1. The Atom

Question 1

What is the basic structure of an atom according to Bohr’s atomic theory?

Answer 1

According to Bohr’s atomic theory, an atom is composed of a nucleus at the center and one or more electrons rotating around the nucleus along different energy orbits. The nucleus is primarily composed of protons and neutrons, collectively called nucleons.

Question 2

How is the electrical charge of the nucleus balanced in an atom?

Answer 2

For an atom of a given element, the number of electrons moving around the nucleus equals the number of protons, balancing the electrical charge of the nucleus.

Question 3

What are the charges of protons, electrons, and neutrons?

Answer 3

Protons are positively charged, electrons are negatively charged and neutrons have no charge.

Question 4

What determines the chemical properties and stability of an atom?

Answer 4

The electron configuration of the atom determines the chemical properties of an element, whereas the nuclear structure characterizes the stability and radioactive decay of the nucleus of an atom.

Question 5

What are the energy shells in an atom according to the Bohr atomic theory?

Answer 5

The Bohr atomic theory states that electrons in an atom rotate around the nucleus in discrete energy orbits or shells. These energy shells, referred to as the K shell, L shell, M shell, N shell, and so forth, are stationary and arranged in order of increasing energy.

Question 6

What happens when there is a transition of an electron from an upper orbit to a lower orbit or vice versa?

Answer 6

When there is a transition of an electron from an upper orbit to a lower orbit, the energy difference between the two orbits is released as the photon radiation. If the electron is raised from a lower orbit to an upper orbit, the energy difference between the two orbits is absorbed and must be supplied for the transition to occur.

Question 7

How are the energy shells and orbitals designated according to the quantum theory?

Answer 7

Each shell is designated by a quantum number n, called the principal quantum number, and denoted by integers. Each energy shell is subdivided into subshells or orbitals, which are designated as s, p, d, f, and so forth. For a principal quantum number n, there are n orbitals in the main shell.

Question 8

How many electrons can each orbital and shell contain?

Answer 8

Each orbital can accommodate a maximum of 2(2l + 1) electrons, and the total number of electrons in a given shell is 2n^2. Thus, the K shell can contain only 2 electrons, the next L shell 8 electrons, the M shell 18 electrons, the N shell 32 electrons, and the O shell 50 electrons.

Question 9

How are the orbitals filled in atoms?

Answer 9

In atoms, the orbitals are filled in order of increasing energy; that is, the lowest energy orbital is filled in first. However, when d orbitals start filling, there are some exceptions to this rule.

Question 10

What determines the chemical properties of an element?

Answer 10

The electronic structure of the atom of an element determines to a large degree the chemical properties of the element.

Question 11

How are elements arranged in the periodic table?

Answer 11

The periodic table has been devised to arrange the groups of elements of similar chemical properties in order of increasing atomic number. Each group contains elements of similar chemical properties, whereas the periods consist of elements having the same number of electron shells but dissimilar chemical properties.

Question 12

What is the valence of an element?

Answer 12

The valence of an element is the tendency of the atom to lose or gain electrons to achieve a stable electron configuration. It is primarily determined by the number of electrons present in the outermost shell, referred to as the valence shell.

Question 13

What is the octet rule in chemical bond formation?

Answer 13

All chemical bond formation is governed by the octet rule, which states that the electronic structure of each atom in a chemical bond assumes ns^2np^6 containing eight electrons, with the exception of hydrogen and lithium atoms, which essentially assume the structure 1s^2.

Question 14

What is an electron volt (eV)?

Answer 14

An electron volt is the energy acquired by an electron accelerated through a potential difference of 1 V. The energy involved in chemical bond formation is of the order of a few electron volts (eV).

Question 15

What is an Electrovalent or Ionic Bond?

Answer 15

An electrovalent or ionic bond is formed by the complete transfer of an electron from the valence shell of one atom to that of another atom. In ionic bonds, two oppositely charged ions are held together in the compound by coulombic forces.

Question 16

How is NaCl formed?

Answer 16

In the formation of NaCl, the sodium atom loses one electron to become Na+, and the chlorine atom receives the electron to become Cl−. Both ions are then held by an electrovalent bond.

Question 17

What is a Covalent Bond?

Answer 17

In covalent bonds, each of the two atoms participating in bond formation contributes one electron to the bond. Both electrons are shared equally by each atom and do not belong exclusively to one atom alone.

Question 18

What is a Coordinate Covalent Bond?

Answer 18

In a coordinate covalent bond, the pair of electrons required for bond formation is donated by only one atom to another that can accommodate two electrons in octet formation. These bonds are also called semipolar bonds.

Question 19

What is Complex Formation?

Answer 19

Metal complexes are produced by coordinate covalent bonds that are formed by the electrons donated by the chemical species having a lone pair of electrons. These complexes can be cationic, anionic, or neutral.

Question 20

What are Ligands?

Answer 20

The molecules containing NH3, −CN, −SH, −COO, −NH2, CO, etc. are called ligands, which may be neutral or ionic in structure. They all possess an unshared pair of electrons that can be donated to a metal ion to form a complex.

Question 21

What influences the stability of a metal complex?

Answer 21

The stability of a metal complex is influenced by the sizes of the metal ion and the ligand and the dipole moment of the ligand molecule. The smaller the size of the metal ion and the ligand, the more stable the coordinate covalent bond. Ligands with larger dipole moments form more stable complexes. The stability of a complex is also increased by chelation and the number of electron donor atoms in the chelating agent.

Question 22

What are the components of an atom’s nucleus?

Answer 22

The nucleus of an atom is composed of protons and neutrons, collectively called nucleons.

Question 23

What are the atomic number, neutron number, and mass number of an atom?

Answer 23

The number of protons in a nucleus is called the atomic number of the atom, denoted by Z. The number of neutrons is denoted by N. The total number of nucleons in a nucleus is referred to as the mass number, denoted by A. Thus, A is equal to Z + N.

Question 24

What is the Bohr liquid drop model of the nucleus?

Answer 24

According to the Bohr liquid drop model, the nucleus is assumed to be spherical and composed of closely packed nucleons, and particle emission by the nucleus resembles evaporation of molecules from a liquid drop.

Question 25

What is the shell model of the nucleus?

Answer 25

In the shell model, nucleons are arranged in discrete energy shells similar to the electron shells of the atom in the Bohr atomic theory. Nuclei containing 2, 8, 20, 50, 82, or 126 protons or neutrons are very stable, and these nucleon numbers are called magic numbers.

Question 26

What factors affect the stability of a nucleus?

Answer 26

Nuclei are less stable if they contain an odd number of protons or neutrons, whereas nuclei with an even number of protons and neutrons are more stable. The ratio of the number of neutrons to the number of protons (N/Z) is also an approximate index of the stability of an atom.

Question 27

What is the nuclear force?

Answer 27

The nuclear force is a binding force that is much stronger than the electrostatic force and binds equally protons and neutrons within the nucleus. The nuclear force exists only in the nucleus and has no influence outside the nucleus.

Question 28

What is the mass defect of a nucleus?

Answer 28

The mass M of a nucleus is always less than the combined masses of the nucleons A in the nucleus. This difference in mass (M–A) is termed the mass defect, which has been used as energy in binding all the nucleons in the nucleus.

Question 29

What is a nuclide?

Answer 29

A nuclide is a distinct species identified by an exact nuclear composition including the mass number A, atomic number Z, and arrangement of nucleons in the nucleus.

Question 30

What are radionuclides?

Answer 30

Radionuclides are a group of nuclides which are unstable or radioactive and decay by spontaneous fission or emission of α-particle, β-particle, and γ-ray.

Question 31

What are isotopes?

Answer 31

Isotopes are nuclides of the same atomic number and exhibit the same chemical properties.

Question 32

What are isotones?

Answer 32

Isotones are nuclides having the same number of neutrons but different atomic numbers.

Question 33

What are isobars?

Answer 33

Isobars are nuclides with the same number of nucleons, that is, the same mass number, but a different number of protons and neutrons.

Question 34

What are isomers?

Answer 34

Isomers are nuclides having the same number of protons and neutrons but differing in energy states and spins.

Question 35

What is the chart of the nuclides?

Answer 35

The chart of the nuclides is a form where both stable and radioactive nuclides are arranged. Each nuclide is represented by a square containing various information such as the half-life, type and energy of radiations, and so forth, of the radionuclide and the neutron capture cross-section of the stable nuclides.