A-Level/ Advanced Higher Chemistry course notes. Concise notes covering atomic orbitals, electronic configurations, and the Periodic Table. Notes include a clear and easy to follow summary of the topic, highlighted key-points, and supporting examples where appropriate.
ATOMIC ORBITALS ELECTRONIC CONFIGURATIONS 31 THE PERIODIC TABLE
◦The discrete lines observed in the atomic emission spectra of elements with more than one
electron can be explained if electrons, like protons, display the properties of both particles and
waves.
◦Electrons within atoms are said to be quantised. This means they can only possess xed
amounts of energy known as quanta. As a result, electrons can be de ned in terms of quantum
numbers.
◦Electrons behave as standing (stationary) waves in an atom. These are waves that vibrate in
time but do not move in space. There are di erent sizes and shapes of standing wave possible
sound the nucleus, known as orbitals.
◦Any orbital, irrespective of size or shape, can hold a maximum of two electrons.
◦The di erent shapes of orbitals are identi ed as s, p, d and f.
All electrons in
an atom can be defined in terms of FOUR QUANTUM NUMBERS The fourquantum
numbers are the PRINCIPLE QUANTUM NUMBER n the ANGULAR MOMENTUM QUANTUMNUMBER L the
MAGNETIC QUANTUM NUMBER M and the SPIN QUANTUM NUMBER S
It is possible to describe any electron in an atom using four quantum numbers:
• The principle quantum number, n, is related to the size of the orbital.
• The angular momentum quantum number, l, is used to represent di erent shapes of orbital and can
have values from zero to n - 1 .
• The orientation of any non-spherical orbital is indicated by the magnetic quantum number, m, and
can have values between - 1 and + 1.
• The spin magnetic number, s, indicates the spin of an electron within an orbital and can have values
of - 1/2 or + 1/2.
The four Quantum Numbers can be used in a wave equation to predict the position and momentum of
the electron.
Atomic orbitals are regions of space where the probability of nding an electron is > 90% (notice not
100%)
Electrons are arranged in shells around the nucleus. The shell nearest to the nucleus is the rst shell
and any electron in the rst shell has the principle quantum number n = 1. Electrons in the second shell
have n = 2. The higher the value of n, then the further the electrons are from the nucleus.
If the emission spectra of elements other then hydrogen are studied using high resolution
spectroscopes, the single lines seen at low resolution often appear as two or three lines very close
together. For example, one yellow line is seen in the emission spectrum of sodium at low resolution.
However, at higher resolutions it can be seen that there are two lines very close together. This suggests
that shells are divided into sub shells.
, PRINCIPLE QUANTUM NUMBER N ANGULAR MOMENT I
Refers to the ENERGY
of the electron Givesindication of the SHAPE
of the orbital
shell atomic orbital
values 0 1 2 n 1 to zero
Give numbers n 1 2 3
Correspondsto subshell withinthe energy
when n 1 electrons are most likely level given letters s p d and f
to be found close to the nucleus
All s orbitals are SPHERICAL
in the side
f IIIa also
c s all p orbitals are DUMBBELLSHAPED
Magnetic Moment m Spin Quantum Number Ms
the ORIENTATION and Describes the SPIN of the electrons
Iffy
can be t or t
L to I
Theneedfor this quantum number arises
from the Pauli Exclusion Principle
An atomic orbital is the volume in space where the probability of nding an electron is more than
90%.
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