Because atomic radius is half the distance between the nuclei of two atoms, how close those atoms are to each other affects atomic radius.Įach of the noble gases has their outermost electron shell completely filled, which means multiple noble gas atoms are held together by Van der Waals forces rather than through bonds. The noble gases are an exception because they bond differently than other atoms, and noble gas atoms don't get as close to each other when they bond. The six noble gases, in group 18 of the periodic table, are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). However, there are a few exceptions to these trends. The two atomic radius trends we discussed above are true for the majority of the periodic table of elements. The potassium atom has an extra electron shell compared to the sodium atom, which means its valence electrons are further from the nucleus, giving potassium a larger atomic radius. So, because of electron shielding, the valence electrons don’t get particularly close to the center of the atom, and because they can’t get that close, the atom has a larger radius.Īs an example, potassium (K) has a larger average atomic radius (220 pm)than sodium (Na) does (180 pm). Electron shielding refers to a decreased attraction between outer electrons and the nucleus of an atom whenever the atom has more than one electron shell. While you may think the valence electrons (those in the outermost shell) would be attracted to the nucleus, electron shielding prevents that from happening. Each new shell is further away from the nucleus of the atom, which increases the atomic radius. For each group you move down, the atom gets an additional electron shell. The second atomic radius periodic trend is that atomic radii increase as you move downwards in a group in the periodic table. And this is true carbon has an average atomic radius of about 70 pm while fluorine’s is about 50 pm.Ītomic Radius Trend 2: Atomic Radii Increase as You Move Down a Group The electrons being pulled closer to the nucleus makes the atom’s radius smaller.Ĭomparing carbon (C) with an atomic number of 6 and fluorine (F) with an atomic number of 9, we can tell that, based on atomic radius trends, a carbon atom will have a larger radius than a fluorine atom since the three additional protons the fluorine has will pull its electrons closer to the nucleus and shrink the fluorine's radius. This means that, as more protons are added, the nucleus gets a stronger positive charge which then attracts the electrons more strongly and pulls them closer to the atom’s nucleus. When an electron is added, a new proton is also added to the nucleus, which gives the nucleus a stronger positive charge and a greater nuclear attraction. Within a period of elements, each new electron is added to the same shell. The first atomic radius periodic trend is that atomic size decreases as you move left to right across a period. At the end of this section is a chart with the estimated empirical atomic radius for each element.Ītomic Radius Trend 1: Atomic Radii Decrease From Left to Right Across a Period Below is a periodic table with arrows showing how atomic radii change to help you understand and visualize each atomic radius trend. One atomic radius trend occurs as you move left to right across the periodic table (moving within a period), and the other trend occurs when you move from the top of the periodic table down (moving within a group). What Are the Atomic Radius Trends? What Causes Them? Hydrogen (H) has the smallest average atomic radius at about 25 pm, while caesium (Cs) has the largest average radius at about 260 pm.
Atomic radii are measured in picometers (one picometer is equal to one trillionth of a meter). A radius is the distance between the center of an object and its outer edge.Īn atomic radius is one-half the distance between the nuclei of two atoms. An atom is a basic unit of a chemical element, such as hydrogen, helium, potassium, etc.
We’ll also discuss exceptions to the trends and how you can use this information as part of a broader understanding of chemistry.īefore we dive into atomic radius trends, let’s review some basic terms. Need information on atomic radius trends? What's the trend for atomic radius? In this guide, we’ll clearly explain atomic radius trends and how they work.
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