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Complete AS + AL AQA Chemistry Notes with Examples
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AQA A-Level Chemistry alevelchemistry.co.uk3.1.1 Atomic structure
SPECIFICATION
‒Structure of an atom
‒Mass number and atomic number
‒Isotopes and calculating Relative Atomic Mass
‒Time of flight (TOF) mass spectrometer
‒Applications of mass spectroscopy
‒Electronic configuration
‒Quantum sub-shells
‒Atomic orbitals
‒Filling of orbitals
‒Electron configurations and periodic table
‒Electron configurations of ions
‒Electronic configurations of period 2 and 3 elements
‒Electronic configuration of d-block elements
‒Periodic trends in ionisation energy
‒Predicting electronic structure using successive ionisation energy
Source: AQA spec
,AQA A-Level Chemistry 3.1.1 Atomic structure
Scientists have discovered 118 elements till now. Each element has different properties.
In this article, the structure of an atom and its mass are discussed. Properties of isotopes
are also discussed.
This article also introduces the concepts about how electrons are arranged in an atom.
The atomic orbitals and their shapes are discussed. The number of atomic orbitals a
principal quantum shell carries is also explained in brief. The order in which electrons are
filled in each atomic orbital is an important concept that links with arrangement of
elements in periodic table.
A. Structure of an atom
An atom consists of a nucleus and electrons orbiting around the nucleus. Structure of an
atom is illustrated in figure 1. A nucleus consists of protons and neutrons. Protons are
particles with positive electrical charge and neutrons carry a neutral electrical charge.
Both protons and neutrons have the same mass. An electron carries a negative electrical
charge and almost has no mass.
Figure 1: Structure of an atom
Relative
Particle Location Relative mass
charge
Proton Nucleus 1 +1
Neutron Nucleus 1 0
Electron Shells around 1ൗ -1
1840
the nucleus
,AQA A-Level Chemistry 3.1.1 Atomic structure
B. Mass number and atomic number
In a neutral atom, the number of protons and the number of electrons are always equal
because their charge is equal in size but opposite in nature. Each element is represented
in the form of MPX, where X is the symbol of the element, M is the mass number or
nucleon number, and P is the proton or atomic number. Mass number is the total number
of protons and neutrons in the atom. Atomic number is the number of protons in the atom.
Mass number M
Atomic number P
X
M=n+p
Mass Number of
Atomic Number of
Element Symbol number neutrons
number (p) protons (p)
(p+n) (n)
23
Sodium 11𝑁𝑎 23 11 11 12
27
Aluminium 13𝐴𝑙 27 13 13 14
40
Calcium 20𝐶𝑎 40 20 20 20
C. Isotopes and calculating Relative Atomic Mass
Isotopes are different forms of the same element having different masses. Isotopes have
the same number of protons but different number of neutrons. As the number of protons
is same as the number of electrons in any atom, all isotopes of an element have same
electronic configuration. Therefore, all isotopes of an element react chemically in same
manner. Because, isotopes of an element have different number of neutrons, their mass
numbers varies. Example:
Isotope Isotopes of boron
10 11
Symbol 5𝐵 5𝐵
Atomic number (p) 5 5
Mass number (n+p) 10 11
Number of neutrons (n) 5 6
, AQA A-Level Chemistry 3.1.1 Atomic structure
Relative atomic mass is the average mass of one atom of an element compared to one-
twelfth of the mass of one carbon-12 atom. Relative isotopic mass is the mass of one
isotope compared to one-twelfth of the mass of one carbon-12 atom. Relative masses
are ratios of two masses and hence do not have any units.
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑎𝑡𝑜𝑚 𝑜𝑓 𝑎𝑛 𝑒𝑙𝑒𝑚𝑒𝑛𝑡
𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝐴𝑟 =
𝑜𝑛𝑒 − 𝑡𝑤𝑒𝑙𝑓𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑐𝑎𝑟𝑏𝑜𝑛 − 12 𝑎𝑡𝑜𝑚
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑎𝑡𝑜𝑚 𝑜𝑓 𝑎𝑛 𝑖𝑠𝑜𝑡𝑜𝑝𝑒
𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑖𝑠𝑜𝑡𝑜𝑝𝑖𝑐 𝑚𝑎𝑠𝑠 =
𝑜𝑛𝑒 − 𝑡𝑤𝑒𝑙𝑓𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑐𝑎𝑟𝑏𝑜𝑛 − 12 𝑎𝑡𝑜𝑚
The relative composition of isotopes of Boron found mass by spectroscopy and is given
in the table below:
Boron-10 Boron-11
Relative
10 11
isotopic mass
Relative
23 100
abundance
Based on this data, the relative atomic mass of Boron is calculated:
𝑠𝑢𝑚 𝑖𝑠𝑜𝑡𝑜𝑝𝑖𝑐 𝑚𝑎𝑠𝑠 × 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 =
𝑡𝑜𝑡𝑎𝑙 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒
10 × 23 + 11 × 100
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 =
123
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑏𝑜𝑟𝑜𝑛 = 10.81
D. Time of flight (TOF) mass spectrometer
A mass spectrometer separates the atoms of a particular sample according to their
masses. When a sample is placed in the mass spectrometer, the sample is converted to
positively charged ions, highly accelerated and reach the detector. When a sample of an
element with isotopes is placed, a mass spectrum with peaks showing their relative
abundances is obtained.
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