atomic properties and the periodic table

Question 1

What is electron penetration?

Answer 1

Penetration refers to the relative electron density of an electron to the nucleus of an atom. proximity of an electron to the nucleus. Electrons in different orbitals have different wavefunctions (y) and consequently different RDFs.
A 2s electron has more electron density near the nucleus than a 2p electron, therefore it is said to be more penetrating.

Question 2

Why is the 2s orbital filled before the 2p orbital?

Answer 2

This is due to penetration. A part of the 2s orbital lies closer to the nucleus (closer than the 1s orbital). The electrons in the 2s orbital therefore penetrate the core 1s electrons. Because the 2s orbital has a minor maximum close to the nucleus, the orbital energy for the 2s is lower than that for the 2p and therefore the 2s orbitals are filled before the 2p orbitals.

Can be seen from the RDF graph for the 2s and 2p orbitals

Question 3

Why is the 4s orbital filled before the 3d?

Answer 3

Although the 3d orbital has its maximum closer to the nucleus than the 4s, the inner lobes of the 4s orbital penetrate the core electrons. With the 3s and 3p orbitals filled, addition of an electron into the 4s (over the 3d) orbital results in less shielding and as a consequence the electron experiences a higher effective nuclear charge and is therefore lower in energy.

Question 4

Shieldling constant guidlines

Answer 4

• Electrons with higher values on n contribute zero to the shielding constant.
• Electrons with the same principal quantum number contribute 0.35 to the shielding constant.
• Electrons with a principal quantum number one below the electron be observed contribute 0.85.
• Electrons with lower n values contribute 1 (they act as a perfect shield).
• If the shell considered is an nd or nf shell then all electrons in the group lower in energy shield by an amount of 1. (d and f orbitals are completely shielded by the lower shells)

Question 5

IE, Zeff, electronegativity, radii

What are the trends in the periodic table?

Answer 5

• The effective nuclear charge, Zeff, increases down a group and across a row.
• Ionisation energy increases across a period (due to increasing Zeff), and decreases down a group (due to increasing value of n which is not fully compensated for by the increase in Zeff)
• Electronegativity increases going across a period and decreases going down a group.
• Covalent radii decreases going across a period (due the increasing Zeff meaning that the electrons are held more tightly, resulting in a contraction of the orbital size and therefore the covalent radius decreases) and increases going down a group ( due to electrons being placed in orbitals with higher principal quantum numbers (n) which lie further from the nucleus).

Question 6

Electrons which have greater penetration experience…

Answer 6

…stronger attraction to the nucleus, less shielding, and therefore experience a larger Effective Nuclear Charge.

Question 7

What is ionisation energy?

Answer 7

The minimum amount of energy required to remove one electron from a gaseous atom, leaving both the electron and the resulting ion without any kinetic energy.

Question 8

What is electron affinity?

Answer 8

The electron affinity (sometimes known as the electron attachment energy) is the energy change on adding an electron to a gaseous atom, ion or molecule. (positive or negative)

Question 9

How can electronegativities be used to explain the electron distribution in a bond?

polarisability

Answer 9

If A and B have the same electronegativity it follows that A and B attract the electrons in the bond equally, and the A–B bond is not polarised.
If B is more electronegative than A, electrons are attracted more to B, giving a polarised bond; the degree of polarisation is proportional to the difference in electronegativity.

Question 10

What is a covalent radii?

Answer 10

Covalent radius = 1/2 internucleus distance in a homonuclear molecule.

Question 11

What is a metallic radius?

Answer 11

The metallic radius is one half of the internuclear distance in a metal.

Question 12

What is a Van der Waals radius?

Answer 12

A Van der Waals radius is one half of the internuclear distance between two non-bonded atoms in neighbouring molecules in the solid or liquid state.

Question 13

What is an ionic radius?

Answer 13

The same as a covalent radius but for ionic compounds.