Content Writer-SME | Updated On - Jun 4, 2024
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Keywords: Periodic table, Atomic radii, Nucleus, Atom, Electrons, Ionic radius, Covalent radius, Metallic radius, Van Der Waals Radius, Bohr radius, Periodic trends
What is Atomic Radii or Atomic Radius?
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The atomic radius is the distance between the nucleus's centre and the outermost shell containing electrons.
- In other words, it is the distance between the nucleus's core and the place where the electron cloud's density is greatest.
- The atomic radius is defined in basic chemistry as the smallest distance between the nuclei of an atom and the atom's outermost shell.
- The atomic radius of atoms generally decreases from left to right across a period.
- Similarly, the atomic radius of atoms increases from top to bottom within a group in the periodic table.
Types of Atomic Radius
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There are three types of atomic radii:
- Covalent Radius
- Ionic Radius
- Metallic Radius
Covalent Radius
In a molecule, a covalent radius is one-half the distance between the nuclei of two covalently linked atoms of the same element.
- As a result, rcovalent = \(\frac{1}{2}\) (internuclear distance between two bonded atoms).
- The bond length is the internuclear distance between two linked atoms.
Therefore,
rcovalent = \(\frac{1}{2}\)( bond length)
Ionic Radius
The ionic radius is the radius of an atom that forms an ionic bond or ion. Atomic bonds limit electrons and nuclei, resulting in ions or atoms with no definite structure.
- The ionic radius is measured in either Armstrong (Å) or picometers (pm).
- The average radius is between 30 and 200 pm.
- The ionic radius is not constant, but varies according to the spin state of the electrons, coordination number, and a variety of other factors.
- The size of an ion increases as the number of coordination numbers increases.
- An ion with a high electron spin state has a larger ionic size than an ion with a low spin state.
- If we consider the ion's charge, the positive ion will be smaller than the negative ion.
Metallic Radius
A metal lattice or crystal is made up of positive kernels or metal ions that are organized in a certain pattern amid a sea of movable valence electrons. Each kernel is attracted by a number of mobile electrons at the same time, and each mobile electron is attracted by a number of metal ions.
- The metallic bond refers to the force of attraction between mobile electrons and positive kernels.
- In the metallic lattice, it is one-half the internuclear distance between two neighbouring metal ions.
- Because the valence electrons are movable in a metallic lattice, they are only weakly attracted by metal ions or kernels.
- A pair of electrons are strongly attracted by the nuclei of two atoms in a covalent connection.
- As a result, a metallic radius is always greater than its covalent radius.
- The metallic radius of sodium, for example, is 186 pm, but its covalent radius, as measured by its vapour, which exists as Na2, is 154 pm.
- Potassium has a metallic radius of 231 pm and a covalent radius of 203 pm.
Other Terms related to Atomic Radius
In addition to the atomic radius and covalent radius, two more types of radii are known in chemistry that are based on the theories of two scientists: Vanderwall and Bohr. Other terms related to atomic radius are given below
Van der Waals Radius
It is half the distance between the nuclei of two identical non-bonded solitary atoms of two adjacent identical atoms belonging to two nearby molecules of a solid-state element.
- When an element is in the solid state, the size of the Van der Waals radius is determined by the atomic packing.
- In the solid state, for example, the internuclear distance between two adjacent chlorine atoms of two nearby molecules is 360 pm.
- As a result, the chlorine atom's Van der Waals radius is 180 pm.
Bohr Radius
The Bohr model of the atom predicted the radius of the lowest-energy electron orbit.
- Bohr's radius applies only to atoms and ions that have a single electron, such as hydrogen.
- Although the model itself is no longer used, the Bohr radius for the hydrogen atom is still considered an important physical constant.
Variation of Atomic Radii in the Periodic Table
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The following are the variations of atomic radii in the periodic table
Variation within the Periodic Table
As we travel from left to right in a period, the Covalent and Van der Waals radii drop as the atomic number increases. The alkali metals on the far left of the periodic table are the biggest in a period.
- The halogens at the far right of the periodic table are the tiniest. Nitrogen has the lowest atomic size.
- After nitrogen, the atomic size of oxygen rises and subsequently falls for fluorine.
- Atoms of inert gases are bigger in size than those of the previous halogens.
- The nuclear charge grows by one unit in each consecutive element as we travel from left to right in a period, but the number of shells remains constant.
- This increased nuclear charge attracts electrons from all shells closer to the nucleus.
- As a result, each individual shell becomes smaller and smaller. As a result, as we travel from left to right in a period, the atomic radius decreases.
- As we get from halogens to inert gases, the atomic radius abruptly rises.
- This is due to the fact that noble gases have entirely filled orbitals. As a result, the inter-electronic is maximized.
- Because they do not form covalent bonds, we express atomic size in terms of vanderwall radius.
- The radius of van der Waals is greater than the radius of covalent bonding.
- As a result, the atomic size of the inert gas in a period is significantly larger than that of the previous halogen.
Variation within a Group
The atomic radii of elements rise as the atomic number in a group increases from top to bottom.
- The primary quantum number grows as we travel along with the group.
- At each following element, a new energy shell is added.
- The valence electrons are moving away from the nucleus.
- As a result, the nucleus's attraction to the electron weakens.
- As a result, the atomic radius grows.
Periodic Trends of Atomic Radii
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In basic chemistry, we may see several patterns in the characteristics (physical and chemical) of elements as we move down a group or across a column or row in the current periodic table. For example, as we travel down a nonmetal group, the reactivity of the elements diminishes, but it increases when we move down the group of representative metals.
- When two atoms are combined, the distance between them may be used to calculate their atomic size.
- Another way to determine the atomic size of a nonmetallic element is to establish a single covalent connection between two atoms and measure the distance between the two atoms.
- The radius determined by this approach is referred to as the element's covalent radii.
- It is known as a metallic radius in the case of metal.
- It is defined as half the total distance between the nuclei of two adjacent metal ions connected by a metallic link.
- X-ray or other spectroscopic methods are used to determine an atom's atomic radius.
- Elements' atomic radii fluctuate in a predictable way across the periodic table.
- This pattern may be explained by examining the nuclear charge and energy level.
- In general, as we travel from left to right in a period, the atomic radius drops and rises as we move down a group.
- This is due to the fact that in periods, the valence electrons are all in the same outermost shell.
- Moving from left to right, the atomic number increases during the same time period, increasing the effective nuclear charge.
- The atomic radius of elements decreases as attractive forces rise.
Sample Questions
Ques. Does atomic radius increase across a period? (2 Marks)
Ans. An atom's size generally decreases as one goes from left to right across a period. In general, the atomic radius reduces across a period and increases down a group.
Ques. What is Bohr radius? (2 Marks)
Ans. The Bohr model of the atom predicted the radius of the lowest-energy electron orbit. Bohr's radius applies exclusively to atoms and ions that have a single electron, such as hydrogen.
Ques. Which element has the largest atomic radius? (1 Mark)
Ans. Francium has the largest atomic radius.
Ques. Why can’t atomic radii be measured directly? (1 Mark)
Ans.
The atomic radii are difficult to calculate because of the uncertainty in the exact position of the outermost electron.Ques. What do you mean by atomic radius and ionic radius? (2 Marks)
Ans. The distance from the centre of the nucleus to the outermost shell of electrons in the atom of any element is called the atomic radius.
- It refers to both covalent as well as metallic radii depending on whether the element is non-metal or a metal.
- The ionic radii can be referred to as the measurements of the distances between the cations and the anions in ionic crystals.
Ques. How does the atomic radius vary in a period and in a group? How can you explain the variation? (3 Marks)
Ans. Atomic radius increases down the group within a group. This is due to the continuous increase in the number of electronic shells or orbit numbers in the structure of atoms of the elements down the group.
Variation Across Period: Atomic radii generally decrease from left to right across a period due to an increase in effective nuclear charge from left to right across a period.
Ques. Why cation is smaller and anions larger in radii than their parent atoms? Explain. (3 Marks)
Ans. A cation is smaller than the parent atom as it possesses fewer electrons whereas its nuclear charge remains the same. The size of an anion is larger than that of the parent atom as the addition of one or more electrons would result in increased repulsion among the electrons and a decrease in effective nuclear charge.
Ques. What are the units for the measurement of atomic radius? (1 Mark)
Ans. The atomic radius can be measured in nanometers, angstroms, or picometers. But generally, it is measured in Angstroms (?).
Ques. What influences the atomic size of an atom? (3 Marks)
Ans. The trends that are observed with atomic size have to do with three factors: these are,
- The number of protons in the nucleus (called the nuclear charge).
- The number of energy levels holding electrons and the number of electrons in the outer energy level.
- The number of electrons held between the nucleus and its outermost electrons.
Ques. Describe the theory associated with the radius of an atom as it: (3 Marks)
a) Gains an electron
b) Loses an electron
Ans. The gain of an electron leads to the formation of an anion. The size of an anion will be larger than that of the parent atom because the addition of one or more electrons will result in increased repulsion among electrons and a decrease in effective nuclear charge. For instance, the ionic radius of fluoride (Fl-) is 136 pm, while the atomic radius of Fluorine (F) is only 64 pm.
The loss of an electron from an atom forms a cation. A cation is smaller than its parent atom. As it has former electrons while its nuclear charge remains the same. For instance- the atomic radius of sodium (Na) is 186 pm and the atomic radius of sodium ion (Na+) is 95 pm.
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