The full electron configuration of bromine is: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5. Label these electrons as core electrons or valence electrons using the drop-down arrows provided: 1s2 Select ] core electrons Select ] valence electrons 2s Select ] core electrons Select ] valence electrons 2p Select ] core electrons Select ] valence electrons 3s Select ] core electrons Select ] valence electrons 3p Select ] core electrons Select ] valence electrons 3d Select ] core electrons Select ] valence electrons 4s Select ] core electrons Select ] valence electrons 4p Select ] core electrons Select ] valence electrons NOTE: We will always show electron configurations with the orbitals shown in order of increasing energy in CHEM 1251. The electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5 would be incorrect for bromine.
The Correct Answer and Explanation is:
Correct Answers:
| Orbital | Label |
|---|---|
| 1s² | Core electrons |
| 2s² | Core electrons |
| 2p⁶ | Core electrons |
| 3s² | Core electrons |
| 3p⁶ | Core electrons |
| 3d¹⁰ | Core electrons |
| 4s² | Valence electrons |
| 4p⁵ | Valence electrons |
Explanation
To classify electrons as core or valence electrons, we need to understand the structure and behavior of atoms. Core electrons are the inner-shell electrons that do not participate in bonding, while valence electrons are in the outermost shell and are involved in chemical bonding and reactivity.
Bromine (Br) has an atomic number of 35, meaning it has 35 electrons in a neutral atom. Its full electron configuration is:
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁵
The highest principal energy level in this configuration is n = 4, which includes the 4s² and 4p⁵ orbitals. These electrons (totaling 7) are in the outermost shell and are therefore considered valence electrons.
The remaining electrons (in the 1s, 2s, 2p, 3s, 3p, and 3d orbitals) are all in lower energy levels and are classified as core electrons. Even though the 3d orbitals come after 4s in terms of energy filling, they are still part of the third principal energy level (n = 3), making them core electrons in this case.
Chemically, the reactivity of bromine—like other halogens—is largely due to its 7 valence electrons (4s² 4p⁵), needing just one more to achieve a stable octet.
It’s also important to note the instructional reminder in the image: Electron configurations must be written in increasing energy order, not just by principal quantum number. Hence, 3d comes after 4s, even though it is part of a lower principal energy level.
Correctly identifying valence electrons is crucial for predicting chemical properties, bonding behavior, and periodic trends such as electronegativity and ionization energy.
