Atoms and the Periodic table

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Learning Check

State the number of valence electrons for each.

A. 2, 8, 5

B. 2, 8, 8, 2

C. 2, 7

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Solution

State the number of valence electrons for each.

A. 2, 8, 5 5

B. 2, 8, 8, 2 2

C. 2, 7 7

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Energy levels are spaced differently, like ladder rungs

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Atomic energy levels are like floors of a house

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State transitions for hydrogen

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Figure 3.8: Atomic orbitals.

Boundary surface diagrams for electron densities of 1s, 2s, 2p, 3s, 3p, and 3d orbitals. For the p orbitals, the subscript letter on the orbital notation (x, y, z) indicates the cartesian axis along which the orbital lies.

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Figure 3.9: Subshell filling order.

Subshells in atoms are filled in order of increasing energy, as this diagram shows. The order of filling is 1s → 2s → 2p → 3s → 3p → 4s → 3d and so on.

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Atomic Subshell Energies

Run the following web animations/movies.

3.3:

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The Orbital Model: Electronic Configurations

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Sample energy level diagram

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Figure 3.10: In this “building-up” version of the periodic table, the lightest elements are at the bottom. Electrons fill subshells from bottom to top in order of energy as the atomic number of the atom increases. The numbers across the top give the number of electrons in each subshell. The ground-state electron configurations of most elements are apparent from their positions in the table. Those that are known to differ from expectation are indicated explicitly.

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Figure 3.10: In this “building-up” version of the periodic table, the lightest elements are at the bottom. Electrons fill subshells from bottom to top in order of energy as the atomic number of the atom increases. The numbers across the top give the number of electrons in each subshell. The ground-state electron configurations of most elements are apparent from their positions in the table. Those that are known to differ from expectation are indicated explicitly.