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Periodic trends

Periodic trends are patterns in the periodic table for different aspects of a certain element.

Major periodic trends include: electronegativity, ionization energy, electron affinity, atomic radius, melting point, and metallic character.

They provide chemists with an invaluable tool to quickly predict an element’s properties.

The text below is adapted freely from Periodic Trends UC Davis ChemWiki

Electronegativity Trends

Electronegativity is an atom’s ability to attract electrons.

It is a qualitative property, made on a simple 4 point scale (the Pauling scale.) These numbers have no units.

Electronegativity

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from youtube.com/watch?v=66NyESIRvwk; GIF made with giphy.com

Ionization Energy Trends

Ionization energy IE is the energy required to remove an electron from a neutral atom in its gaseous phase.

It is the opposite of electronegativity:

The lower IE is, the more readily the atom becomes a cation.

The higher IE is, the more unlikely it is the atom becomes a cation.

Elements on the right side of the periodic table have a higher IE because their valence shell is nearly filled.

Elements on the left side have low IE because of their willingness to lose electrons and become cations.
Thus, ionization energy increases from left to right on the periodic table.

ionization energy

ionization energy 2

Electron Affinity Trends

The ability of an atom to accept an electron.

Unlike electronegativity, electron affinity is a quantitative measurement: it measures the energy change occurring when an electron is added to a neutral gas atom.

The more negative the value, the higher an atom’s affinity for electrons.

Electron affinity trend 1

Electron affinity trend 2

from youtube.com/watch?v=66NyESIRvwk; GIF made with giphy.com

Atomic Radius Trends

Atomic radius =   1/2 the distance between the nuclei of two atoms

caution: not all atoms are normally bound together in the same way:

Some bound by covalent bonds. Some attracted to each other in ionic crystals. Some held in metallic crystals.

Nevertheless, it is possible for most elements to form covalent molecules, in which two like atoms are held together by a single covalent bond.

The covalent radii of these molecules are often referred to as atomic radii. This distance is measured in picometers.

Atomic radius patterns are observed throughout the periodic table.

Atomic size gradually decreases from left to right across a period of elements.

This is because, within a period, all electrons are added to the same shell. However, at the same time, protons are being added to the nucleus, making it more + charged.

Effect of increasing proton number > than effect of increasing electron number; therefore, there is a greater nuclear attraction.

This means that the nucleus attracts the electrons more strongly, pulling the atom’s shell closer to the nucleus. The valence electrons are held closer towards the nucleus of the atom. As a result, the atomic radius decreases.

Melting Point Trends

There are subtle trends, but they are not as regular as the other trends. The reasons are subtle/complicated – so we’ll just skip over this for now. For details and a graph please see  Is there a trend in melting points on the periodic table?

Metallic Character Trends

The metallic character of an element is basically how easily an atom can lose an electron.

From right to left across a period, metallic character increases.

Happens because the attraction between valence electron and the nucleus is weaker, enabling an easier loss of electrons.

Metallic character increases as you move down a group – because the atomic size is increasing.

When the atomic size increases, the outer shells are farther away. The principle quantum number increases and average electron density moves farther from nucleus. The electrons of the valence shell have less attraction to the nucleus and, as a result, can lose electrons more readily. This causes an increase in metallic character.

Metallic character trend

from Metals, Nonmetals, and Metalloids – ChemWiki UC Davis

 

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