SPS 101_Lecture 1_Atomic Structure.pdf

UDS – SPPS Department of Pharm. Chemistry Course: SPS 101 Principles of Chemistry Lecture: Chemical Structure Lecturer: Abdallah Yakubu (PhD) 25 January 2024 SPS 101 Principles of Chemistry_2023-2024...

UDS – SPPS Department of Pharm. Chemistry Course: SPS 101 Principles of Chemistry Lecture: Chemical Structure Lecturer: Abdallah Yakubu (PhD) 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 1 Course Outline Chemical structure Chemical bonding Molecular shapes Periodic table Acids, bases and salts Bioinorganic chemistry 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 2 Course Objectives The objectives of this course are to: Describe the atomic structure, chemical bonding and molecular shapes. Understand the arrangement of elements in the Periodic Table and explain the periodic properties of elements. Know and describe weak acids and bases and determine hydrogen ion concentration. Recognize the importance of metals in biological systems. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 3 Course Delivery and Assessments Teaching and learning would be delivered face-to-face in classrooms or through an online medium where suitable. Assessment would be conducted for continuous assessment (via quizzes, assignments, mid-trimester exams, etc) and end of trimester exams. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 4 Reference Reading Materials Nivaldo J. Tro. Principles of Chemistry: A molecular approach, Pearson Education, Inc. , New Jersey, 2010. Ebbing, D. D. and Gammon, S. D. General Chemistry, 7th edition, Houghton Mifflin, New York, 2002. Raymond Chang; Kenneth A. Golds. Chemistry, McGraw-Hill Education, New York, 2015. Any other relevant chemistry textbook will be helpful. The University Library Online chemistry resource materials 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 5 Classical Atomic Structure An atom is composed of three elementary particles: proton, electron and neutron. The proton has a positive charge, the electron has a negative charge, and the neutron has no charge. Protons and neutrons are located in a small region of space at the center of the atom, called the nucleus, and electrons are spread out about the nucleus at some distance from it. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 6 Classical Atomic Structure Electron Nucleus Classical structure of the atom 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 7 Classical Atomic Structure The classical model of an atom describes the structure of the atom based on Rutherford and Bohr’s models. Rutherford Atomic Model An atom consist of a positively charged, dense and very small nucleus containing protons and neutrons. The entire mass of the atom is concentrated in the nucleus. The size of the nucleus is very small compared to the size of the atom. The nucleus is surrounded by negatively charged electrons. An atom is electrically neutral because the number of protons is equal to the number of electrons. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 8 Classical Atomic Model The electrons move around the nucleus in circular paths at very high speed. These circular paths of the electrons are called orbits. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 9 Bohr’s model of the Hydrogen atom A model that depicts electrons as moving around the nucleus in circular orbits at specific, fixed radii from the nucleus. Bohr assumed that the energy of each orbit is fixed or quantized and called these orbits as stationary states or energy levels. r r 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 10 Bohr’s model of the Hydrogen atom Thus, the energy of the electron is quantized, and the electron is restricted to certain allowed energy levels unless it gains or loses a certain amount of energy. According to Bohr, an electron in an atom can move from one energy level to another by absorbing or emitting specific energies or frequencies. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 11 Bohr’s model of the Hydrogen atom Bohr showed that the allowed energy of the H electron is given as: 𝟏 𝑬𝒏 = −𝑹𝑯 ( 𝟐 ) 𝒏 where the 𝑹𝑯 is the Rydberg constant = 2.18 × 10−18 J, n is an integer (n = 1, 2, 3…) called the principal quantum number. Question: Calculate the frequency and the wavelength for H electron transition from n = 3 to n = 1 levels. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 12 Bohr’s model of the Hydrogen atom Bohr assumed that when an atom is in its ground state, it does not absorb or emit electromagnetic radiation. However, when the atom absorbs or emits electromagnetic radiations with specific energies, the electrons undergo a transition from one E absorbed energy state to another. E emitted Absorption Emission 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 13 Bohr’s model of the Hydrogen atom Thus, each frequency of radiation absorbed or emitted as a result of the transition from a higher energy state to lower energy state correspond to a spectral line. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 14 Quantum Theory of the Atom The quantum theory of atoms is governed by the works of Louis de Broglie (1892–1987), Werner Heisenberg (1901–1976) and Erwin Schrödinger, (1887–1961). The theory explains how electrons exist in atoms and how those electrons determine the chemical and physical properties of elements. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 15 de Broglie relation Louis de Broglie proposed that particles of matter e.g. electrons moving through space exhibit wave properties. The wavelength associated with a particle moving through space is related to its kinetic energy. The faster the electron is moving, the higher its kinetic energy and the shorter its wavelength. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 16 de Broglie relation According to de Broglie’s proposal, a particle with mass (m) and velocity (ν) has an associated wavelength (λ) expressed as: 𝒉 𝛌= 𝐦𝐯 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 17 de Broglie relation Example: Calculate the wavelength of an electron traveling with a speed of 2.65 x 106 m/s. (h = 6.626 x10-34J.s me- = 9.11 x10-31kg). h λ= mv 6.626 × 10−34 −𝟏𝟎 𝛌= −31 6 = 𝟐. 𝟕𝟒 × 𝟏𝟎 𝐦 9.11 × 10 × 2.65 × 10 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 18 Heisenberg uncertainty principle 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 19 Heisenberg uncertainty principle A problem arose when electrons were demonstrated to be wave-like: How can the “position” of a wave be specified? The precise location of a wave cannot be specified because a wave extends in space. To describe the problem of trying to locate a subatomic particle that behaves like a wave, Werner Heisenberg formulated what is now known as the Heisenberg uncertainty principle. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 20 Heisenberg uncertainty principle The Heisenberg uncertainty principle states that ‘it is not possible to know at the same time both the precise position and momentum of a particle. If the position of the particle is known more precisely, its momentum measurement must become less precise. Expressed mathematically, the uncertainty principle is: ∆𝑥 ∗ ∆𝑝 ≥ = ℎ 4𝜋 where Δx and Δp are the uncertainties in position and momentum, respectively, and ℎ is Planck’s constant. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 21 The Schrödinger Equation In 1926 Erwin Schrödinger developed a complex differential equation to describe the behaviour and energy of an electron in an atom. Η𝜓 = 𝐸𝜓 This equation describes the wave properties (𝜓) of the electron at various points in space. The quantity 𝜓 is known as the wave function. The wave function is called an atomic orbital, a region of space where there is a high probability of finding an electron in an atom. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 22 The Schrödinger Equation The relative energy of an atomic orbital, its shape and its orientation in space can be deduced from the wave function. The electron cloud diagram, shows how 𝜓 2 for the H electron in its ground state (n = 1) varies moving out from the nucleus. Regions of high electron density represent a high probability of locating the electron, whereas the opposite holds for regions of low electron density. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 23 The Schrödinger Equation A representation of the electron density distribution surrounding the nucleus in a hydrogen atom. It shows a high probability of finding the electron closer to the nucleus. The electron cloud diagram, shows how 𝜓 2 for the H electron in its ground state (n = 1) varies moving out from the nucleus. The depth of the color is proportional to the probability of finding the electron at a given point. The dotted circle encloses the volume within which there is a 90% probability of finding the electron. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 24 The Schrödinger Equation The wave function, is described by three quantum numbers: the principal quantum number n, the angular momentum quantum number l and the magnetic quantum number ml. A wave function corresponding to a particular set of the three quantum numbers (e.g. n = 2, l = 1, ml = 0) is associated with an electron occupying an atomic orbital. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 25 Quantum Numbers Quantum numbers are used to describe the position and energy of electrons in an atom. The four quantum numbers used to completely describe the properties of electrons in an atom are the: Principal quantum number, denoted by n. Orbital angular momentum quantum number (or azimuthal quantum number), denoted by l. Magnetic quantum number, denoted by ml. Electron spin quantum number, denoted by ms. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 26 Principal Quantum Number (n) The principle quantum number n describes the average distance of the orbital from the nucleus and the energy of the electron in an atom. It designates the principal electron shell of the atom. It can have positive integers of n = 1, 2, 3, and so on. The value n = 1 denotes the innermost electron shell of an atom, which corresponds to the lowest energy state (or the ground state) of an electron. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 27 Angular momentum QN The angular momentum quantum number l, determines the shape of the atomic orbital. The allowed l values for a given n value are positive integers from 0 up to n – 1. That is, l = 0, 1, 2,..., (n – 1). Thus for n = 1, l = 0; For n = 3, l = 0, 1, 2. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 28 Angular momentum QN Each l value indicates an s, p, d, or f subshell which represent a particular shape of the atomic orbital. Larger values of l produce more complex shapes. l 0 1 2 3 4 Orbital s p d f g 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 29 Angular momentum QN s-Orbital Value of l = 0. Spherical in shape. Radius of sphere increases with increasing value of n. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 30 Angular momentum QN p-Orbital Value of l = 1. It has a dumb-bell shape. Have two lobes with a node between them. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 31 Angular momentum QN d-Orbital Value of l is 2. Four of the five orbitals have 4 lobes; the other resembles a p orbital with a doughnut around the center. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 32 Angular momentum QN For a given n, the energy of an orbital increases with l. Thus for multi- electron atoms, the energy is dependent on n as well as l. Within a given principal level (same value of n), sublevels increase in energy in the order: ns < np < nd < nf < ng … Increasing orbital energy n value: 1 2 3 4 l sublevels: 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f Increasing orbital energy 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 33 Magnetic Quantum Number (ml) The magnetic quantum number ml determines the orientation of atomic orbitals in space relative to the nucleus. Allowed values of ml are all integers from –l to +l. For example, if the l = 2, then ml will have values of -2, -1, 0, 1 and 2. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 34 Magnetic Quantum Number (ml) The number of ml values is equal to the number of atomic orbitals in a subshell. The number of orbitals in a given subshell is equal to (2l + 1). An s subshell has only one orbital [2(0) +1 = 1], a p subshell has three orbital [2(1) +1 = 3] and so on. For l =1 (p subshell), ml = -1, 0 and +1; there are three different p- orbitals. The three p-orbitals have the same size, shape and energy but different orientations in space. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 35 Magnetic Quantum Number (ml) Each subshell consists of one or more atomic orbitals. The number of orbitals in a given subshell is equal to (2l + 1). For l = 1 (p subshell), there are three orbitals [2(1) +1 = 3] and ml will have values of -1, 0 and +1. The three p-orbitals have the same size, shape and energy but different orientations in space. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 36 Spin Quantum Number (ms) The spin quantum number describes the spin and magnetic behaviour of electrons. The electron in an atom does not only revolves around the nucleus but also rotates around its own axis. The rotation of an electron around its own axis is called a spin. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 37 Spin Quantum Number (ms) An electron can either spin clockwise (spin up) or anticlockwise (spin down). The probability of rotation in one direction is 12. Therefore, the ms can only have two values: + 12 or − 12. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 38 Spin Quantum Number (ms) Two electrons that have the same spin are said to be parallel, whereas 1 1 electrons with different spins (one + and the other − ) are called 2 2 paired. According to the electromagnetic theory, a spinning charge generates a magnetic field, and it is this motion that causes an electron to behave like a magnet. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 39 Electron Configuration of Atoms Electron configuration of an atom shows how electrons are distributed among the various atomic orbitals in order to understand electronic behavior of the atom. The EC of atoms is governed by three principles namely: Aufbau principle Pauli’s exclusion principle Hund’s rule of multiplicity 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 40 Electron Configuration of Atoms Aufbau Principle The Aufbau Principle states that when electrons fill into atomic orbitals, the electrons occupy the lowest energy orbitals before the highest energy orbitals. It describes the order in which electrons occupy orbitals in atoms. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 41 Electron Configuration of Atoms Aufbau Principle The Principle is used to build up the ground state electron configuration of atoms. The order in which atomic subshells are filled in a many-electron atom is given as: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < ……….. Write the ground state electron configuration for atoms of the following elements. 1H, 2He, 6C, 8O, 10Ne, and 11Na 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 42 Electron Configuration of Atoms Pauli Exclusion Principle The Pauli Exclusion Principle states that “No more than two electrons can occupy the same orbital in an atom, and these electrons must have opposite spins”. It means that if two electrons are in the same atomic orbital, then they must have opposite spins. In other words, only two electrons may occupy the same atomic 1 orbital, and these electrons must have opposite spins (i.e. spin up + ; 2 1 or spin down − ). 25 January 2024 2 SPS 101 Principles of Chemistry_2023-2024 Academic Year 43 Electron Configuration of Atoms By applying the exclusion principle, the electron configuration and orbital diagram for hydrogen, helium and neon can be written and drawn respectively as follows: Atom Electron configuration Orbital diagram 1H 1s1 2He 1s2 10Ne: 1s2 2s2 2p6 1s 2s 2p 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 44 Electron Configuration of Atoms Hund’s Rule Hund’s rule states that degenerate orbitals are filled with one electron each before any electrons are paired. When two electrons occupy separate orbitals of equal energy, the repulsive interaction between them is lower than when they occupy the same orbital because the electrons are spread out over a larger region of space. 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 45 Electron Configuration of Atoms When two electrons occupy separate orbitals of equal energy, the repulsive interaction between them is lower than when they occupy the same orbital because the electrons are spread out over a larger region of space. For examples 8O: 1s2 2s2 2px22py12pz1 1s 2s 2px 2py 2pz 10Ne: 1s2 2s2 2px22py22pz2 1s 2s 2px 2py 2pz 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 46  End of Lecture 25 January 2024 SPS 101 Principles of Chemistry_2023-2024 Academic Year 47