Periodic Table Trends

periodic table

Today, we are going to learn about how the periodic table is organized and the important information that it provides.

periodic table

Scientists discovered that if they arranged the elements according to their atomic number, properties of the elements would occur in a regular and repeating pattern. This pattern is known as the periodic trends.

Before we delve into the periodic trends, let’s familiarize ourselves with a few new terms.

Nuclear charge The nuclear charge is equal to the number of protons in the nucleus of an atom
Valence electrons Valence electrons are electrons in the highest energy level and they are furthest away from the nucleus.
Core electrons The inner electrons that are not valence electrons are called core electrons.
Bonding electrons Bonding electrons are electrons involved in chemical bonding.


When chemical reactions take place, atoms play tug of war with the valence electrons.
When two atoms form a bond, a pair of electrons is involved. These electrons are called bonding electrons.


Depending on which atom has a stronger pull, the pair of bonding electrons will move closer to that atom.


Periodic trends

The major trends that we will learn in this topic include: atomic radius, electronegativity, ionization energy, and electron affinity.

Atomic radius trend

Not all atoms are the same size. The size of an atom is dictated by how far the valence electrons are from the nucleus of the atom. An atom with a higher nuclear charge tends to attract the valence electrons closer to itself.

  • Across each row, the energy level does not change but the atomic number increases.
  • The higher the atomic number, the higher the nuclear charge.
  • More nuclear charge means the electrons are pulled closer to the nucleus, and hence reducing the atomic radius.

The atomic radius of atoms generally decreases from left to right across a period.

Atomic radius trend

Going down the group, a different trend unfolds. As the atomic number increases, the atomic radius also increases.

    • Going down a group, although the number of protons increases, the number of core electrons also increases.
    • Electron repulsion results as the electrons push each other farther apart.
    • Due to the electron repulsion, the attractive forces of the nucleus is reduced.
    • This is called the “shielding” effect.
    • “Effective nuclear charge” is a term used to describe the reduced charge felt by the electron due to shielding.

K atom and Li atom

        • K has more energy levels than Li.
        • Due to the presence of several layers of electrons, it reduces the attraction between the outer electron and the nucleus of the atom.
        • Hence the valence electron for the K atom is less tightly held, it’s further away and the atomic radius is larger than Li which has fewer energy levels.

      In general, due to more energy levels, atomic radius increases down a group in the periodic table.

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