Question

For each element, tell how many electrons are in each energy level and write the electron dot structure. (Chapter 5) \(\begin{array}{ll}{\text { a. Kr }} & {\text { d. B }} \\ {\text { b. Sr }} & {\text { e. Br }} \\ {\text { c. } P} & {\text { f. Se }}\end{array}\)

Short answer

a. Krypton (Kr): 18 electrons in first two energy levels, 18 electrons in third and fourth energy levels. Electron dot structure: \[\text{..}\]\[\text{.Kr.}\]\[\text{..}\] b. Strontium (Sr): 18 electrons in first two energy levels, 18 electrons in third and fourth energy levels, 2 electrons in fifth energy level. Electron dot structure: \[\text{.Sr.}\] c. Phosphorus (P): 10 electrons in first two energy levels, 5 electrons in third energy level. Electron dot structure: \[\text{..}\]\[\text{.P.}\] d. Boron (B): 2 electrons in first energy level, 3 electrons in second energy level. Electron dot structure: \[\text{.B.}\] e. Bromine (Br): 18 electrons in first two energy levels, 17 electrons in third and fourth energy levels. Electron dot structure: \[\text{..}\]\[\text{.Br.}\] f. Selenium (Se): 18 electrons in first two energy levels, 16 electrons in third and fourth energy levels. Electron dot structure: \[\text{..}\]\[\text{.Se.}\]

Step-by-step solution

a. Krypton (Kr) #

Step 1: Find the electron configuration. Krypton has an atomic number of 36, so its electron configuration is \[1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^2\ 3d^10\ 4p^6\]. Step 2: Determine the number of electrons in each energy level. Krypton has 18 electrons in the first two energy levels and 18 electrons in the third and fourth energy levels. Step 3: Draw the electron dot structure. The electron dot structure for Krypton, with 8 valence electrons, is: \[\text{..}\] \[\text{.Kr.}\] \[\text{..}\]

b. Strontium (Sr) #

Step 1: Find the electron configuration. Strontium has an atomic number of 38, so its electron configuration is \[1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^2\ 3d^10\ 4p^6\ 5s^2\]. Step 2: Determine the number of electrons in each energy level. Strontium has 18 electrons in the first two energy levels, 18 electrons in the third and fourth energy levels, and 2 electrons in the fifth energy level. Step 3: Draw the electron dot structure. The electron dot structure for Strontium, with 2 valence electrons, is: \[\text{.Sr.}\]

c. Phosphorus (P) #

Step 1: Find the electron configuration. Phosphorus has an atomic number of 15, so its electron configuration is \[1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^3\]. Step 2: Determine the number of electrons in each energy level. Phosphorus has 10 electrons in the first two energy levels and 5 electrons in the third energy level. Step 3: Draw the electron dot structure. The electron dot structure for Phosphorus, with 5 valence electrons, is: \[\text{..}\] \[\text{.P.}\]

d. Boron (B) #

Step 1: Find the electron configuration. Boron has an atomic number of 5, so its electron configuration is \[1s^2\ 2s^2\ 2p^1\]. Step 2: Determine the number of electrons in each energy level. Boron has 2 electrons in the first energy level and 3 electrons in the second energy level. Step 3: Draw the electron dot structure. The electron dot structure for Boron, with 3 valence electrons, is: \[\text{.B.}\]

e. Bromine (Br) #

Step 1: Find the electron configuration. Bromine has an atomic number of 35, so its electron configuration is \[1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^2\ 3d^10\ 4p^5\]. Step 2: Determine the number of electrons in each energy level. Bromine has 18 electrons in the first two energy levels and 17 electrons in the third and fourth energy levels. Step 3: Draw the electron dot structure. The electron dot structure for Bromine, with 7 valence electrons, is: \[\text{..}\] \[\text{.Br.}\]

f. Selenium (Se) #

Step 1: Find the electron configuration. Selenium has an atomic number of 34, so its electron configuration is \[1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^2\ 3d^10\ 4p^4\]. Step 2: Determine the number of electrons in each energy level. Selenium has 18 electrons in the first two energy levels and 16 electrons in the third and fourth energy levels. Step 3: Draw the electron dot structure. The electron dot structure for Selenium, with 6 valence electrons, is: \[\text{..}\] \[\text{.Se.}\]

Key concepts

Energy Levels of Electrons

In atoms, electrons are organized in regions around the nucleus called energy levels or electron shells, each with a distinct energy. Think of them as the rungs on a ladder, where the electrons climb up one step at a time — they can't hover between two. The first level is closest to the nucleus and has the lowest energy; electrons here are tightly bound to the nucleus. As you move to higher levels, the energy increases and electrons are less tightly bound.

Each energy level can hold a specific number of electrons. The first level holds up to 2 electrons, the second and third up to 8 each, and so on, with the capacity increasing in a pattern given by the formula 2n², where 'n' represents the energy level's number. This organization dictates an element's chemical properties and is key to understanding how atoms bond with each other.

For instance, Krypton (Kr) has its electrons distributed across four energy levels, adhering to the principle of electron capacity per level. Understanding each element's energy levels is fundamental in predicting how it will react with other elements, making this concept a cornerstone of chemistry.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a vital role in chemical reactions and bonding. They are the 'social butterflies' of an atom, interacting with other atoms to form compounds. An atom's stability and reactivity are determined by these electrons. Atoms typically seek an outer shell resembling that of a noble gas, either by sharing, gaining, or losing electrons, leading to the formation of molecules and ionic compounds.

The number of valence electrons determines an element's group on the periodic table and its chemical behavior. For example, elements in group 1, like hydrogen and sodium, have 1 valence electron and are highly reactive. In contrast, noble gases like Helium and Neon have full outer shells and are largely unreactive.

In the examples provided, Boron (B) has 3 valence electrons, implying it can form three bonds with other elements. Phosphorus (P), with 5 valence electrons, is more versatile in its bonding capabilities.

Electron Dot Structures

Electron dot structures, also known as Lewis dot structures, are a simplified visual representation depicting the valence electrons around an element's symbol. Each dot corresponds to a valence electron, and they are positioned around the symbol representing the nucleus to illustrate how these electrons might participate in bonding.

Creating the electron dot structure is a three-step process: First, ascertain the element's electron configuration. Next, calculate the valence electrons. Finally, arrange the valence electrons around the element's symbol according to the octet rule — where atoms seek eight electrons in their outer shell for maximum stability, mimicking noble gases.

For example, the electron dot structure for Strontium (Sr), with its two valence electrons, would be indicated by two dots placed around the 'Sr' symbol. On the other hand, Bromine (Br), with seven valence electrons, shows one lone pair and five unshared electrons. By learning to draw and interpret these structures, students better understand the potential bonding scenarios an atom can engage in.