2.4 Periodic Trends

Learning Objectives

  1. Understand how certain properties of atoms vary based on their relative position on the periodic table.

 

Arranging the periodic table by electron configuration allows us to observe trends within elements across groups and periods. Let us discuss some important chemical properties which can be observed.

Atomic radii

The atomic radius is an indication of the size of an atom. While atoms don’t have a size in the traditional sense (due to the fluctuating position of electrons), they behave as if they have a radius, particularly with reference to bonding. Atomic radii increase from the top to the bottom of a group of the periodic table. As [latex]n[/latex] increases when proceeding down a group, orbitals become larger, increasing atomic radii (see Figure 2.4.1).

A diagram of atomic radii in the s and p blocks of the periodic table, showing varying sizes of atoms with numbers indicating their radii in picometres. In the top right corner, helium has the smallest size radii at 37 picometres, with elements getting bigger the closer in proximity to caesium, in the bottom left of the periodic table, which has a radii of 265 pictometres. There are some notable exceptions, like tellurium and polonium found within group 16.
Figure 2.4.1: Atomic Radii trends on the Periodic Table. Although there are some reversals in the trend (e.g., see Po in the bottom row), atoms generally get smaller as you go across the periodic table and larger as you go down any one column. Numbers are the radii in picometres. Image attribution: “Atomic Radii Trends on the Periodic Table” by David W. Ball © CC BY-NC-SA (Attribution-NonCommercial-ShareAlike).

Atoms as Balls: Why the approximation?

While atoms aren’t small balls floating around in space, it can be handy for us to treat them as such! In constructing 3D models of compounds, the relative sizes of molecules should be taken into consideration. You don’t need to know the specifics, but should be able to take an educated guess at what is bigger on the periodic table. Consider [latex]H_2O[/latex], how big are the hydrogen atoms compared to the oxygen?

Diagram of the molecular structure of ice, depicting water molecules connected in a three dimensional pattern. Each molecule consists of two smaller white spheres representing hydrogen atoms bonded to a larger red sphere representing an oxygen atom, with dashed lines indicating hydrogen bonds between molecules. The red balls have double the radius of the white balls.
Figure 2.4.2: Water as a solid (ice). Note the size difference between the (white) hydrogen atoms and the (red) oxygens. With the radius roughly doubled — the area of the circle should increase by a factor of 4. Image attribution: Chem&121: Introduction to Chemistry Copyright © 2023 by Lake Washington Institute of Technology is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

In chapter 7.2, Thermochemistry Essentials, we will talk about collision theory — in which atoms and molecules are seen as whole spherical objects that can collide with one another. In tertiary studies, we can mathematically work out the chances of a collision occurring through the size of the cross-section of a molecule. It is much easier to determine the probability of a sphere colliding than a complex object. For simple compounds, such as diatomic molecules comprised of a large atom and a relatively small atom, this approximation works.

Naturally, simplifications like this always bring about inaccuracy in our answers — however these are acceptable at this stage in your chemical career!

Ionisation energy

The first ionisation energy is the minimum energy required to remove one electron from a neutral atom. The second and third ionisation energies are the quantities necessary to remove the second and third electron from the atom, respectively. Ionisation energy increases when moving from left to right across a period as electrons are bound tightly. When proceeding down a group, the first ionisation energy decreases as an electron in a higher energy level is easier to remove. The trend in the first ionisation energy is the inverse of the atomic radii (see Figure 2.4.3). As atomic radii increase, ionisation energy decreases.

Periodic table diagram showing atomic spheres sized according to ionization energies, from helium with the highest to caesium with the lowest.<a href="https://rmit.pressbooks.pub/rmitchemistrybridgingcourse/chapter/2-4-the-periodic-trends/">View accessible transcript here.
Figure 2.4.3 “Ionisation Energy on the Periodic Table.” Values are in kJ/mol. View accessible transcript here. Image attribution: “Ionization Energy on the Periodic Table” by David W. Ball © CC BY-NC-SA (Attribution-NonCommercial-ShareAlike).

Electronegativity

Electronegativity is the power of an atom in a molecule to attract electrons. The larger the value, the larger the electron-attracting ability. Atoms with higher electronegativity form anions, whereas atoms with smaller electronegativity form cations (see Figure 2.4.4). Electronegativity decreases from top to bottom and increases from the left to the right of the periodic table — similar to the trend of ionisation energy (see Figure 2.4.3). Electronegativity is an important elemental property, as it dictates the types of bonds that can form between elements.

Diagram of s and p blocks of the periodic table displaying electronegativity values of the elements. The diagram has two arrows indicating increasing electronegativity, starting from top left to the right and again from bottom left to the top. The electronegativity values decrease from Flourine in group 7a, which has the highest electronegativity at 3.98; down to francium in group 1 with the lowest at 0.7. Noble gases do not have an electronegativity value.
Figure 2.4.4: Electronegativity of elements. Image attribution: Periodic table of elements © Torsu – stock.adobe.com.

Key Takeaways

  • Due to the construction of the periodic table, a number of trends can be seen moving across it.
  • Atomic radius of atoms increases from the top to the bottom of a group, and gets larger as one moves from the right to the left.
  • Ionisation energy, the energy needed to remove an electron, decreases as atomic radius increases.
  • Electronegativity, the power of an atom to attract electrons within molecules, increases alongside ionisation energy

 

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