Electronic arrangement in atoms • In an atom, electrons are located in energy shells around the nucleus. • These energy shells are arranged in quantum numbers (represented by the letter n), with quantum number 1 being the closest to the nucleus, quantum number 2 being the next in that order until infinity. • The quantum numbers are further split into subshells. The subshells are represented by letters 𝒔 , 𝒑 , 𝒅 and 𝒇. Quantum number Subshells in it 1 1s 2 2s 2p 3 3s 3p 3d 4 4s 4p 4d 5 5s 5p 5d 5f • A subshell consists of one or more orbitals. An orbital is a region in an atom which can occupy a maximum of two electrons. Subshell s p d f Number of orbitals has. 1 3 5 7 Maximum number of electrons 2 6 10 14 • Since quantum number 1 consists of a 1s subshell only it can contain a maximum of two electrons, quantum number 3 on the other hand can have a maximum of 18 electrons (2 from 3s, 6 from 3p, 10 from 3d) Electronic configuration • Electrons are filled in the atom from the lowest energy orbital 1s, closest to the nucleus, to the highest. Order of filling an atom • the energy increases in the order 𝟏𝒔 𝟐𝒔 𝟑𝒔 𝟑𝒑 𝟒𝒔 𝟑𝒅 𝟒𝒑 𝟓𝒔 as shown by the direction of the arrows NB note that the energies of orbitals from different quantum numbers may overlap e.g the 𝟒𝒔 is filled before the 𝟑𝒅 and the 𝟓𝒔 is filled before the 𝟒𝒅 and the 𝟒𝒇. Examples • The numbers at the top indicate the number of electrons in that subshell. • The display of electrons in boxes has been placed by the side, these are very helpful in chemistry • NB note that when filling a subshell with more than one orbital like the p or d , each orbital is occupied by a single electron before pairing is done, 𝑝3 in nitrogen thus has one electron in each of the three orbitals. • the noble gas configuration (also called the shorthand configuration) is also used e.g [𝐴𝑟]4𝑠 2 is the configuration of Ca since it has 20 electrons and 𝐴𝑟 has 18 Ground state and excited state • the energy levels of electrons are quantized, this means that every energy shell has a specific amount of energy associated with it. • If an electron is to occupy a particular orbital it must have the exact energy requirements of that particular orbital, not more or less. If by any means the electron gains or loses energy then it automatically moves away from that orbital to another. • The location of the electron when it has not lost or gained energy is known as its ground state. If an electron gains energy, it goes to higher energy levels, in this case it is said to be in an excited state. As always, the state of being excited does not last forever and the electrons usually lose the energy they gained and go back again to where they belong, the ground state! Shapes of orbitals • the s orbitals are spherical, the difference between the 1s and the 2s orbital is simple that it has a larger radius which in turn is smaller than that of a 3𝑠 in that order. There are three 𝑝 orbitals, 𝑝𝑥 , 𝑝𝑦 𝑎𝑛𝑑 𝑝𝑧 , each of them is dumbbell in shape but have different orientations. Radicals • a radical is any specie that contains one or more unpaired electrons e g O is a radical as it has two unpaired electrons (check its configuration above) • Radicals are very reactive and unstable; they exist for a very short period in nature as they will quickly react to form other more stable compounds. Blocks in the periodic table • The periodic table consists of the s-block, the d-block and the p block elements. In the s block elements e.g Ca, the outer orbital being filled is an s orbital , in the d-block elements eg Fe it is the d orbital that is being filled and for the p block elements eg N , it is the p orbitals being field.
