Download The Aufbau principle determines an atom`s electron

The Aufbau principle determines an atom's electron configuration by
adding electrons to atomic orbitals following a defined set of rules.
LEARNING OBJECTIVES [ edit ]
Write electron configurations for elements in standard notation.
Memorize the rules for filling atomic orbitals in an Aufbau diagram.
KEY POINTS [ edit ]
The Madelung rule defines the order in which atomic orbitalsare filled with electrons. Electrons
fill orbitals starting at the lowest available energy state before filling higher states.
Aufbau procedure: Determine number of electrons for the atomof interest. Fill available orbitals
starting with the lowest-energy levels first and avoid pairing electrons in a single orbital until it is
necessary.
Electron configuration notation describes the energy levels, orbitals, and the number of electron.
The number and letter describe the energy level and orbital respectively, and the superscript
number shows how many electrons are in that orbital.
The Aufbau principle works well for the first 18 elements but then becomes less useful.
TERM [ edit ]
Pauli Exclusion Principle
The quantum mechanical principle that no two identical fermions (particles with half-integer
spin) may occupy the samequantum state simultaneously.
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Atoms Are Built Up by Adding Electrons
Although the nucleus of an atom is very dense, the electrons around it can take on a variety of
positions which can be summarized as an electron configuration. An element's electron
configuration can be represented using energy level diagrams, or Aufbau diagrams. The
Aufbau principle (from the GermanAufbau meaning "building up, construction") describes a
model-building method in which an atom is "built up" by progressively adding electrons. As
electrons are added, they assume the most stable shells with respect to the nucleus and the
electrons already present.
Filling in an Aufbau Diagram
The order in which orbitals are filled is given by the Madelung rule. The rule is based on the
total number of nodes in theatomic orbital, n + ℓ, which is related to the energy. In this
context, n represents the principal quantum number and ℓ represents the azimuthal quantum
number. The values ℓ = 0, 1, 2, 3 correspond to the s, p, d, and f labels, respectively. According
to the principle, electrons fill orbitals starting at the lowest available energy states before
filling higher states (e.g., 1s before 2s).
The Madelung energy ordering rule
Order in which orbitals are arranged by increasing energy according to the Madelung Rule. Each
diagonal read arrow corresponds to a different value of n + l.
An Aufbau diagram uses arrows to represent electrons. When there are two electrons in an
orbital, the electrons are called an electron pair. Electron pairs are shown with arrows
pointing in opposite directions. According to the Pauli Exclusion Principle, two electrons in
an orbital will not spin the same way. That is, an Aufbau diagram uses arrows pointing in
opposite directions. An arrow pointing up denotes an electron spinning one way and an
arrow pointing downwards denotes an electron spinning the other way. If the orbital only has
one electron, this electron is called an unpaired electron.
Aufbau diagram for lithium
The electron configuration of lithium, shown on an Aufbau diagram
The following steps detail how to draw an Aufbau diagram:
1. Determine the number of electrons that the atom has.
2. Fill the s orbital in the first energy level (the 1s orbital) with the first two electrons.
3. Fill the s orbital in the second energy level (the 2s orbital) with the second two electrons.
4. Put one electron in each of the three p orbitals in the second energy level (the 2p
orbitals) and then if there are still electrons remaining, go back and place a second
electron in each of the 2p orbitals to complete the electron pairs.
5. Continue in this way through each of the successive energy levels until all the electrons
have been drawn.
Aufbau diagram for fluorine
An Aufbau diagram showing the electron configuration of fluorine.
Electron Configuration Standard Notation
A special type of notation is used to write an atom's electron configuration. The notation
describes the energy levels, orbitals, and the number of electrons in each. For example, the
electron configuration of lithium is 1s22s1. The number and letter describe the energy level
and orbital, and the number above the orbital shows how many electrons are in that orbital.
Using standard notation, the electron configuration of fluorine is 1s22s22p5.
Limitations to Aufbau
The Aufbau principle is based on the idea that the order of orbital energies is fixed—both for
a given element and between different elements. This assumption is approximately true—
enough for the principle to be useful—but not physically reasonable. It models atomic orbitals
as "boxes" of fixed energy into which at most two electrons can be placed. However, the
energy of an electron in an atomic orbital depends on the energies of all the other electrons of
the atom.
In a hydrogen-like atom, which only has one electron, the s-orbital and the p-orbitals of the
same shell in the Aufbau diagram have exactly the same energy. However, in a real hydrogen
atom, the energy levels are slightly split by the magnetic field of the nucleus. Because each
atom has a different number of protons in its nucleus, the magnetic field differs, which alters
the pull on each electron. In general, the Aufbau principle works very well for the ground
states of the atoms for the first 18 elements, then decreasingly well for the following 100
elements.
Interactive: Energy Levels of a Hydrogen Atom
The likely location of an electron around the nucleus of an atom is called an orbital. The shape of an
orbital depends on the energy state of the electron. A neutral hydrogen atom has one electron. Click in
the boxes to set the energy of that electron and see the orbital shape describing where you are likely to
find that electron around the nucleus.