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Electron Configurations
The arrangement of electrons in an atom is known as the atom’s electron configuration
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The
lowest-energy arrangement of the electrons for each element is called the element’s ground-state electron configuration
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Then electrons are added to the orbitals one by one
according to three basic rules
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According to the Aufbau principle, an electron occupies the
lowest-energy orbital that can receive it
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The 2s orbital is the next highest
energy, then the 2p orbitals
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The second rule reflects the importance of the spin quantum
number
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The principal, angular momentum, and
magnetic quantum numbers specify the energy, shape, and orientation of an orbital
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According to the Pauli Exclusion Principle, an orbital can hold two
electrons of opposite spin states
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The direction of the arrow
indicates the electron’s spin state
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In this way, electron-electron repulsion Is minimized so that the
electron arrangements have ht lowest energy possible
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orbitals must have the same spin state
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IN a ground-state
hydrogen atom, the single electron is in the lowest energy orbital, the 1s orbital
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Orbital Notation
In orbital notation, an unoccupied orbital is represented by a line, ___, with the orbital’s name written underneath the line
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An orbital containing two electrons is represented as , showing
the electrons paired and with opposite spin states
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Electron-Configuration Notation
Electron-configuration notation eliminates the lines and arrows of orbital notation
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The hydrogen configuration is represented by 1s2
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Noble-Gas Notation
Neon is a member of the Group 18 elements
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To simplify sodium’s notation, the symbol for neon, enclosed in square brackets, is used to
represent the complete neon configuration: [Ne] 1s2 2s22p6
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Follow the diagonal arrows
from the top to get the order
in which atomic orbitals are
filled according to the Aufbau
principle

After the outer octet is filled in neon, the next electron enters the s sub level in the n=3 main energy level
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As in neon, the highest-occupied energy level of argon has
an octet of electrons
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A noble-gas
configuration refers to an outer main energy level occupied, in most cases, by eight electrons
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It starts with the 4s orbital, then to the 3d
orbitals, and finally to the 4p orbitals
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However,
they start at the 5s orbital, then to the 4d orbitals, and finally to the 5p orbitals
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The seventh period is incomplete and consists largely of synthetic elements

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