Atomic Structure Flashcards

Question

Plank’s quantum theory

Answer 1

Planck assumed that radiation could be sub-divided into discrete chunks of energy. He suggested that atoms and molecules could emit or absorb energy only in discrete quantities ie discontinuous manner and not in a continuous manner. He gave the name quantum to the smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic radiation The energy (E ) of a quantum of radiation is proportional to its frequency

Answer 2

Hot objects emit electromagnetic radiations over a wide range of wavelengths. At high temperatures, an appreciable proportion of radiation is in the visible region of the spectrum. As the temperature is raised, a higher proportion of short wavelength (blue light) is generated.

Answer 3

when an iron rod is heated in a furnace, it first turns to dull red and then progressively becomes more and more red as the temperature increases. As this is heated further, the radiation emitted becomes white and then becomes blue as the temperature becomes very high. This means that red radiation is most intense at a particular temperature and the blue radiation is more intense at another temperature.

Answer 4

An ideal body, which emits and absorbs radiations of all frequencies uniformly, is called a black body and the radiation emitted by such a body is called black body radiation. A black body is also a perfect radiator of radiant energy.

Answer 5

In practice, no such body exists. Carbon black approximates fairly closely to black body. A good physical approximation to a black body is a cavity with a tiny hole, which has no other opening. Any ray entering the hole will be reflected by the cavity walls and will be eventually absorbed by the walls.

Answer 6

Quantum theory of radiation and classical laws of physics It is also based on or valid for particle nature of electron

Answer 7

1: The electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits, stationary states or allowed energy states. These orbits are arranged concentrically around the nucleus. 2: The energy of an electron in the orbit does not change with time. However, the electron will move from a lower stationary state to a higher stationary state when required amount of energy is absorbed by the electron or energy is emitted when electron moves from higher stationary state to lower stationary state. The energy change does not take place in a continuous manner. 3: The angular momentum of an electron is quantised. 4: Thus an electron can move only in those orbits for which its angular momentum is integral multiple of h/2π. That means angular momentum is quantised. Radiation is emitted or obsorbed only when transition of electron takes place from one quantised value of angular momentum to another. Therefore, Maxwell’s electromagnetic theory does not apply here that is why only certain fixed orbits are allowed.

Answer 8

(1) Bohr's theory does not explain the spectrum of multi electron atom/ion. (2) Why the Angular momentum of the revolving electron is equal to nh/2 has not been explained by Bohr's theory. (3) Bohr inter related quantum theory of radiation and classical laws of physics without any theoretical explanation. (4) Bohr's theory does not explain the fine structure of the spectral lines. Fine structure of the spectral line is obtained when spectrum is viewed by spectroscope of more resolution power. (5) Bohr's theory does not explain the splitting of spectral lines in the presence of magnetic field (Zeeman's effect) or electric field (Stark effect)

Answer 9

The principal quantum number determines the size and to large extent the energy of the orbital. The principal quantum number also identifies the shell. With the increase in the value of ‘n’, All the orbitals of a given value of ‘n’ constitute a single shell of atom Size of an orbital increases with increase of principal quantum number ‘n’. In other words the electron will be located away from the nucleus. Since energy is required in shifting away the negatively charged electron from the positively charged nucleus, the energy of the orbital will increase with increase of n.

Answer 10

Azimuthal quantum number. ‘l’ is also known as orbital angular momentum or subsidiary quantum number. It defines the three-dimensional shape of the orbital. l can have n values ranging from 0 to n – 1 Each shell consists of one or more sub-shells or sub-levels. The number of sub-shells in a principal shell is equal to the value of n. For example in the first shell (n = 1), there is only one sub-shell which corresponds to l = 0.

Answer 11

Magnetic orbital quantum number. ‘ml’ gives information about the spatial orientation of the orbital with respect to standard set of co-ordinate axis. Value of m ranges from -l to +l including the zero

Answer 12

True some of the lines actually occur in doublets (two lines closely spaced), triplets (three lines, closely spaced) etc. This suggests the presence of a few more energy levels than predicted by the three quantum numbers.

Answer 13

An electron spins around its own axis, much in a similar way as earth spins around its own axis while revolving around the sun. In other words, an electron has, besides charge and mass, intrinsic spin angular quantum number. Spin angular momentum of the electron — a vector quantity, can have two orientations relative to the chosen axis. These two orientations are distinguished by the spin quantum numbers ms which can take the values of +½ or –½. These are called the two spin states of the electron and are normally represented by two arrows, ↑ (spin up) and ↓ (spin down). Two electrons that have different ms values (one +½ and the other –½) are said to have opposite spins. An orbital cannot hold more than two electrons and these two electrons should have opposite spins.

Answer 14

1. The energy of electrons in atoms is quantized (i.e., can only have certain specific values), for example when electrons are bound to the nucleus in atoms. 2. The existence of quantized electronic energy levels is a direct result of the wave like properties of electrons and are allowed solutions of Schrödinger wave equation. 3. Both the exact position and exact velocity of an electron in an atom cannot be determined simultaneously (Heisenberg uncertainty principle). The path of an electron in an atom therefore, can never be determined or known accurately. That is why, as you shall see later on, one talks of only probability of finding the electron at different points in an atom. 4. An atomic orbital is the wave function ψ for an electron in an atom. Whenever an electron is described by a wave function, we say that the electron occupies that orbital. Since many such wave functions are possible for an electron, there are many atomic orbitals in an atom. These “one electron orbital wave functions” or orbitals form the basis of the electronic structure of atoms. In each orbital, the electron has a definite energy. An orbital cannot contain more than two electrons. In a multi-electron atom, the electrons are filled in various orbitals in the order of increasing energy. For each electron of a multi-electron atom, there shall, therefore, be an orbital wave function characteristic of the orbital it occupies. All the information about the electron in an atom is stored in its orbital wave function ψ and quantum mechanics makes it possible to extract this information out of ψ. 5. The probability of finding an electron at a point within an atom is proportional to the square of the orbital wave function i.e., |ψ|2 at that point. |ψ|2 is known as probability density and is always positive. From the value of |ψ|2 at different points within an atom, it is possible to predict the region around the nucleus where electron will most probably be found.

Answer 15

The word ‘aufbau’ in German means ‘building up’. The building up of orbitals means the filling up of orbitals with electrons. The principle states : In the ground state of the atoms, the orbitals are filled in order of their increasing energies.

Answer 16

Wolfgang Pauli (1926). Pauli’s exclusion principle According to this principle : No two electrons in an atom can have the same set of four quantum numbers. Pauli exclusion principle can also be stated as : “Only two electrons may exist in the same orbital and these electrons must have opposite spin.” The restriction imposed by Pauli’s exclusion principle on the number of electrons in an orbital helps in calculating the capacity of electrons to be present in any subshell.

Answer 17

This rule deals with the filling of electrons into the orbitals belonging to the same subshell (that is, orbitals of equal energy, called degenerate orbitals). It states : pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied.

Answer 18

Applied only on species that aren’t like hydrogen For hydrogen like species energy is proportional to ‘n’ (n+l) for different values of this sum the energy is high for the one with high value of sum For (n+l) same then compare with ‘n’ then energy is proportional to ‘n’

Answer 19

Both statement are incorrect Absorption spectrum consist of some dark spaces separated by some bright lines Emission spectra consists of bright lines always

Answer 20

It is the pattern of light emitted or absorbed by an atom due to electron transitions between energy levels.

Answer 21

1. Absorption Spectrum – Dark lines on a bright background (electrons absorb energy). 2. Emission Spectrum – Bright lines on a dark background (electrons release energy).

Answer 22

It consists of dark lines in a continuous spectrum where specific wavelengths are absorbed by electrons moving to higher energy levels.

Answer 23

✔ Unique for each element. ✔ Dark lines on a bright background. ✔ Absorbed wavelengths match those in the emission spectrum.

Answer 24

It consists of bright lines on a dark background, produced when electrons fall back to lower energy levels, releasing energy as light.

Answer 25

✔ Unique for each element. ✔ Bright lines on a dark background. ✔ Represents specific electron transitions.

Answer 26

✔ Continuous Spectrum – All wavelengths present (e.g., sunlight). ✔ Line Spectrum – Only specific wavelengths appear (e.g., hydrogen emission spectrum).

Answer 27

The hydrogen spectrum consists of five series of spectral lines corresponding to electron transitions to different energy levels.

Answer 28

✔ Each element has unique energy levels. ✔ The possible electron transitions differ for each element. ✔ This results in unique absorption and emission lines.