Problem 110 - Chemistry Matter and Change

Question

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

Step-by-Step Solution

Answer

Kr: 8 valence electrons; Sr: 2; P: 5; B: 3; Br: 7; Se: 6. Electron dot structures vary by valence electrons.

Step 1: Understanding Electron Configuration

Determine the number of electrons in each element according to its atomic number. Krypton (Kr) has 36 electrons, Strontium (Sr) has 38 electrons, Phosphorus (P) has 15 electrons, Boron (B) has 5 electrons, Bromine (Br) has 35 electrons, and Selenium (Se) has 34 electrons.

Step 2: Assigning Electrons to Energy Levels

For each element, distribute the electrons into energy levels according to the order of filling: first energy level can hold up to 2 electrons, second can hold up to 8, third can hold up to 18, and the fourth can hold up to 32 electrons, following the order dictated by the Aufbau principle.

Step 3: Krypton Electron Distribution

Krypton (Kr): 1st level - 2 electrons, 2nd level - 8 electrons, 3rd level - 18 electrons, 4th level - 8 electrons, totaling 36 electrons.

Step 4: Strontium Electron Distribution

Strontium (Sr): 1st level - 2 electrons, 2nd level - 8 electrons, 3rd level - 18 electrons, 4th level - 8 electrons, 5th level - 2 electrons, totaling 38 electrons.

Step 5: Phosphorus Electron Distribution

Phosphorus (P): 1st level - 2 electrons, 2nd level - 8 electrons, 3rd level - 5 electrons, totaling 15 electrons.

Step 6: Boron Electron Distribution

Boron (B): 1st level - 2 electrons, 2nd level - 3 electrons, totaling 5 electrons.

Step 7: Bromine Electron Distribution

Bromine (Br): 1st level - 2 electrons, 2nd level - 8 electrons, 3rd level - 18 electrons, 4th level - 7 electrons, totaling 35 electrons.

Step 8: Selenium Electron Distribution

Selenium (Se): 1st level - 2 electrons, 2nd level - 8 electrons, 3rd level - 18 electrons, 4th level - 6 electrons, totaling 34 electrons.

Step 9: Drawing Electron Dot Structures

For electron dot structures, only the valence electrons (outermost shell electrons) are shown with dots around the element symbol. For Kr, Sr, P, B, Br, and Se, their valence shell electron counts are 8, 2, 5, 3, 7, and 6 respectively.

Step 10: Electron Dot Structures

Krypton (Kr): Kr, with 8 dots around to represent valence electrons (Stable; no dots typically needed). Strontium (Sr): Sr with 2 dots. Phosphorus (P): P with 5 dots. Boron (B): B with 3 dots. Bromine (Br): Br with 7 dots. Selenium (Se): Se with 6 dots.

Key Concepts

Energy Levels

In atomic theory, energy levels are the fixed distances from the nucleus where electrons are most likely to be found. Each level can be seen as a circular track around the nucleus where electrons orbit. Electrons fill these levels starting from the lowest energy level first and move to higher levels as the lower ones get filled, much like passengers filling up seats from the front to the back of a bus.
This distribution is guided by the following capacities:

• The first energy level can hold up to 2 electrons,
• the second can hold up to 8 electrons,
• the third can hold 18, and
• the fourth can hold 32 electrons.

As you move to higher energy levels, the amount of energy the electrons possess increases. This is crucial for understanding chemical bonding and reactivity, such as why neon is inert or why fluorine is highly reactive.

Aufbau Principle

The Aufbau Principle is a guideline for the sequence in which the atomic orbitals are filled with electrons. It is derived from the German word "Aufbau," which means "building up."
According to this principle:

• Electrons occupy the lowest energy orbitals first before moving to higher levels.
• This order of filling is generally determined by increasing energy, not just increasing distance from the nucleus.
• The sequence is verified through experimental data and is used to predict electron configurations.

The typical order of filling is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, and so on, continuing through the sequence. Sometimes, you will see deviations caused by specific atom properties, but the principle helps establish a foundation for understanding electron arrangements and their interaction during chemical reactions.

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They are crucial because they determine how an element will interact or bond with others.

• These electrons play a fundamental role in the chemical properties of the element.
• For elements that participate in bonds, it's often the valence electrons that are shared or transferred.
• This outer shell can hold a maximum of 8 electrons (known as an octet), and atoms will often seek to complete their valence shell to achieve a stable electronic configuration.

In the periodic table, elements with the same number of valence electrons are typically grouped together in columns, showing similar chemical behavior. For instance, all halogens have 7 valence electrons which is why they are highly reactive, just needing one more electron to complete their octet.

Electron Dot Structures

Electron dot structures, also known as Lewis dot structures, are a simple way to represent the valence electrons of an element. These structures help to visualize the electrons involved in bonding.
Here is how to create a dot structure:

• Write the symbol of the element.
• Place dots around the symbol to represent the valence electrons.
• A maximum of two dots are placed on each side of the symbol (top, bottom, left, right) before pairing up on the sides.

For example, for an element like phosphorus (P) with 5 valence electrons, you would write P and place 5 dots around it. Learning to create these structures assists in understanding how atoms pair up during chemical reactions and what types of bonds they can form.