Introduction to Atomic Radius
The radius of an atom of a chemical element is a measure of the atom's size. The meaning of it is said as the typical distance which is from the center of the nucleus till the boundary of the atom which is surrounding the electrons. There are three widely used definitions which are used for atomic radius:they are the ionic radius, Van der Waals radius, and covalent radius.
According to the definition these terms may be applied only to isolated atoms in condensed matter as well covalently which have bonding in molecules or even within ionized and excited states as well. And its value can also be obtained by the experimental measurements, or computed from models of theory. The radius value can even depend on the atom's state.
It is said that the electrons do not have definite orbits or sharply defined ranges. The position of the molecules can be described as probability which has distributions that gradually taper off as one that moves away from the nucleus. And that too without a sharp cutoff. In condensed molecules and matter the cloud of electrons of the atoms usually overlap to some extent, and some of the electrons can even roam over a large region encompassing two or even more atoms.
In most of the definitions of the radii of isolated atoms which are neutral atoms range between 30 and 300 pm that is trillionths. Or even between 0.3 and 3 ångströms. The radius of atoms is more than 10,000 times the radius of its nucleus that is 1–10 pm and less than 1/1000 of the wavelength of visible light that is 400–700 nm.
Covalent Radius
Here letter r covalent is defined as = ½ that is the internuclear distance between two bonded atoms. The internuclear distance which is between two bonded atoms is called the bond length.
Van Der Waals Radius
It is one the distance which is half the distance between the nuclei of two similar non-bonded isolated atoms or even two adjacent identical atoms belonging to two neighboring molecules of an element which are in the solid-state. The weakest forces which are also known as the van der wall magnitude of the radius are dependent on the packing of the atoms when the element is in the solid-state.
An example of the internuclear that is the distance which is between two adjacent atoms of chlorine in the solid-state which is 360 pm. So the Van der Waals radius of the chlorine atom is said to be 180 pm.
Metallic Radius
A crystal contains positive kernel ions which are arranged in a pattern which is definite in a sea of mobile electrons which are valence.The force of attraction which is between electrons that are basically mobile and the positive kernels is also called the metallic bond. It is said to be one of the half internuclear distances which is between the two adjacent metal ions in the lattice metallic. In a metallic lattice the valence electrons are mobile or we can say they are free to move therefore they are only weakly attracted by the metal ions or kernels.
In a bond-like covalent bond, there is a pair of electrons which is strongly attracted by the nuclei of two atoms. That is why a metallic radius is always longer than its covalent radius.
General Trends in Atomic Radii in the Elements of the Periodic Table
The way the atomic radius varies with increasing atomic number is referred to here as the general trends. This trend can be explained by the arrangement of electrons in fixed shells around the nucleus of an atom. The general rule is that shells fill in the order of increasing radius, since the negatively charged electrons are attracted by the positively charged protons in the nucleus. As the atomic numbers increase(and therefore the total number of protons) along each row of the periodic table, the additional electrons go into the same outermost shell; whose radius gradually contracts, due to the increasing nuclear charge.
In elements such as that of a noble gas, the outermost shell of the atom is completely filled; therefore, the additional electron of the next alkali metal enters into the next outer shell, which explains the sudden increase in the atomic radius.
Thus we notice that for an atom in the periodic table, increasing charge in the nucleus is partially counterbalanced by the increasing number of electrons in its orbit, a phenomenon that is called the "shielding effect". This effect accounts for the size of atoms which gradually increases down each column in the periodic table.
However, there are few notable exceptions. They are the d-block contraction and the f-block contraction (also known as lanthanide contraction). Lanthanide contraction refers to the much smaller size of the 5d block of elements than one would expect which happens due to the poor shielding of the 4f electrons.
As a thumb rule, the atomic radii decrease across the periods as the total protons in the nucleus increases. There is greater attraction experienced by the orbiting electrons, which draws them closer to the protons, decreasing the size of the atom. Therefore, the atomic radius decreases.
Down the groups, atomic radius increases because there are more energy levels and hence a greater space between protons and electrons. In addition to this, electron shielding effect is also observed which causes attraction to decrease and the remaining (valence) electrons can go farther away from the positively charged nucleus. This causes the size, or atomic radius, to increase.
Explanation
The shells which are present are generally filled in order of radius which is increasing since the negatively charged electrons are attracted by the positively charged protons in the nucleus. The additional electrons go into the same shell which is the outermost shell whose radius contracts graduall. And this happens mostly due to the increasing nuclear charge. In a noble gas if we keenly observe, the outermost shell is completely filled and therefore the additional electron of the next alkali metal will go into the next outer shell for the increase in the atomic radius.
This explains why the size of atoms usually increases down each column. There is one notable exception for this and is known as the lanthanide contraction that is the 5d block of elements which are much smaller than one would expect which is due to the weak shielding of the 4f electrons.
Notes
The Difference between experimental and empirical data: “Empirical data basically means that they are originating or we can say they are based on observation or experience that are relying on observation or experience alone often without due regard for system and theory data". It basically means that we measure it through physical observation and a lot of experiments which are generating the same results. Note that the values are not calculated by a formula. But, often the empirical results then become an equation of estimation. Experimental data which are on the other hand are only based on theories.
FAQs on Atomic Radii
1. What is meant by the term atomic radius?
Atomic radius is a measure of the size of an atom. Since an atom doesn't have a sharp boundary, it is typically defined as half the distance between the nuclei of two identical atoms that are bonded together. For example, in a chlorine molecule (Cl₂), the atomic radius would be half the measured distance between the two chlorine nuclei.
2. What are the main types of atomic radii defined in chemistry?
There are three primary types of atomic radii, each based on the chemical environment of the atom:
- Covalent Radius: Half the distance between the nuclei of two atoms joined by a covalent bond.
- Van der Waals Radius: Half the distance between the nuclei of two non-bonded, adjacent atoms in their solid state. It describes the distance at which atoms are held by weak intermolecular forces.
- Metallic Radius: Half the distance between the nuclei of two adjacent metal ions in a metallic crystal lattice.
3. Why does atomic radius generally decrease when moving from left to right across a period?
The atomic radius decreases across a period because the effective nuclear charge increases. As you move from left to right, protons are added to the nucleus, but electrons are added to the same outermost energy shell. This stronger positive charge from the nucleus pulls the electron cloud closer, thereby reducing the atom's size.
4. Why does atomic radius increase when moving down a group in the periodic table?
Atomic radius increases down a group primarily because a new electron shell is added for each subsequent element. This new, higher energy level is located farther from the nucleus. Additionally, the inner electrons create a shielding effect, which reduces the pull of the nucleus on the outermost electrons, allowing the atom to expand.
5. What is the difference between atomic radius and ionic radius?
The key difference is that atomic radius measures a neutral atom, while ionic radius measures an ion. A cation (positive ion) is always smaller than its parent atom because it has lost one or more electrons, reducing electron-electron repulsion and often an entire shell. An anion (negative ion) is always larger than its parent atom because the addition of electrons increases electron-electron repulsion, causing the electron cloud to expand.
6. What are the primary factors that determine the atomic radius of an element?
The size of an atom, or its atomic radius, is influenced by three main factors:
- Nuclear Charge: The number of protons in the nucleus. A higher charge pulls electrons closer, decreasing the radius.
- Number of Electron Shells: The principal energy level of the outermost electrons. More shells mean a larger radius.
- Shielding Effect: The repulsion between inner-shell electrons and outer-shell electrons, which 'shields' the outer electrons from the full nuclear charge.
7. How is Van der Waals radius different from covalent radius?
The main difference lies in the type of interaction being measured. Covalent radius is measured between two atoms that are chemically bonded, sharing electrons. In contrast, Van der Waals radius is measured between two non-bonded atoms that are merely adjacent, such as in a solid crystal of a noble gas. Consequently, the Van der Waals radius of an atom is always larger than its covalent radius because the forces holding non-bonded atoms apart are weaker than the length of a chemical bond holding them together.
8. What is the Lanthanide Contraction and how does it affect atomic radii?
The Lanthanide Contraction is the greater-than-expected decrease in atomic and ionic radii of the elements following the lanthanide series. This occurs because the electrons added to the 4f orbitals are very poor at shielding the outer electrons from the increasing nuclear charge. As a result, the effective nuclear charge increases significantly, pulling the electron shells closer and making atoms like Hafnium (Hf) surprisingly similar in size to Zirconium (Zr), the element directly above it.
9. How is the size of an atom measured and what units are used?
The size of an atom cannot be measured directly like a solid sphere. It is determined experimentally, often using techniques like X-ray crystallography or spectroscopy to measure the internuclear distance between atoms in a solid or a molecule. The atomic radius is then calculated from this distance. The common units used are the picometre (pm), where 1 pm = 10⁻¹² m, and the Angstrom (Å), where 1 Å = 10⁻¹⁰ m.
