Shells, Bohr model

The illustrations that depict the electron configurations of the atoms of each element are based on the Bohr model of quantum energy shells. 

An atom is composed of a nucleus of protons and neutrons surrounded by an electron cloud. Theoretically, electrons may be found at any distance from the nucleus, although they preferentially rotate around low-energy orbits or levels. Within a single level, various sublevels can be distinguished. [The term level corresponds to electron shell in the Bohr model. The terminological analogy is shell K = level I (n = 1) shell L = level II (n = 2) shell M = level III (n = 3) shell N = level IV ( = 4) and so on.] Electron levels are established according to four quantum numbers ... 

Quantum Number (Principal). A quantum number that, in the old Bohr model of the atom, determined the energy of an electron in one of the allowed orbits around the nucleus, In the theory of quantum mechanics, the principal quantum number is used most commonly to describe the atomic shell in which tlie elections are located, In a somewhat general way, it is related to the energy of the electronic states of an atom, The symbol for the principal quantum number is n. In x-ray spectral terminology, a -shell is identical to an n = 1 shell, and an L-shell to an n = 2 shell, etc. 

The hydrogen atom with the symbol H is the smallest atom. It contains one proton and one electron in the K-shell. The model of the atom in Fig. 3.1 which meets all 6 requirements for building an atom is called the Bohr atomic model after Niels Henrik David Bohr, the... 

According to an early theory about the atom, the atom looks like a mini solar system. The nucleus of the atom would be the Sun and the electrons are the orbiting planets. This model of the atom is called the Bohr model. It is named for the Danish physicist, Niels Bohr, who proposed electron shells in 1913. The Bohr model is very useful for understanding how atoms work, but it does not answer some questions about the behavior of all atoms. 

Figure 3.2 In this Bohr model of a nitrogen atom, electrons orbit the nucleus much like planets orbit the Sun. The electrons are located in shells that increase in energy as their distance from the nucleus increases. In a nitrogen atom, there are two electrons in the first shell and five electrons in the second shell.

The principal quantum number n may have positive integer values (1, 2, 3,. ..). n is a measure of the distance of an orbital from the nucleus, and orbitals with the same value of n are said to be in the same shell. This is analogous to the Bohr model of the atom. Each shell may contain up to 2n2 electrons. 

Theoreticians thought that stable heavier elements might be in prospect. The stability of a nucleus (based on a model of nuclear stability analogous to that of the Rutherford-Bohr model of electronic structure) is determined by the inter-nucleon forces (nucleons are protons and neutrons), an attractive force between all nucleons and a Coulombic repulsion force between protons, the latter becoming proportionately more important as the number of protons increases. Extra stability is associated with filled shells of nucleons, magic numbers for neutrons they are 2,8,20,28,50,82,126,184, and 196 and for protons they are 2, 8, 20, 28, 50, 82, 114, and 164. 

BohrPT Bohr Model of the Atom - Four Quantum Numbers -Electron Configuration of the Atom - Electron Shells - Shapes of Orbitals - Wave Nature of the Electron - Wave Functions, Radial and... 

Such snapshots would show that the volume in which it is 90% probable that the s electron would be found is spherical. The volume is given the name atomic orbital, usually abbreviated to orbital. The orbital of a Is electron is called a Is-orbital, and it possesses a radius of about 100pm (100 X lO m). However, the single radius at which the Is electron is most likely to be found is at 52.9 pm (the Bohr radius of the n = 1 shell). But - and this is the crucial difference between the wave model and the Bohr model - the wave model states there is always a chance that the electron will be somewhere outside this radius (Fig. 3.12). 

The simple Bohr model of the atom, in which each shell is capable of holding... 

Although the Bohr model correctly predicts the number of electrons in each energy shell according to the 2w calculation with n = the number of the energy shell, the quantum wave model more accurately describes the location probability of the electrons in three-dimensional space. 

The principal quantum number defines the shell in which a particular orbital is found and must be a positive integer ( = 1, 2, 3, 4, 5,.. . ). When n= 1, we are describing the first shell when n = 2, the second shell and so on. Because there is only one electron in hydrogen, all orbitals in the same shell have the same energy, as predicted by the Bohr model. However, when atoms have more than one electron, the negative charges on the electrons repel one another. This repulsion between electrons causes energy differences between orbitals in a shell. This difficulty was one reason the Bohr model had to be replaced. 

Figure 7.18 (a) Plot of electron density in the hydrogen 1s orbital as a function of the distance from the nucleus. The electron density falls off rapidly as the distance from the nucleus increases, p) Boundary surface diagram of the hydrogen 1s orbital, p) A more realistic way of viewing electron density distribution is to divide the Is orbital into successive spherical thin shells. A plot of the probability of finding the electron in each shell, called radial probability, as a function of distance shows a maximum at 52.9 pm from the nucleus. Interestingly, this is equal to the radius of the innermost orbit in the Bohr model. 

Niels Bohr proposed a theory for the electronic structure of hydrogen based on the idea that the electrons of atoms move around atomic nuclei in fixed circular orbits. Electrons change orbits only when they absorb or release energy. The Bohr model was modified as a result of continued research. It was found that precise Bohr orbits for electrons could not be determined. Instead, the energy and location of electrons could be specified in terms of shells, subshells, and orbitals, which are indicated by a notation system of numbers and letters. 

The modified Bohr model, or shell model, of electronic structure provides an explanation for the periodic law. The rules governing electron occupancy in shells, subshells, and orbitals result in a repeating pattern of valence-shell electron arrangements. Elements with similar chemical properties turn out to be elements with identical numbers and types of electfons in their valence shells. 

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