Chemistry Unit 1.1 - Atomic Structure

Notes on Atomic Structure

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Chemistry Unit 1.1: Atomic Structure
Definitions
Relative atomic mass - the mass of an atom relative to 1/12th the mass of a carbon-12 atom.
Relative isotopic mass - the mass of an isotope of an element relative to 1/12th the mass of a
carbon-12 atom.
Relative molecular mass - the mass of a molecule relative to 1/12th the mass of a carbon-12 atom.
Molar mass - the mass of 1 mole of a substance.
First ionisation energy - the energy required to remove 1 mole of electrons from 1 mole of gaseous
atoms to from 1 mole of 1+ ions:
Second (and higher) ionisation energy - the energy required to remove 1 mole of electrons from 1
mole of gaseous 1+ ions to form 1 mole of 2+ ions:
First electron affinity - the energy change when 1 mole of electrons is added to 1 mole of gaseous
atoms to form 1 mole of 1- ions:
Second (and higher) electron affinity - the energy change when 1 mole of electrons is added to 1 mole
of gaseous 1- ions to form 1 mole of 2- ions:
Mass spectrometry
The mass spectrometer is a machine which compares relative atomic weights by measuring the mass to
charge ratio. This mass spectrometer works in 5 phases:
1. Vaporisation ­ The sample is vaporised so that individual atoms/molecules separate.
2. Ionisation ­ The gaseous sample is bombarded with electrons. This generally causes an electron to
be knocked off each atom, to form gaseous ions with a 1+ charge; however, occasionally two or
more electrons may be knocked off the atom. Sometimes fragmentation occurs, i.e. a molecule
breaks down into different parts.
3. Acceleration ­ An electric field is applied and causes the ions to speed up.
4. Deflection ­ A magnetic field deflects the ions. Heavier ions and ones with less charge are deflected
less; hence the result measures the mass:charge ratio.
5. Detection ­ A detector is used to register how many ions are being deflected through a specific
angle, i.e. how many ions have a given mass:charge ratio.
Some uses of the mass spectrometer include radioactive dating, space research, steroid detection, and in
the pharmaceutical industry.
Electronic configuration
Electrons are arranged in different energy levels (shells) within the atom, and the ionisation energy is
determined by which shell it is in, whether there are other electrons nearby, and the charge of the nucleus.
The first and successive ionisation energies provide evidence for the existence of shells and orbitals within
atoms.
Each shell is split into subshells, referred to as s, p, d and f subshells. The first (innermost) shell has only an s
subshell; the second has an s subshell and a p subshell; the third has s, p and d subshells; thereafter, all shells
have s, p, d and f subshells. Subshells are further split up into orbitals; each orbital can hold up to two
electrons. An s subshell has one orbital, a p subshell has three orbitals, a d subshell has five orbitals, and an f
subshell has seven orbitals. The electrons in an orbital must have an opposing spin; an orbital is sometimes
diagrammatically represented by a box and electrons by half-arrows, one pointing upwards and one pointing
downwards to represent the different spins.

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When electron subshells are "filling up" (i.e. electrons are being added to them), each orbital in that subshell
must have one electron before any orbital can take a second. Electrons fill shells in the following order:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p...
This can be easily remembered using the following table, which should be read down the diagonals, as
indicated by the arrows.…read more

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