Md. Imran Nur ManikLecture

Department of PharmacyNorthern University

Bangladesh

Structure of Atom

Distribution of Electrons in AtomsThe electrons are arranged among the known elements in seven main energy level is designated by the principal quantum numbers n=1, 2, 3, 4, 5, 6 and 7. These principal levels are divided into sub-levels indicated by s, p, d, and f.The first energy level (n=1) which has only one sub-level is designated as 1s. The second energy level (n=2) has two sub-levels designated as 2s and 2p, the third (n=3) energy level has three sub-levels designated as 3s, 3p and 3d and the fourth energy level n=4 has four sub-levels 4s, 4p, 4d and 4f.The sub-levels are further divisible into orbitals. An s sub-level is made up of one orbital; a p sub-level - three orbitals; a d sub-level-five orbitals and f sub-level - seven orbitals.Each electron orbital can accommodate a maximum of two electrons of opposed spins. Thus, one s orbital can hold a maximum of two electrons. The three p orbitals can hold a maximum of 6 electrons etc.

Electronic Configuration of AtomAccording to quantum mechanics, the arrangement of electrons of an atom in different orbitals is called the electronic configuration.

This electronic configuration follows some rules i.e. • Pauli’s Exclusion Principle• Aufbau’s law• Hund’s rule of maximum multiplicity

Pauli’s Exclusion PrincipleThe number of electrons to be filled in various orbitals is restricted by the exclusion principle given by the Austrian scientist Wolfgang Pauli. Pauli’s exclusion principle states that, ‘no two electrons in an atom can have the same set of values for all four quantum numbers.’This principle can also be stated as-“only two electrons may exist in the same orbital and they must have opposite spin.” It means that the two electrons of the same atom can have the same values for three of their quantum numbers, but the fourth quantum number must be different for the two electrons. Thus, two electrons may have orbits of the same size, shape and orientation in space provided they have opposed spins.For example, helium has two electrons. According to Pauli’s exclusion principle, the values of the quantum numbers for these two electrons will be as follows-

For electron 1, e1: n = 1; l = 0; m = 0; s = +1/2For electron 2, e2: n =1; l = 0; m = 0; s = -1/2

Basic concept(read only)Consider the second shell (n = 2), there being four orbitals, one s orbital (l = 0) and three p

orbitals (l = 1), the possible number of electrons having different set of quantum numbers can be asfollows :

The total number of electrons that can be accommodated in second shell is equal to 2 + 6 = 8.Similarly it can be shown that the maximum number of electrons in the third and fourth shells is equalto 18 and 32 respectively.

BASIC CONCEPT (READ ONLY)ENERGY DISTRIBUTION AND ORBITALS within each energy level ‘n’, the various sublevels (different l values orbitals) exhibit slightly different energies. The orbitals at a principal level n get split up and come to possess different energies, which increase in the same order as the various values of l. Thus for a particular principal level, the energy of the sublevels is in the order s < p < d < f.The energy levels of 3s, 3p and 3d orbitals are different even though they belong to the same shell n = 3. However, it may be noted that the energy of electrons in the same orbital is the same. Thus all 3d orbitals (3dxy, 3dyz, 3dzx, ……….) or 4p orbitals (4px, 4py, 4pz) are at the same level of energy, irrespective of their orientation. It is also noteworthy from the above diagram that the order of increase of energy values of various orbitals approximately follows the sequence given below : 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5dIt is, therefore, clear that the 3d orbital electrons belonging to a lower shell (n = 3) possess more energy than 4s orbital electrons which belong to higher shell (n = 4). The 3d orbitals lie at a higher energy level than 4s orbital.

Aufbau’s Law Aufbau is a German word which means build up or construction. It is for this reason that the principle is also often called building up principle or construction principle. This principle states as followsThe orbitals are filled up with electrons in the increasing order of their energy. The principle can also be restated as, ‘During electronic configuration, electrons first go to the orbital of low energy and then systematically to the orbital of higher energy.’ This happens because the orbital with lower energy is more stable.The energy of various orbitals of an atom a multi-electron atom increases as1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s ......

The energy of an orbital is determined by the sum of principal quantum number (n) and the azimuthal quantum number (l). This rule is called (n + l) rule. There are two parts of this rule: (a) The orbitals with the lower value of (n + l) have lower energy than the orbitals of higher (n + l) value. And hence is filled up first with electrons.(b) When two or more orbitals have the same value of (n + 1), the orbital with lower value of n is lower in energy and hence is filled up first with electrons.Example: Considering 4s and 3d orbital:For the 4s orbital, the value of, n=4 and the value of l=0 (s). The total value of (n+l)= 4+0= 4Again, for the 3d orbital, the value of, n=3 and the value of l=2 (d). The total value of (n+l)= 3+2= 5So, electrons will go to 4s orbital first between 4s and 3d orbital.On the other hand, If the two orbital have the same value of (n+l) then electrons will go to that orbital first which has lower principal quantum number.

Considering 3d and 4p orbital:For the 3d orbital, the value of n=3 and the value of l=2 (d). The total value of (n+l) = 3+2=5For the 4p orbital, the value of n=4 and the value of l=1(p). The total value of (n+l) = 4+1=5

Hence, electrons will go to the 3d orbital first.

BASIC CONCEPT (READ ONLY)It is a common practice to denote an orbital by a horizontal line or a circle or square and an electron by an arrow over it. The direction of the arrow indicates the spin, an upward arrow representing a clockwise spin while the downward arrow stands for the anticlockwise direction of spin. When there are more than one orbitals in a subshell (degenerate orbitals), they are shown by an equivalent number of horizontal lines at the same energy level. Let us now describe the electron configuration of first ten elements.

Hund’s Rule of Maximum Multiplicity

This rule is applied to the electronic configuration in the orbitals of same energy. The distribution of electron in a set of degenerate orbitals like px, py, pz set; dxy, dyz, dzx, dz

2, dx2, dy

2 set etc. of a given sub-shell takes place in such a way to give the maximum number of unpaired electrons and these electrons must have the same direction of spins.This rule can also be stated as- ‘The orbital of a given sub-shell is first filled singly and then the pairing of electrons in each orbital begins.’

Illustration of the principle:In order to

illustrate the principle, let us consider the electronic configuration of oxygen atom whose atomic number is eight. According to this principle the degenerated 2px, 2py and 2pz orbitals will be first filled singly with electrons numbered as 5, 6 and 7. These 3 electrons will have the same spin. Electron number 8 which is the last electron will pair with the electron number 5 in 2px orbital.

Stable configuration:It’s been observed that half-filled or fully filled orbitals with same energy have more stable configuration. That’s why np3, np6, nd5, nd10, nf7 or nf14 orbitals are more stable.For this reason, d5s1 configuration occurs rather than d4s2 and d10s1 occurs rather than d9s2.Example, Cr(24)=1s2 2s2 2p6 3s2 3p6 4s2 3d4 (Aufbau’s law)=1s2 2s2 2p6 3s2 3p6 4s1 3d5 (Stable configuration)Here, the latter one occurs in case of Chromium to acquire more stable configuration.