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Topic 2: Atomic Structure
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Key facts
A compound is a substance made by chemically combining two or more elements.
It has different properties from its constituent elements.
The mass number (A) is the number of protons plus the number of neutrons in an
atom. As it gives the total number of nucleons in the nucleus, it is sometimes called
the nucleon number.
Isotopes are atoms of the same element with different mass numbers.
When an atom loses electrons, a positive ion is formed and when it gains electrons,
a negative ion is formed. Positive ions are called cations and negative ions are
called anions.
The relative atomic mass of an element (Ar) is the average mass of an atom of the
element, taking into account all its isotopes and their relative abundance,
compared to one atom of carbon-12.
The distance between two successive crests (or troughs) is called the wavelength
(𝜆). The frequency (𝑣) of the wave is the number of, waves which pass a point in
one second. The wavelength and frequency are related by the equation 𝑐 = 𝑣𝜆
where c is the speed of light.
A continuous spectrum shows an unbroken sequence of frequencies, such as the
spectrum of visible light.
A line emission spectrum has only certain frequencies of light as it is produced by
excited atoms and ions as they fall back to a lower energy level.
A line absorption spectrum is a continuous spectrum except for certain colors which
are absorbed as the atoms are excited to higher energy levels.
The energy of a photon of electromagnetic radiation is directly proportional to its
frequency and inversely proportional to its wavelength. It can be calculated from
the Planck equation (𝐸 = ℎ𝑣), which is given in section 1 of the IB data booklet.
The first ionization energy of an element is the minimum energy needed to remove
one mole of electrons from one mole of gaseous atoms in their ground state.
The electrons in the Bohr model occupy orbits, which are circular paths. An orbital,
which is a wave description of the electron, shows the volume of space in which the
electron is likely to be found.
The Pauli Exclusion Principle states that no more than two electrons can occupy any
one orbital, and if two electrons are in the same orbital they must spin in opposite
directions.
Hund’s rule: If more than one orbital in a sub-level is available, electrons occupy
different orbitals with parallel spins.
When the transition metal atoms form ions they lose electrons from the 4s sublevel before the 3d sub-level.
The ns and np sub-levels are filled for elements in Period n. However the (n-1)d sublevel is filled for elements in Period n.