Summary Edexcel A-Level Chemistry: Topic 1 - Atomic Structure and the Periodic Table (Notes and Exam-style questions)

Atomic structure and the Periodic Table
Definitions
The atomic number is the number of protons in an atom.
The mass number is the total number of protons and neutrons in an atom.
The relative molecular mass is the mass of a molecule relative to the mass of a
carbon-12 atom.
The relative formula mass is the mass of an ionic compound relative to the mass of a
carbon-12 atom.
The relative ionic mass is the mass of an ion relative to the mass of a carbon-12
atom.
Mass spectrometry
Mass spectrometry provides a method of measuring the mass of atoms and
molecules. It is done using a mass spectrometer and occur in four stages:
 Ionisation – Atom or molecule is ionised by kicking at least 1 electron (most of
the time just 1 electron) off by firing electrons at it.
 Acceleration – The ions are accelerated so they all have the same kinetic
energy.
 Deflection – The ions are then deflected by a magnetic field according to their
masses. The lighter they are, the more they get deflected. The more the ion is
charged, the more it gets deflected.
 Detection – The beam of ions passing through the machine is detected
electrically.
The mass spectra of chlorine (Cl2) has three peaks around an m/z of 70: 70 for a
molecule made up of two Cl-35 atoms (this molecule occurs 9/16 times on average),
72 for a molecule made up of one Cl-35 atom and one Cl-37 atom (this molecule
occurs 6/16 times on average), and 74 for a molecule made up of two Cl-37 atoms
(this molecule occurs 1/16 times on average). Peaks occur at lower m/z ratios (35
and 37) because of 35Cl+ and 37Cl+. The graph below shows the mass spectrum of
chlorine.

, The mass spectra of bromine (Br2) has three peaks around an m/z of 160: 158 for a
molecule made up of two Br-79 atoms (this molecule occurs 1/4 times on average),
160 for a molecule made up of one Br-79 atom and one Br-81 atom (this molecule
occurs 2/4 times on average), and 162 for a molecule made up of two Br-81 atoms
(this molecule occurs 1/4 times on average). Peaks occur at lower m/z ratios (79 and
81) because of 79Br+ and 81Br+. The graph below shows the mass spectrum of
bromine.




Ionisation energies
First ionisation energy is the energy required to remove one mole of electrons from a
gaseous atom, to make it into an ion with a 1+ charge.
Successive ionisation energies are the energy required to remove one mole of
electrons from a gaseous ion of a various charge.
Ionization energies are affected by the number of protons (which affects the nuclear
charge), electron shielding, and the electron sub-shell from which the electron is
removed.
Going across a period, the number of protons increases, and so the nuclear charge
increases, increasing the attraction of the outer shell electrons to the nucleus. The
atomic radius also decreases because of this, causing the outer shell electrons to be
more strongly attracted to the nucleus, whilst the electron shielding remains constant
causing the first ionisation energies increase. The second ionisation energy is higher
than the first ionisation energy because there are less electrons left on the shell,
making the attraction of the outer shell electrons to the nucleus higher. When the
electron is being removed from a full outer shell of electrons, the ionisation energy is
very high and much higher compared to the previous ionisation energy because
there is one less shell and so less electron shielding, and the electrons are being
removed from a full outer shell of electrons, which is very electronically stable. These
trends in first ionisation energy provides evidence for the existence of sub-shells,