Question

For each element, indicate the number of valence electrons, core electrons, and unpaired electrons in the ground state: (a) carbon, (b) phosphorus, (c) neon.

Short answer

(a) Carbon: 4 valence electrons, 2 core electrons, 2 unpaired electrons. (b) Phosphorus: 5 valence electrons, 10 core electrons, 3 unpaired electrons. (c) Neon: 8 valence electrons, 2 core electrons, 0 unpaired electrons.

Step-by-step solution

(a) Carbon

For carbon, we need to determine its electron configuration. Carbon has 6 electrons (since its atomic number is 6). The electron configuration is 1s² 2s² 2p². - Valence electrons are the electrons in the outermost shell, so carbon has 4 valence electrons (2s² 2p²). - Core electrons are the electrons in the inner shells, so carbon has 2 core electrons (1s²). - Unpaired electrons are electrons that are not paired with another electron in an orbital. In carbon, there are 2 unpaired electrons (2p²).

(b) Phosphorus

For phosphorus, we need to determine its electron configuration. Phosphorus has 15 electrons (since its atomic number is 15). The electron configuration is 1s² 2s² 2p⁶ 3s² 3p³. - Valence electrons are the electrons in the outermost shell, so phosphorus has 5 valence electrons (3s² 3p³). - Core electrons are the electrons in the inner shells, so phosphorus has 10 core electrons (1s² 2s² 2p⁶). - Unpaired electrons are electrons that are not paired with another electron in an orbital. In phosphorus, there are 3 unpaired electrons (3p³).

(c) Neon

For neon, we need to determine its electron configuration. Neon has 10 electrons (since its atomic number is 10). The electron configuration is 1s² 2s² 2p⁶. - Valence electrons are the electrons in the outermost shell, so neon has 8 valence electrons (2s² 2p⁶). - Core electrons are the electrons in the inner shells, so neon has 2 core electrons (1s²). - Unpaired electrons are electrons that are not paired with another electron in an orbital. Neon has a full outer shell, and all the electrons are paired, so it has 0 unpaired electrons.

Key concepts

Valence Electrons

Understanding valence electrons is critical as they determine an element's chemical properties and its ability to bond with other elements. Valence electrons are indeed the electrons present in the outermost shell of an atom. These electrons play a pivotal role in chemical reactions as they can be gained, lost, or shared during chemical bonding. For example, carbon, as mentioned in the exercise, has four valence electrons, which makes it tetravalent and allows it to form various organic compounds by making four bonds with other atoms.

In a step-by-step approach to identifying valence electrons, one must first find the atom's electron configuration and then count the electrons in the highest energy level, often referred to as the 'valence shell'. Students can remember that in main-group elements, the number of valence electrons corresponds to the group number in the periodic table.

Core Electrons

Core electrons, on the other hand, are the electrons that occupy the inner shells of an atom. These are not involved in chemical bond formation or reactions as the valence electrons are. Instead, core electrons serve to shield the positively charged nucleus, affecting the size of the atom and the energy required to remove an electron from an atom (ionization energy).

Identifying Core Electrons

As seen with carbon and neon in the exercise, identifying core electrons involves counting all electrons in an element's electron configuration except those in the valence (outermost) shell. For instance, even though phosphorus has fifteen electrons in total, only ten of these are core electrons as these occupy the 1s, 2s, and 2p orbitals, which are at lower energy levels than the valence electrons occupying the 3s and 3p orbitals.

Unpaired Electrons

Unpaired electrons are those that exist alone in an orbital when a subshell is not completely filled. These unpaired electrons are significant because they provide an atom with magnetic properties and are also available for bonding. The arrangement of electrons within an orbital follows Hund's rule, which states that every orbital in a subshell is singularly occupied by one electron before any orbital is doubly occupied, and all electrons in singularly occupied orbitals have the same spin.

For instance, as seen in the exercise, carbon has two unpaired electrons in the 2p orbitals, making it paramagnetic. However, neon has all electrons paired and is therefore diamagnetic. An easy way to determine the number of unpaired electrons is by looking at the electron configuration of an atom and noting the number of orbitals that do not have electron pairs.