Atomic and Electronic Structure 1 PDF

MED-102 General Chemistry Atomic and Electronic Structure 1  Learning Objectives (LOBs) Describe atomic structure. Identify subatomic particles and describe their properties. Perform calculations related to isotopic species. Recognize atomic orbitals of s, p, and d type visually. Ide...

MED-102 General Chemistry Atomic and Electronic Structure 1  Learning Objectives (LOBs) Describe atomic structure. Identify subatomic particles and describe their properties. Perform calculations related to isotopic species. Recognize atomic orbitals of s, p, and d type visually. Identify the spatial aspects of atomic orbitals controlled by quantum numbers.  Atomic Theory of Matter Democritus was the first to propose an atomic theory for matter Atom (άτομο) means literally an object that cannot be cut into smaller parts. It is the smallest piece of matter, in the chemical sense.  Atoms can be “visualized” Scanning Tunnelling Electron Microscope (STEM) images Simplified Structure of the Atom Bohr Model - As an introduction this is useful but not valid Properties of subatomic particles Proton – Electrical charge = +1 – Mass = 1 atomic mass unit (amu) Neutron – Electrical charge = 0 – Mass = 1 atomic mass unit (amu) Electron – Electrical charge = -1 – Mass = 0.00055 atomic mass units (amu)  Electron Cloud Model Since electrons move at 80% of the speed of light, they form a cloud-like sphere around the atom – the electron cloud  Electron Cloud – Analogy to a Fan Electrons are difficult to locate because they are spinning very fast around the nucleus, like the spinning blades of a fan  Atomic Symbols Atomic symbols enable us to determine the number of subatomic particles in a particular atom Atomic number = number of protons Mass number = number of nucleons (protons + neutrons) The number of electrons = number of protons in a neutral atom Atomic Symbols for Charged Atoms In some atoms the number of protons and the number of electrons are not equal. These are called ions. Positive ions = cations Negative ions = anions  Exercise on Atomic Symbols Consider the following atomic symbol: How many protons, neutrons and electrons are there in this symbol? Exercise on Atomic Symbols - Solution Consider the following atomic symbol: How many protons, neutrons and electrons are there in this symbol? The subscript 27 is the atomic number = number of protons The superscript is the mass number = protons + neutrons Therefore, 59 – 27 = 32 = number of neutrons The overall charge is positive. 2 more protons than electrons. Therefore, number of electrons = 27 – 2 = 25  Isotopes The atomic number (proton number) defines the element. No two different elements can have the same atomic number. Within an element, we can have different atoms, all with the same atomic number, but with different mass numbers, i.e. a different number of neutrons.  Attributes of isotopes A stable isotope has a mass and a natural abundance (% in nature) An unstable (radioactive) isotope also has a half-life Carbon-14 is radioactive with a half-life of 5730 years and is used in the carbon-dating method for ancient samples  Average Atomic Mass AM is the weighted average of the different isotopes of an element, taking into account the isotopic mass and the natural abundance of each isotope  Average Atomic Mass The average atomic mass of an element is what is displayed in the Periodic Table  Exercise on isotopes Consider a hypothetical element X. It has two isotopes, with the following characteristics: Isotope 1: Mass = 20.00 amu Natural Abundance = 40% Isotope 2: Mass = 22.00 amu What is the Average Atomic Mass of this element?  Exercise on isotopes Consider a hypothetical element X. It has two isotopes, with the following characteristics: Isotope 1: Mass = 20.00 amu Natural Abundance = 40% Isotope 2: Mass = 22.00 amu What is the Average Atomic Mass of this element? First, we know that the sum of the natural abundances must equal 100%. Therefore, the natural abundance of Isotope 2 must be 60% AAM = 20.00  0.40 + 22.00  0.60 = 21.20 amu Electronic structure of atoms As a first approximation, electrons can be thought of moving in orbits (shells) around the nucleus  Example – Fluorine atom 9 protons 10 neutrons 9 electrons 5-Minute Break  Electronic Structure and Quantum Mechanics The electronic structure of an atom is very important as it determines the chemical behaviour of the element Quantum mechanics is the branch of science that deals with matter on the sub-microscopic level It gives an accurate description of how atoms and molecules behave It can also be employed to give an accurate description of how the electrons in an atom behave  Quantum Mechanical Picture The electrons do occupy space around the nucleus, but this space does not resemble orbits like the Moon orbiting around the Earth We must apply the principles of Quantum Theory to determine the true picture. We consider the electrons acting also as waves We must solve the Schrodinger Equation or wave equation for electron motion Solution of this equation defines regions of space around the nucleus where it is likely to find electrons. These regions are known as orbitals. Orbitals have three characteristics: – (a) unique shape – (b) unique size – (c) unique 3D orientation in space.  Orbitals Solution of S.E. gives probabilities of finding the electrons in specified regions of space around the nucleus The mathematical solutions are called Orbitals Think of orbitals as specific electron cloud formations  Orbitals – Revision Slide WATCH: https://www.youtube.com/watch?v=qVtMKejyBiE Recall that nuclei are positively charged, and electrons are negatively charged. By Coulomb’s Law, opposite charges attract. The electron stays near the nucleus because of this attraction to the positive nucleus. Moving the electron further away from the nucleus takes energy, as the electron is attracted to the positive nucleus. This is the reason that as you move away from the central nucleus, it is less and less likely to find the electron there. Thus, the distribution of electron density thins out as we move further away from the nucleus. Describing Orbitals: Quantum Numbers Orbitals have specific shapes, sizes and three-dimensional orientations For each of these three attributes there is a specific quantum number A quantum number is just a number. It is a numerical index that provides a specific description for a given orbital  Shape of orbitals Orbital angular momentum (azimuthal) quantum number l  Size of orbitals Principal quantum number n  Size of orbitals p orbitals also grow in size as n increases 3D-Orientation of orbitals Magnetic quantum number, ml  Summary for Revision - 1 The first reference to the atomic structure of matter was in ancient Greece. This atomic theory was abandoned and revived in the late 19th century. Nowadays, we can actually ‘see’ atoms using the Scanning Tunneling Electron Microscope. The simplest model of an atom has a central nucleus and electrons moving in orbits around the nucleus. This model, however, is not correct. There are three types of subatomic particles making up atoms: protons (positive charge), electrons (negative charge), and neutrons (no charge). Most of the mass of the atom results from the protons and neutrons in the nucleus. The electron cloud model of the atom is the most realistic of the simple models. Atomic (or isotopic) symbols are useful in counting the number of subatomic particles. Charged atoms, called ions, have different numbers of protons and electrons. Cations are positively charged, and anions are negatively charged. Isotopes are atoms of the same element having different masses (different number of neutrons). Isotopes have a unique mass and a unique natural abundance. The average atomic mass is calculated by taking the weighted average of the masses of all the isotopes. A simplistic picture of the atom has a central nucleus and electrons moving around the nucleus in shells.  Summary for Revision - 2 Quantum mechanics is used to provide an accurate model for the atom. Quantum mechanics predicts that electrons exist around the nucleus in areas called orbitals. Quantum mechanics gives the probability of finding the electrons in the regions around the nucleus. Orbitals are described by a set of three quantum numbers. These describe the shape, the size and the orientation of the orbital. The Principal Quantum Number, n, gives the size of the orbital. The orbitals increases in size as n increases. The Azimuthal Quantum Number, l, gives the shape of the orbital. Different values of l correspond to different shapes. s-type orbitals are spherically shaped. p-type orbitals are shaped like dumbbells having two lobes, d-type orbitals generally have 4 lobes.