Chapter 2 Atomic Structure By Mounia Elamrani Blei / Odian’s General, Organic, and Biochemistry

Objectives Percent composition Dalton’s atomic Theory Constant composition of matter and conservation of mass Atomic mass Atomic structure Periodic table and properties of elements Electron configuration and the valence shells Octet rule Mounia Elamrani Chapter 2 – The Atomic Structure

2.1 The early atomic theory The concept of the atom had limited scientific usefulness until the discovery of two important laws in 18th and 19th centuries: The Law of conservation of mass: No detectable gain or loss of mass occurs in chemical reactions. Mass is conserved. The Law of constant composition: In a given chemical compound, the elements are always combined in the same proportions by mass. Mounia Elamrani Chapter 2 – The Atomic Structure

2.1 Percent composition The usual form for describing the relative masses of the elements in a compound is a list of percentages by mass, called the percent composition It is the number of grams of the element in 100 g of the compound and can be calculated using: Mounia Elamrani Chapter 2 – The Atomic Structure

Example Analysis of 4.800g of niacin, one of the B-complex vitamins, yields 2.810g of C, 0.1954g of H, 0.5462g of N, and 1.249g of O. Calculate the mass percent of each element in the compound. Mounia Elamrani Chapter 2 – The Atomic Structure

2.2 Dalton’s Atomic Theory Matter consists of tiny particles called atoms. In chemical reactions, the atoms rearrange but they do not themselves break apart. In any sample of a pure element, all the atoms are identical in mass and other properties. The atoms of different elements differ in mass and other properties. In a given compound the constituent atoms are always present in the same fixed numerical ratio. Mounia Elamrani Chapter 2 – The Atomic Structure

2.3 Atomic Mass It follows from Dalton’s Atomic Theory that atoms of an element have a constant, characteristic atomic mass or atomic weight For example, for any sample of hydrogen fluoride: F-to-H atom ratio: 1 to 1 F-to-H mass ratio: 19.0 to 1.00 This is only possible if each fluorine atom is 19.0 times heavier than each hydrogen atom Mounia Elamrani Chapter 2 – The Atomic Structure

2.3 Atomic Mass Units (amu, u) Atoms have extremely small masses. The mass of the heaviest known atom is about 4 x 10-22 g. We define the atomic mass unit, amu, using Carbon-12 as the standard: 1 atom of carbon-12 = 12 u (exactly) 1 u = 1/12 of mass of 1 atom of carbon-12 (exactly) This definition results in the assignment of approximately 1 u for the mass of hydrogen (the lightest atom) Mounia Elamrani Chapter 2 – The Atomic Structure

2.4 The structure of atoms Experiments have been performed that show atoms are comprised of subatomic particles There are three principal kinds of subatomic particles: Proton (p) – carries a positive charge, found in the nucleus, mass≃1u Electron (e) – carries a negative charge, found outside the nucleus, about 1/1800 the mass of a proton, mass≃0u Neutron (n) – carries no charge, found in the nucleus, a bit heavier than a proton, mass≃1u Mounia Elamrani Chapter 2 – The Atomic Structure

2.4 Atomic number and mass number The number of protons in the nucleus is called the atomic number (Z), and is unique for each element. Z = number of protons The mass number (A) of an atom is the total number of particles in the nucleus: A = (number of protons) + (number of neutrons) The charge is given by: Charge = number of proton - number of electrons Mounia Elamrani Chapter 2 – The Atomic Structure

Exercises What is the charge of an atom containing 9p, 10n, and 9e? What is the charge of an ion containing 12p, 13n, and 14e? Which element has 22p and 26n, and what is its approximate mass? Mounia Elamrani Chapter 2 – The Atomic Structure

2.5 Isotopes It turns out that most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses Atoms of the same element with different masses are called isotopes Isotopes have the same atomic number (Z) but different mass (A) Mounia Elamrani Chapter 2 – The Atomic Structure

2.5 Counting Particles in Isotopes Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 #P _______ _______ _______ #N _______ _______ _______ #E _______ _______ _______ Mounia Elamrani Chapter 2 – The Atomic Structure

2.5 Calculating an element’s average atomic mass We average the masses of isotopes using their masses and relative abundances to give the average atomic mass of an element: Naturally occurring chlorine is a mixture of two isotopes: Contribution Mass (u) Abundance (%) Isotope 0.7577 * 34.9689 = 26.50 u 34.9689 75.77 Cl-35 0.2423 * 36.9659 = 8.957 u 36.9659 24.23 Cl-37 (Rounded) Total = 35.46 u Mounia Elamrani Chapter 2 – The Atomic Structure

Exercises What is the symbolic notation for the isotopes of oxygen that contain 7 and 8n, respectively? Calculate the number of protons and neutrons in the two isotopes of nitrogen Magnesium consists of three isotopes of masses 24.0amu, 25.0amu, and 26.0amu with abundances of 78.70%, 10.13%, and 11.17% respectively. Calculate the average atomic mass of Mg. Mounia Elamrani Chapter 2 – The Atomic Structure

2.6 The Periodic Table It Arranges elements by increasing atomic number Elements are arranged in numbered rows called periods The vertical columns are called groups or families (group labels vary) Elements with similar physical and chemical properties, are placed in vertical columns, or groups. Li, Na, K: are soft, very reactive metals. He, Ne, Ar: are non reactive gases. Mounia Elamrani Chapter 2 – The Atomic Structure

2.6 Some important classifications A groups = representative elements or main group elements I A= alkali metals II A = alkaline earth metals VI A = chalcogens VII A = halogens VIII = noble gases B groups = transition elements Inner transition elements = elements 58-71 and 90-103 58 – 71 = lanthanide elements 90 – 103 = actinide elements Mounia Elamrani Chapter 2 – The Atomic Structure

2.6 Modern Periodic Table Note: Placement of elements 58 – 71 and 90 – 103 saves space Mounia Elamrani Chapter 2 – The Atomic Structure

2.6 Metals, Nonmetals, and Metalloids… Tend to shine (have metallic luster) Can be hammered or rolled into thin sheets (malleable) and can be drawn into wire (ductile) Are solids at room temperature (except Hg) and conduct electricity Mounia Elamrani Chapter 2 – The Atomic Structure

2.6 Metals, Nonmetals, and Metalloids… Lack the properties of metals At room T can be solid, liquid, or gas React with metals to form (ionic) compounds Metalloids Have properties between metals and nonmetals (semiconductors) Mounia Elamrani Chapter 2 – The Atomic Structure

2.6 Metals, Nonmetals, and Metalloids… Mounia Elamrani Chapter 2 – The Atomic Structure

2.7 Electron Organization Within The Atom Some early scientists thought that electrons were orbiting around the nucleus Later scientist discovered, through the emission of light by the elements, that electrons have energy states It all started with atomic emission spectra... Mounia Elamrani Chapter 2 – The Atomic Structure

Continuous Spectrum vs. Line Spectrum Visible Light Spectrum Rb Na K Hydrogen Line Spectrum Mounia Elamrani Chapter 2 – The Atomic Structure

2.7 Electromagnetic Radiation and Energy Radiation carries energy through space Light is a form of electromagnetic radiation Electromagnetic radiation is characterized by its wave nature The frequency , ν (in Hz or s-1), of a radiation is the number of “waves”, or complete vibrations, in one second Each color of the rainbow has a specific frequency Visible light range: 8x104 to 4x104 Hz The higher the energy of the radiation, the higher its frequency, ν Mounia Elamrani Chapter 2 – The Atomic Structure

2.7 Electromagnetic Radiation and Energy Mounia Elamrani Chapter 2 – The Atomic Structure

2.7 Atomic Energy States Excited State Emission Absorption Planck said that energy can only be absorbed or released from atoms in fixed amounts called quanta Atoms absorb energy first, then they release it in the form of a radiation (or color if in the visible range) When energy is absorbed, the e- “jumps” to the excited state When energy is released, or emitted, the e- “drops” to the ground state Excited State Absorption Emission Ground State Mounia Elamrani Chapter 2 – The Atomic Structure

2.8 The Quantum Mechanical Atom Complex mathematical theory that was able to predict a variety of atomic properties The detailed structure of the atom can explain the periodicity of the chemical properties of elements Elements can be built by adding electrons and protons to the nuclei There are rules guiding the way the electrons are organized Mounia Elamrani Chapter 2 – The Atomic Structure

2.8 Bohr Model First model of the electron structure Gives levels where an electron is most likely to be found Incorrect today, but a key in understanding the atom Mounia Elamrani Chapter 2 – The Atomic Structure

Shells, Subshells, and Orbitals Shells Contain electrons that are similar in energy and distance from nucleus: Low energy electrons are closest to the nucleus Identified by a whole number (1, 2, 3…) called the principal quantum number (n)….. The first shell (n=1) is lowest in energy, the 2nd level is next in energy (n=2) and so on: 1<2<3<4 Subshells are locations within a shell: s<p<d<f Atomic Orbitals define the region in space where electrons are most likely to be. Mounia Elamrani Chapter 2 – The Atomic Structure

Number of Electrons Maximum number of electrons in any level = 2n2 Mounia Elamrani Chapter 2 – The Atomic Structure

Order of Electron Filling All electrons in the same energy level have similar energy. Shell 1 2 electrons Shell 2 8 electrons Shell 3 18 electrons (8 first, later 10) Order of filling for the first 20 electrons Shell 1 2 3 4 2e 8e 8e 2e Mounia Elamrani Chapter 2 – The Atomic Structure

Electron Configuration Lists the shells containing electrons Written in order of increasing energy Element \ Shell 1 2 3 He 2 C 2 4 F 2 7 Ne 2 8 Al 2 8 3 Cl 2 8 7 Mounia Elamrani Chapter 2 – The Atomic Structure

Exercises A. The electron configuration for sulfur 1) 2,6 2) 8,2,6 3) 2, 8, 6 B. The element in period 3 with two electrons in the outermost energy level 1) Mg 2) Ca 3) Na Mounia Elamrani Chapter 2 – The Atomic Structure

Valence Electrons Valence or outer shell, or outermost electrons, are the electrons filling the last level of energy in a given atom Indicate the number of valance electrons: A. O 1) 4 2) 6 3) 8 B. Al 1) 13 2) 3 3) 1 C. Cl 1) 2 2) 5 3) 7 Mounia Elamrani Chapter 2 – The Atomic Structure

Periodic Law All the elements in a group have the same electron configuration in their outermost shells Example: Group 2A Be 2, 2 Mg 2, 8, 2 Ca 2, 8, 8, 2 Specify if each pair has chemical properties that are similar (1) or different (2): A. Cl and Br B. 2 - 5 and 2 - 8 - 7 C. 2 - 4 and 2 - 8 – 4 D. P and S E. O and S Mounia Elamrani Chapter 2 – The Atomic Structure

9. Electron Configurations and the Periodic Table - Note that the 3d orbital fills after the 4s orbital. - Similarly, the 4f orbital fills after the 5d orbital. Si V Mounia Elamrani Chapter 2 – The Atomic Structure

Electron Configurations and the Periodic Table There is a shorthand way of writing electron configurations Write the core electrons corresponding to the filled Noble gas in square brackets. Write the valence electrons explicitly. Example, P: 1s22s22p63s23p3 but Ne is 1s22s22p6 Therefore, P: [Ne]3s23p3. Mounia Elamrani Chapter 2 – The Atomic Structure

Electron Configurations There are a couple exceptions due to more stable electron configuration: Cr group prefers to have a half filled nd orbital with 5 electrons Cr: [Ar] 4s2 3d4 becomes Cr: [Ar] 4s1 3d5 Cu group prefers to have a completely filled nd orbital with 10 electrons Cu: [Ar] 4s2 3d9 becomes Cu: [Ar] 4s1 3d10 The electron configurations (using noble gas abbreviations) are given for the Group 2A and Group 3A elements. The relationship between the group number and the number of valence shell electrons is illustrated. Mounia Elamrani Chapter 2 – The Atomic Structure