Lesson Plan - Get It!
- How good are you at making predictions?
- How does that fit in with all those symbols and numbers in the periodic table?
The periodic table organizes elements based on the number of protons in the nucleus of the atom. This results in patterns within the table that allow scientists to predict chemical element behavior. These trends can be used to determine bonding probability, reactivity, and other characteristics.
If you missed or want a refresher on previous The Periodic Table Related Lessons, find them in the right-hand sidebar.
There are several periodic trends, or patterns, that you will learn in this lesson. It will help you if you access a periodic table during this lesson, like the Periodic Table of Elements: LANL, from Los Alamos National Laboratory.
Remember that columns are called groups and rows are called periods. As we move across a period, an electron is added to the atom as the number of protons increases. This pattern leads to several of the trends we will discuss.
Trends are patterns in chemical behavior that occur in the periodic table because elements are organized based on atomic structure. We will look at several trends during this lesson: electronegativity, ionization energy, electron affinity, and atomic radius.
The first trend is based on the ability of an atom to attract electrons from other atoms, or electronegativity. This quality determines how easily an element can bond with other elements in the periodic table. It is based on the number and organization of electrons in the atom — how many are closer or farther away from the nucleus. Elements on the left-hand side of the table have fewer electrons in the outer shells and are more likely to give up electrons, therefore having a lower electronegativity value. Nonmetals, on the right-hand side, typically have larger values, suggesting high bonding potential. Scientists use electronegativity values to predict if a chemical bond will form between two elements.
Electronegativity follows a pattern on the table, increasing in a period as you move from left to right. That means that nitrogen has a larger value than boron or lithium. It decreases as you move down a group, so calcium would have a lower value than magnesium.
- Would fluorine or chlorine have a larger electronegativity value?
If you guessed fluorine, you were right!
Another trend we find on the periodic table is based on the amount of energy needed to remove an electron from an atom, called ionization energy. It is related to electronegativity, because elements on the left-hand side of the periodic table generally give up electrons during bonding.
- Do you think ionization energy follows the same pattern as electronegativity? Why or why not?
Jot down your answer before reading on!
Ionization energy does follow the same trend, increasing across a period and decreasing down a group. Think about why that occurs: If an element has more electrons in the outer shell, it will take more energy to remove them instead of just adding an electron to fill the shell.
- Does barium or cesium have a larger ionization energy?
- What about gold and silver?
Atoms can give up electrons, but they can also accept them. Electron affinity is defined as the likelihood that an atom will accept electrons from other elements. This trend is based on the changing size of the atomic nucleus. As you move across a period, the number of protons increases, which also increases the number of electrons. However, the atom actually gets smaller in size.
- Why do you think this happens?
Make a prediction on a sheet of paper.
Image by Cdang and Adrignola, via Wikimedia Commons, is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Protons have a stronger positive force than the electrons pushing out in shells, and this causes the atom to pull tighter towards the center. This trend is shown above in the image provided by WikiMedia Commons. That force of attraction results in chemical behaviors like electron affinity. Nonmetals, which have more protons, have higher electron affinity because the electrons are actually closer to the nucleus, pulling new electrons in with great force. Metallic elements have lower electron affinity values because they are farther away from the nucleus. Electron affinity follows the same pattern as ionization energy and electronegativity, increasing across a period and decreasing down a group.
As you have learned, many of these trends depend on changes in the atomic nucleus. The atomic radius follows the complete opposite pattern on the periodic table — it decreases across a period and increases down a group. This occurs because of the strong attraction between protons and electrons in the atom, which draws the outer shells closer to the nucleus as protons are added to the atom. As we move down a group, we add a new shell layer of electrons with each row, increasing the overall available size of the atom.
- Do you think potassium or calcium has a larger radius?
- What about calcium and strontium?
These periodic trends help scientists make predictions about how elements will behave individually and in the context of chemical bonds. They provide information about possible reactions and help us understand why elements may not react. Ionization energy, electronegativity, and electron affinity all involve bonding and follow the same trend on the periodic table, increasing across a group and decreasing down a period. While these trends help us understand how elements relate to one another, they are based on the atomic nucleus that determines the atomic radius. The atomic radius follows a different trend on the table, decreasing across a period and increasing down a group.
List five ways you think a scientist could use the periodic trends on the periodic table in their work.
Write them down on a sheet of paper before moving to the Got It? section, where you will review some physical characteristic trends on the periodic table.