Define Atomic Radius and Atomic Size
An atom is made of three tiny subatomic particles, which are protons, neutrons, and electrons. Protons and neutrons constitute the atom's centre and are together known as the nucleus. The electrons fly around and above the nucleus in a smaller cloud. Electrons carry a negative charge, protons have a positive charge, and neutrons are neutral. If an atom is neutral, it has an equal number of electrons and protons; but not mandatorily the equal number of neutrons.
Define Atomic Size
Atomic size refers to the distance between the nucleus's centre in an atom and its outermost shell. In introductory chemistry, one can define atomic radius as the shortest distance between the atom's nucleus and its outermost shell.
Radius and Atomic Radius
The radius of a circle is the distance from the centre point to the circle's edge. This distance stays the same anywhere on the circle due to its radial symmetry. In an atom, the radius of the nucleus is almost negligible due to its smaller size.
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Atomic radius is half of the distance between the adjacent atoms of the same element in any molecule. It is difficult to measure the atomic radius of chemical elements as the atomic size is of the order 1.2 x 10⁻¹⁰ m. The electron cloud acts as the atom's shell and does not have a fixed shape, making it tough to determine the atomic size.
For precisely measuring the radius of an atom, one can use Heisenberg Uncertainty Principle. According to it, the atom's radius is calculated based on the distance between the nuclei of two of its bonded atoms. Thus, the bonds formed by an atom help in determining its radii. The radii of the atoms vary based on their bond formations; thus, there is no fixed radius of an atom. There are different types of atomic radii, which are atomic radius, Bohr radius, covalent radius, metallic radius, Van Der Waals radius.
Common Types of Radius with Respect to Bonds
1. Van Der Waals Radius:
Van Der Waals radius is the measurement of an atom's size that does not bind chemically, ionically, or covalently. It is defined as half of the closest distance of the two equal and non bonded atoms.
2. Ionic Radius:
Ionic radius definition refers to the radius of an atom that forms an ionic bond or an ion. Ionic radii are measured in Armstrong or Picometers, and this radius ranges from 30 to 200 pm. In these, the electrons and nucleus are restricted by atomic bonds, so the atoms or ions do not have any specific shape. The ionic radii are not static, and they differ according to the spin state of the electrons, their coordination number, and other parameters. As the coordination number increases, the ionic size also rises. And, the ions having a higher spin state of the electron have a larger ionic size.
3. Metallic Radius:
It is the atoms' radii that are joined by a metallic bond. The metallic radius is half of the total distance between the two adjacent atoms' nuclei, which belong to a metallic cluster.
4. Covalent Radius:
It is defined as the atom's radius under a covalent bond with another atom of a similar element. The covalent radius of an atom is determined by measuring the bond lengths between the two covalently bonded atoms. In this case, if the two atoms are on the same side, then the covalent radius is half of its bond length.
5. Bohr Radius:
This is a physical constant equal to the most probable distance between the electron and nucleus of the hydrogen atom in its ground state. The Van der Waals radius of a hydrogen atom is 120 pm, and the atomic radius of a hydrogen atom is 53 pm.
FAQs on Atomic Radius
1. What is atomic radius and how is it measured for different elements?
Atomic radius is defined as the distance from the center of an atom's nucleus to the outermost electron shell. It is typically measured using half the distance between the nuclei of two bonded atoms of the same element. Measurement methods can include spectroscopy, x-ray diffraction, or calculations based on the Heisenberg Uncertainty Principle.
2. Explain the main types of atomic radii found in Physics.
Types of atomic radii include:
- Covalent radius: Half the distance between two covalently bonded atoms of the same element.
- Metallic radius: Half the distance between nuclei of two adjacent atoms in a metallic lattice.
- Van der Waals radius: Half the closest possible distance between two non-bonded atoms.
- Ionic radius: Size of an atom's ion, often depends on its charge and coordination number.
3. How does the atomic radius trend change across a period and down a group according to the Modern Periodic Table?
As you move across a period from left to right, the atomic radius decreases due to increasing nuclear charge pulling electrons closer. Moving down a group, the atomic radius increases as new electron shells are added, expanding the atom's size.
4. What is the difference between atomic radius and ionic radius?
Atomic radius refers to the size of a neutral atom, measuring from the nucleus to its outermost electron shell. Ionic radius is the radius of an atom after it has gained or lost electrons to become an ion. Cations have smaller ionic radii than their atomic radii, while anions have larger ionic radii compared to their neutral atoms.
5. Why is it difficult to measure the exact atomic radius of an atom?
The electron cloud surrounding the nucleus does not have a definite boundary, making it difficult to pinpoint the exact size of an atom. Moreover, the electron distribution is influenced by the type of chemical bonds and interactions present, so there is no single, absolute atomic radius for an element.
6. How does the atomic radius affect the physical and chemical properties of elements?
The atomic radius influences reactivity, ionization energy, and electronegativity. Smaller atoms generally hold electrons more tightly, leading to higher ionization energies and stronger attraction for shared electrons in bonds, while larger atoms are usually more reactive in alkali metals.
7. What factors cause the variation in ionic radii among ions of the same element?
Variation in ionic radii depends on factors such as:
- The ion's charge (higher positive charge reduces size, negative charge increases size).
- The coordination number (number of atoms or ions surrounding the ion).
- The spin state of the electrons.
8. Can you compare the Bohr radius with the Van der Waals radius for hydrogen? What does each represent?
The Bohr radius is the most probable distance between the nucleus and the electron in a hydrogen atom in its ground state (about 53 pm). The Van der Waals radius is the effective size of a hydrogen atom when not bonded (about 120 pm), representing the closest approach to another atom without bonding.
9. How would atomic radius play a role when predicting the outcome of a chemical reaction?
The atomic radius determines how easily an atom can lose or gain electrons. A larger atomic radius means the outermost electrons are held less tightly, often making such atoms more reactive, especially in metals. This can influence reaction rates and product formation in chemical reactions, as seen in trends across groups like alkali metals or halogens.
10. What common student misconceptions exist regarding atomic size and radius trends in the periodic table?
Common misconceptions include:
- Believing the atomic radius remains the same within a period or group.
- Assuming that atomic and ionic radii always change in the same way for all ions.
- Ignoring the effect of added shells (principal quantum number) when moving down a group.
