Reviewing important periodic trends in chemistry


Artistic version of the periodic table of elements. (credits Southern University at Shreveport)

As the first semester of the academic 2020 to 2021 school year comes to a close, teachers have fallen into a rhythm and are making their way through the academic material for the year. Although many teachers have had to make significant alterations to their plans as far as distance learning lesson plans and assessments go, ultimately they are still responsible for providing their students with a complete education of their subject.

Specifically in the science department, chemistry teachers are responsible for teaching their students about important topics such as the periodic table of elements in the periodic trends that coincide with it, concepts that can be foundational to science courses they take in the future. 

This is the first thing that many students learn about in AP Chemistry at Redlands East Valley High school. While learning the many periodic trends displayed on the periodic table that help scientists and chemists analyze elemental properties can be challenging, science students recognize the importance.

Allison Bermudez, a junior at Redlands East Valley High School and AP Chemistry student said that understanding periodic trends helps you “understand the force of the subatomic particles and their relation to being on the periodic table.”

Three major periodic trends that students study are: ionization energy, electron affinity and atomic radius. 

Ionization energy is the energy required to remove an electron from an atom or ion. Five key factors can help understand this complex concept:

  1. Ionization energy decreases going down a column because electrons are farther away from the nucleus, meaning they have less Coulombic attraction. 
  2. The inner core electrons shield the outermost electrons from the attractive forces of the protons in the nucleus.
  3. The electron is not bounded as tightly and requires less energy to be removed.  
  4. Ionization energy increases going across a period because the atomic radius decreases and the number of protons increases.  
  5. This increases the Coulombic attraction between particles, which means more energy is required to remove the electron.

Breaking down key processes into parts can help students not only understand the term at hand, but strengthen a solid chemistry foundation as the class intensifies. 

Shannon Cockerill, junior AP Chemistry student and ASB president at Redlands East Valley says, “Understanding periodic trends is like the starting point of chemistry. . .You can’t get through anything else without it.” 

Electron affinity, is another concept that can be difficult to understand at first, but helps with future material. Electron affinity refers to the energy change associated with the addition of an electron to an atom. A few key concepts that students must understand:

  1. If the addition of an electron is exothermic (the atom releases energy when an electron is added), then the electron affinity value is negative.  
  2. If the addition of an electron is endothermic (the atom absorbs energy when the electron is added), then the electron affinity value is less negative (more positive).
  3. An element that has a greater electron affinity will have a larger negative value.  
  4. Electron affinity decreases going down a group because electrons are being added to higher energy levels farther from the nucleus.  It increases going across a period because electrons are being added and filling the valence shell of the atom.
  5. Nonmetals have a greater electron affinity than metals.

A student-generated image of the periodic trends. The direction of the arrow indicates that the specific periodic trend is increasing in that direction. (Isaac Mejia and Arianna Ghalambor/Ethic Media)

Marin Mohr, a freshman in her first year of honors chemistry, said,  “It’s kind of complicated, but I can see how it can apply to many different aspects of chemistry. I’m trying my best to make sure I really learned and understand it.” 

The atomic radius of a chemical element is the measured size of the atoms, usually the mean or typical distance from the center of the nucleus to the boundary of the surrounding shells of electrons. The size of atoms is important when trying to explain the behavior of atoms or compounds.  One of the ways to express the size of atoms is with the atomic radius.  This data helps scientists understand why some molecules fit together and why other molecules have parts that get too crowded under certain conditions. 

  1. The size of an atom is defined by the edge of its orbital. However, it is impossible to know both the position and momentum of an electron so orbital boundaries do not have a definite location.  
  2. In order to standardize the measurement of atomic radii, the distance between the nuclei of two identical atoms bonded together is measured. The atomic radius is defined as one-half the distance between the nuclei of identical atoms that are bonded together. Atomic radii have been measured for elements, with the units of measurement being picometers, equal to 10−12 meters. For example, the internuclear distance between the two hydrogen atoms in an H2 molecule is measured to be 74 pm. Therefore, the atomic radius of a hydrogen atom is 74/2 = 37 pm. 
  3. The atomic radius decreases from top to bottom across a period. The atomic radius of atoms typically increases from top to bottom within a group. 
  4. There are two types of atomic radii: ionic and covalent. In a neutral atom, the atomic and ionic radius are the same, but many elements exist as anions or cations. If the atom loses its outermost electron (positively charged or cation), the ionic radius is smaller than the atomic radius because the atom loses an electron energy shell. 
  5. Covalent radius is half of the internuclear separation between the nuclei of two single-bonded atoms of the same species (homonuclear). While van der Waals radius is used to define half of the distance between the closest approach of two non-bonded atoms of a given element. 

These periodic trends are very valuable. They provide answers to many different questions regarding concepts such as photoelectron spectroscopy, lattice energy values and chemical bond length, etc.  They are essential for understanding chemistry and provide a strong foundation for further study.

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