valence electrons. (c) The atom with the given electron configuration has 4 valence electrons.

Step-by-step solution

(a) Assessing the statement about valence electrons and atomic number.

The atomic number of an element represents the number of protons in its nucleus. The valence electrons are the electrons in the outermost energy level of an atom. The statement claims that the number of valence electrons is equal to the atomic number. This statement is not always correct. For example, lithium has an atomic number of 3 but has only 1 valence electron in its outermost energy level. So, the statement is false.

(b) Determining the number of valence electrons in nitrogen.

Nitrogen has an atomic number of 7. Its electron configuration is 1s^2 2s^2 2p^3. The valence electrons are those present in the outermost energy level, which is the second level for nitrogen. There are 2 electrons in the 2s subshell and 3 electrons in the 2p subshell. Therefore, nitrogen has 2 + 3 = 5 valence electrons.

(c) Calculating the number of valence electrons in the given atom.

The given electron configuration is 1s^2 2s^2 2p^6 3s^2 3p^2. The outermost energy level is the third level. In the third level, there are 2 electrons in the 3s subshell and 2 electrons in the 3p subshell. So, the atom has 2 + 2 = 4 valence electrons.

Key concepts

Electron Configuration

Electron configuration describes how electrons are distributed in an atom’s orbitals. Electrons orbit the nucleus in specific regions known as shells, and these shells contain subshells or orbitals where electrons reside. In a typical electron configuration, each element is denoted by a sequence of numbers and letters which reveal the filling of these orbitals.
For example, the electron configuration 1s\(^{2}\) 2s\(^{2}\) 2p\(^{6}\) 3s\(^{2}\) 3p\(^{2}\) signifies the distribution of electrons over different energy levels and sublevels. The numbers indicate the principal energy level, while the letters (s, p, d, f) denote the types of sublevels.
These configurations can help us determine an element’s chemical properties, including how it interacts with other elements. Understanding the way electrons fill these spaces can also aid in predicting how an atom will bond in chemical reactions.

Atomic Number

The atomic number of an element is a fundamental property that signifies the number of protons in the nucleus of an atom. Each element on the periodic table is uniquely defined by this number.
For instance, nitrogen, with an atomic number of 7, has 7 protons in its nucleus. This number also determines the order of the element in the periodic table and is typically equal to the number of electrons in a neutral atom.
The atomic number provides insights into the identity of the element and its position in the periodic table. Although closely related to the atoms' electronic configuration, the number of valence electrons does not necessarily equal the atomic number, as they are only the electrons in the outermost energy shell.

Energy Levels

Energy levels, or electron shells, are regions around the nucleus where electrons reside, each with a certain capacity. The energy close to the nucleus is lower, with the energy increasing as it moves outward.
Electrons fill these levels based on the "aufbau principle," starting from the lowest energy level, typically denoted as level 1. As more electrons are added, they occupy higher levels like 2, 3, and so on, consisting of one or more subshells (s, p, d, f).
Understanding energy levels is crucial for determining an atom's electron configuration and predicting how electrons can interact with those of other atoms during formation of chemical bonds. Thus, knowing the outermost energy level is valuable in defining an element's valence electrons, which are pivotal in chemical reactions.