Although the electron density distribution is different in the 2s and 2p orbitals, an electron has the same energy when it is in the 2s orbital as when it is in the 2p orbital for the H atom. For many-electron atoms, the situation is different. The energy of the orbitals increases as follows:

Although the electron density distribution is different in the 2s and 2p orbitals, an electron has the same energy when it is in the 2s orbital as when it is in the 2p orbital for the H atom. For many-electron atoms, the situation is different. The energy of the orbitals increases as follows: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d

Energy level order 5s 5p 5d 5f 5g 4s 4p 4d 4f 3s 3p 3d 2s 2p 1s Energy

Energy level order 5s 5p 5d 5f 5g 4s 4p 4d 4f 3s 3p 3d 2s 2p 1s Energy

The difference in the energy levels arises because in a many-electron atom, the electrostatic interaction between an electron and the nucleus is influenced by the presence of other electrons.

The difference in the energy levels arises because in a many-electron atom, the electrostatic interaction between an electron and the nucleus is influenced by the presence of other electrons. This is particularly the case for electrons that spend most of their time farther away from the nucleus.

Electronic Configurations

Electronic Configurations The four quantum numbers can be employed to label an electron in any orbital in any atom.

Electronic Configurations The four quantum numbers can be employed to label an electron in any orbital in any atom. For example, the four quantum numbers for a 2s orbital are:

Electronic Configurations The four quantum numbers can be employed to label an electron in any orbital in any atom. For example, the four quantum numbers for a 2s orbital are: n = 2, l = 0, ml = 0, ms = ½

Electronic Configurations The four quantum numbers can be employed to label an electron in any orbital in any atom. For example, the four quantum numbers for a 2s orbital are: n = 2, l = 0, ml = 0, ms = ½ or n = 2, l = 0, ml = 0, ms = -½

Electronic Configurations The four quantum numbers can be employed to label an electron in any orbital in any atom. For example, the four quantum numbers for a 2s orbital are: n = 2, l = 0, ml = 0, ms = ½ or n = 2, l = 0, ml = 0, ms = -½ A shorthand notation is frequently employed: (n, l, ml, ms)

For the 2s example, the quantum numbers are either

For the 2s example, the quantum numbers are either Note that the value of ms has no effect on the energy, size, or shape of an orbital.

Electronic Configuration: The distribution among the atomic orbitals of all the electrons in an atom.

Electronic Configuration: The distribution among the atomic orbitals of all the electrons in an atom. The following ten examples apply to the ground states of the atoms.

1. Hydrogen: The electron must be in the lowest energy level, which is described by a 1s orbital.

1. Hydrogen: The electron must be in the lowest energy level, which is described by a 1s orbital. The electronic configuration of the H atom is: 1s1

1. Hydrogen: The electron must be in the lowest energy level, which is described by a 1s orbital. The electronic configuration of the H atom is: 1s1 value of principal quantum number n

1. Hydrogen: The electron must be in the lowest energy level, which is described by a 1s orbital. The electronic configuration of the H atom is: 1s1 value of principal quantum number n value of angular momentum quantum number l

1. Hydrogen: The electron must be in the lowest energy level, which is described by a 1s orbital. The electronic configuration of the H atom is: 1s1 value of principal quantum number n value of angular momentum quantum number l denotes the number of electrons in the orbital (when the number is a 1, it is sometimes omitted – being understood to be present)

Alternatively, the electronic configuration can be represented by the following orbital diagram:

2. Helium: To assign the electrons to orbitals for any multi-electron atom requires the use of the Pauli Exclusion Principle.

2. Helium: To assign the electrons to orbitals for any multi-electron atom requires the use of the Pauli Exclusion Principle. Pauli Exclusion Principle: No two electrons in the same atom can have the same four quantum numbers.

2. Helium: To assign the electrons to orbitals for any multi-electron atom requires the use of the Pauli Exclusion Principle. Pauli Exclusion Principle: No two electrons in the same atom can have the same four quantum numbers. The Pauli Exclusion Principle limits the number of electrons that can be placed in each orbital.

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½)

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½)

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½) 2p 6 electrons

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½) 2p 6 electrons (2, 1, 1, ½) (2, 1, 1, -½)

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½) 2p 6 electrons (2, 1, 1, ½) (2, 1, 1, -½) (2, 1, 0, ½) (2, 1, 0, -½)

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½) 2p 6 electrons (2, 1, 1, ½) (2, 1, 1, -½) (2, 1, 0, ½) (2, 1, 0, -½) (2, 1, -1, ½) (2, 1, -1, -½)

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½) 2p 6 electrons (2, 1, 1, ½) (2, 1, 1, -½) (2, 1, 0, ½) (2, 1, 0, -½) (2, 1, -1, ½) (2, 1, -1, -½) 3d 10 electrons

1s only 2 electrons (1, 0, 0, ½) or (1, 0, 0, -½) 2p 6 electrons (2, 1, 1, ½) (2, 1, 1, -½) (2, 1, 0, ½) (2, 1, 0, -½) (2, 1, -1, ½) (2, 1, -1, -½) 3d 10 electrons 4f 14 electrons

For the He atom, the electronic configuration is 1s2 The orbital diagram is: He 1s

For the He atom, the electronic configuration is 1s2 The orbital diagram is: He 1s The two electrons occupying the same orbital are said to be “paired”.

For the He atom, the electronic configuration is 1s2 The orbital diagram is: He 1s The two electrons occupying the same orbital are said to be “paired”. One value of ms corresponds to and the other value of ms corresponds to .

Note that both the diagrams would be ruled out by the Pauli Exclusion Principle (both electrons in each box have the same values of ms as well as the same values of n, l, and ml).

3. Lithium Since only two electrons can be placed in the 1s orbital, the third electron must enter the 2s orbital.

Energy level order 5s 5p 5d 5f 5g 4s 4p 4d 4f 3s 3p 3d 2s 2p 1s Energy

3. Li Electronic configuration is 1s22s1 The orbital diagram is: Li 1s 2s

3. Li Electronic configuration is 1s22s1 The orbital diagram is: Li 1s 2s The lithium atom has one unpaired electron. A substance which contains unpaired electrons is paramagnetic.

3. Li Electronic configuration is 1s22s1 The orbital diagram is: Li 1s 2s The lithium atom has one unpaired electron. A substance which contains unpaired electrons is paramagnetic. Paramagnetic: The weak attraction to a magnet of a substance containing unpaired electrons.

Comparison of the energies of 1s22s1 versus 1s22p1

Comparison of the energies of 1s22s1 versus 1s22p1 Which is lower in energy, and can we rationalize why this is so, for a multi-electron atom?

Comparison of the energies of 1s22s1 versus 1s22p1 Which is lower in energy, and can we rationalize why this is so, for a multi-electron atom? The 1s orbital is filled and electrons in this orbital lie close to the nucleus.

Comparison of the energies of 1s22s1 versus 1s22p1 Which is lower in energy, and can we rationalize why this is so, for a multi-electron atom? The 1s orbital is filled and electrons in this orbital lie close to the nucleus. Both the 2s and 2p orbitals are “larger”, and an electron in either of these two orbitals will lie on average, further away from the nucleus than an electron in the 1s orbital.

An electron in either the 2s or 2p orbital is said to be shielded from the nucleus by the 1s electrons.

An electron in either the 2s or 2p orbital is said to be shielded from the nucleus by the 1s electrons. That is, an electron in a 2s or 2p orbital in the lithium atom would not see a nuclear charge of +3.

An electron in either the 2s or 2p orbital is said to be shielded from the nucleus by the 1s electrons. That is, an electron in a 2s or 2p orbital in the lithium atom would not see a nuclear charge of +3. The shielding reduces the electrostatic attraction between the protons in the nucleus and the electron in the 2s or 2p orbitals.

From experiment it can be shown that the electron density for the 2s electron near the nucleus is greater than for an electron in a 2p orbital.

From experiment it can be shown that the electron density for the 2s electron near the nucleus is greater than for an electron in a 2p orbital. For this reason, the 2s orbital is said to be more penetrating than the 2p orbital.

From experiment it can be shown that the electron density for the 2s electron near the nucleus is greater than for an electron in a 2p orbital. For this reason, the 2s orbital is said to be more penetrating than the 2p orbital. For the same principal quantum number n, the penetrating power decreases in the order s > p > d > f ….

From experiment it can be shown that the electron density for the 2s electron near the nucleus is greater than for an electron in a 2p orbital. For this reason, the 2s orbital is said to be more penetrating than the 2p orbital. For the same principal quantum number n, the penetrating power decreases in the order s > p > d > f …. Since the stability of an electron in an orbital is determined by how strongly it is attracted by the nucleus, it follows that an electron in a 2s orbital will be lower in energy than an electron in a 2p orbital.

4. Beryllium For the Be atom, the electronic configuration is 1s2 2s2 The orbital diagram is: Be 1s 2s

4. Boron For the B atom, the electronic configuration is 1s2 2s22p1

4. Boron For the B atom, the electronic configuration is 1s2 2s22p1 The 2p orbital must be used since no more than 4 electrons can be placed in the 1s and 2s orbitals.

4. Boron For the B atom, the electronic configuration is 1s2 2s22p1 The 2p orbital must be used since no more than 4 electrons can be placed in the 1s and 2s orbitals. The orbital diagram is: B 1s 2s 2p

Note that the unpaired electron for the B atom may be in the 2px, 2py, or 2pz orbital. This follows from the fact that the three 2p orbitals are equivalent in energy.

6. Carbon The first 4 electrons will be 1s22s2 which leaves 2 electrons to be placed in the available 2p orbitals.