Chapter 1 Atomic Structure

Transcription

1 Chapter 1 Atomic Structure CHEM 511 Chapter 1 page 1 of 15 What is inorganic chemistry? The periodic table is made of elements, which are made of...? Define: atomic number (Z): Define: mass number (A): Why is nucleon number a better term for A? From general chemistry we recall that elements of a certain kind contain the same number of protons, but may or may not contain the same number of neutrons. Isobars: Isotones: How can you roughly tell, by looking at the periodic table, if an element has just one isotope? Is there a trend with these elements?

2 CHEM 511 Chapter 1 page 2 of 15 Out of 270 stable isotopes, the breakdown is: # of Isotopes A Z 161 even even 55 odd even 50 odd odd 4 even odd What do we notice about nuclear stability? From where did all of these atoms originate? Theory of the Big Bang. ~14 billion years ago the universe was ~10 9 Kelvin and concentrated in a very small space. It exploded and as the material moved outward, it cooled to form H atoms. In stars, H atoms combine to form atoms with higher atomic numbers...but there is a limit, especially based on the type of star that is present. Binding energy: the difference in energy between the nucleus and the same number of protons and neutrons as individual particles. Fusion/fission implications

3 CHEM 511 Chapter 1 page 3 of 15 EX. Look at the energy involved in the alpha decay of a 235 U nucleus. 235 U has a mass of amu 4 He has a mass of amu 231 Th has a mass of amu Besides the nucleus, what other atomic structures exist? How do we know? Johann Rydberg derived an important equation in the 1880 s:

4 CHEM 511 Chapter 1 page 4 of 15 The Niels Bohr contribution... The Schrödinger contribution... Wavefunction (Ψ, psi): a mathematical function describing properties (energy, momentum, spin, etc.) of an electron based on location Probability density (Ψ 2 ): a function describing the probable location of an electron Where are the electrons? In orbitals. To define an orbital, we need three quantum numbers:

5 CHEM 511 Chapter 1 page 5 of 15 To define an electron, we need an additional quantum number: The wavefunction (Ψ) gives rise to the probability density (Ψ 2 ) which then can be used to determine the radial distribution function (the total probability of finding the electron in a spherical shell around the nucleus). Images from your book are plotted relative to a0, the Bohr radius (52.9 pm for H) 4πr 2 R(r) 2 Radial distribution plot for 2p & 3p Probability plots for other orbitals 4πr 2 R(r) 2

6 Node prediction: There are two types of nodes: nodal planes (aka angular nodes) and radial nodes. CHEM 511 Chapter 1 page 6 of 15 EX. Determine the number of angular and radial nodes for each orbital. (a) 5d orbital (b) 8g orbital Angular variations of atomic orbitals The shapes of orbitals are called boundary surfaces: a region in space in which there is a high (~90%) chance of finding a specific electron You will be responsible for knowing the shapes of s, p, and d orbital (including the correct axes and the location of nodes!)

7 Many electron atoms In a hydrogen atom (1 electron), all orbitals of the same n have the same energy CHEM 511 Chapter 1 page 7 of 15 Introduction of just one additional electron changes this and "splits" the energy of the orbitals. Why? Effective nuclear charge (Zeff): That portion of the total nuclear charge that is experienced by a given electron. Slater s Rules for determining effective nuclear charge (empirically derived): Write the electron configuration in the following groupings: (1s), (2s, 2p), (3s, 3p), (3d), (4s, 4p), (4d), (4f), (5s, 5p), etc. Electrons higher than the electron of interest do not contribute to the shielding factor Electrons in s- and p-orbitals: o Each electron in (ns, np) contribute S = 0.35 o Each electron in the n-1 shell contribute S = 0.85 o Each electron in the n-2 or lower shells contribute S = 1.00 Electrons in d- or f-orbitals o Each of the other electrons in the other d- or f-orbitals contributes S = 0.35 o Each of the electrons in a lower group contributes S = 1.00 Why useful? It helps to determine electron configuration or how tightly bound an electron is to an atom!

8 CHEM 511 Chapter 1 page 8 of 15 EX. Determine the Zeff for the outermost electron in the following electron configurations: (a) 1s 2 2s 2 2p 6 (b) 1s 2 2s 2 2p 5 3s 1 Values of Zeff for representative elements given in Table 1.2, page 17 Brief review of electron configuration The ground state (lowest energy) electron configuration is determined by the Aufbau principle Filling orbitals is straight forward until the d-block elements

9 CHEM 511 Chapter 1 page 9 of 15 Determine the electron configuration of: B P Ti Cr Ni Cu Se Bi To create cations, remove electrons To create anions, add electrons Determine the electron configuration of: Na + P 3- Mn 4+ Xe - Fe 3+ Fe 2+

10 CHEM 511 Chapter 1 page 10 of 15 The Classification of the Elements The elements in the periodic table can be viewed in various ways (e.g., by properties), but also by location. The three main categories of elements according to the periodic table: main group elements transition metal elements rare earth elements aka inner transition elements Important families (i.e., groups or congeners) Note the row names of the rare earth elements.

11 CHEM 511 Chapter 1 page 11 of 15 Atomic properties (i.e., Periodic Trends) Atomic radius: a measurement of the unionized form of an atom. metallic radius: Usual method for metals is to measure the distance between nuclei in the solid and divide by 2 covalent radius: For nonmetals, measure the distance between nuclei of a binary molecule and divide by 2 ionic radius: a measurement of an ion's size (note atomic radius definition), usually derived from the distance between an oxygen nucleus and a metal ion (an approximation only!)

12 CHEM 511 Chapter 1 page 12 of 15 Trends within the periodic table? Note: transition metals in the 5th and 6th periods are nearly the same size!! The period 6 atoms have 32 MORE electrons than the period 5 atoms. How can they be the same size? Atomic Radii (pm) Period 5 Zr Nb Mo Tc Ru Rh Pd Ag Period 6 Hf Ta W Re Os Ir Pt Au What do we know about the relative size of cations to parent atoms? anions to parent atoms? Why?

13 CHEM 511 Chapter 1 page 13 of 15 Ionization energy (Ei, I, or ΔionH): Energy needed to remove an electron from an isolated atom in the gas phase. Technically there is a difference between ionization energy and ionization enthalpy (T = 0 K vs T = 298 K), but mathematically the difference is small enough to usually be ignored. What are the general periodic trends for Ei1? Down a group? Across a period? Electron Affinity (Eea) and electron-gain enthalpy (ΔegH): The energy gained or released when an electron is added to the valence shell of an isolated atom in the gas phase. Eea is the negative of ΔegH; Eea is assumed to be at 0 K, ΔegH is assumed to be at 298 K. Whether the electron stays on the atom is a function of the lowest unfilled (or partially filled) orbital. This is one of the so-called frontier orbitals. The FO include the highest filled orbital and the lowest unfilled (or half-filled) orbital.

14 CHEM 511 Chapter 1 page 14 of 15 Electronegativity Symbolized with the Greek letter chi, The most electronegative element? The least electronegative element? (most electropositive) Definition? Trends in the periodic table? Three scales used: Pauling ( P) Most commonly used Assigned a value of ~4 for F and scaled others ( A- B) = 0.102( = the difference in bond energy between A-B and the arithmetic mean of A-A and B-B bond energies Mulliken ( M) Based on ionization energy and electron affinity If an element has a high ionization energy AND a high electron affinity, the element will be electronegative. Allred-Rochow ( AR) Dependent on the Zeff and radius High Zeff and small size leads to high electronegativity Constants chosen to give values close to P

15 Polarizability:, the ability of an atom to have its electron cloud distorted. CHEM 511 Chapter 1 page 15 of 15 Related to the frontier orbitals: if FOs are close, then the atom is generally easily distorted. Fajan s Rules Small, highly charged cations have polarizing ability Large, highly charged anions are easily polarized Cations with non-noble gas electron configurations are easily polarized (especially important for d-block elements)