CHEM 511 Chapter 1 page 1 of 14

Chapter 1

Atomic Structure

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?

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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 ~109 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

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EX. Look at the energy involved in the alpha decay of a 235U nucleus. 235U has a mass of 235.043915 amu 4He has a mass of 4.002603 amu 231Th has a mass of 231.036291 amu

Besides the nucleus, what other atomic structures exist?

How do we know?

Johann Rydberg derived an important equation in the 1880’s:

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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 (Ψ2): a function describing the probable location of an

electron

Where are the electrons? In orbitals. To define an orbital, we need three

quantum numbers:

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To define an electron, we need an additional quantum number:

The wavefunction (Ψ) gives rise to the probability density (Ψ2) 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)

Radial distribution plot for 2p & 3p Probability plots for other orbitals

4π

r2R

(r)2

4π

r2R

(r)2

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Node prediction:

There are two types of nodes: nodal planes (aka angular nodes) and radial nodes.

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!)

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Many electron atoms

In a hydrogen atom (1 electron), all orbitals of the same n have the same energy

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!

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EX. Looking at gallium, we see these Zeff values. How do we interpret these values?

1s = +30.65

2s,2p = +26.85

3s,3p = +19.75

3d = +9.85

4s,4p = +5.00

EX. Determine the Zeff for the outermost electron in the following electron configurations:

(a) 1s22s22p6 (b) 1s22s22p53s1

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

1.

2.

3.

Filling orbitals is straight forward until the d-block elements

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Be sure you can determine the electron configuration of elements and ions!

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)

1 2 15

16 17 18

11 13 14

Note the row names of the rare earth elements.

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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!)

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Trends within the periodic table? Here is Table 1.3, pg 23

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

160 147 140 135 134 134 137 144

Period 6 Hf Ta W Re Os Ir Pt Au

159 147 141 137 135 136 139 144

Does this type of phenomena occur elsewhere?

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What do we know about the relative size of cations to parent atoms? anions to parent atoms? Why?

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?

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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.

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.

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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

Polarizability: , the ability of an atom to have its electron cloud distorted.

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)