P2
Atoms, Molecules and Ions
This unit attempts to explain the macroscopic observations made in the first chapter in
terms of the microscopic structure of the substances, that is, the behaviour of their
particles and the bonds that exist between them. Symbols and electron-dot structures
are used to represent atoms and ions.
Particles in solids
Very orderly arrangement of closely packed particles. The
particles vibrate about fixed positions
Particles in liquids
Less orderly arrangement and the particles are not as closely
packed. The particles have both translational and vibrational
motion
Particles in gases
Particles are widely separated and have no orderly
arrangement because of rapid translational motion.
Atom
The smallest particle of an element that is still recognisable
as that element
Molecule
Smallest particle of a substance that is capable of separate
existence.
Symbols of elements
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First letter of the name of the element (C for carbon)
First (upper case) and second (lower case) letters of the
name of the element (Ba for barium)
First (upper case) and another (lower case) letters of the
name of the element when the first two letters are the
same (Cl for chlorine and Cr for chromium)
Derived from the Latin name of the element (Na for
sodium (natrium))
Monatomic molecules
Elements that exist as single atoms (helium)
Diatomic molecules
Elements or compounds that have molecules consisting of
two atoms (hydrogen (H2) or carbon monoxide (CO))
Electron
Negatively charged particle with a mass of 1/2000 of the
mass of a hydrogen atom
Proton
Positively charged particle with a mass almost equal to that
of a hydrogen atom and a charge equal in magnitude but
opposite in sign to that of an electron
Neutron
Neutral particle with mass equal to that of a proton
Atomic number (Z)
Number of protons in the nucleus of every atom of an
element
Mass number (A)
Number of protons and neutrons in the nucleus of an atom
of an element
Electron configuration
How the electrons are arranged in energy levels around the
nucleus of an atom
Energy levels
Electrons exist in energy levels (1st, 2nd, 3rd, etc). The
electrons in an energy level have the same energy.
Stable electron
configuration
All elements, except the Noble gases, tend to undergo
chemical reactions to obtain the same electron configuration
as the nearby Noble gases.
Valence electrons
Electrons in the highest energy level.
Valence shell
The outermost energy level
Periodic Table
Table in which the elements are listed in order of increasing
atomic number with elements having similar electron
configurations (chemical properties) in the same vertical
columns.
Groups
Vertical columns of elements with similar electron
configuration and chemical properties
Periods
Horizontal rows of elements in which electrons are being
added to the same energy level.
Ionic bonding
Transfer of electrons between atoms
Covalent bonding
Sharing of electrons between pairs of atoms.
Ions
Positively (cations) or negatively (anions) charged particles
Ionic compounds
Exist as infinite arrays of positive and negative ions not
discrete molecules
Covalent compounds
May exist as discrete molecules or covalent lattices
Electron-dot structures
The symbol or formulae of elements or compounds that
show the valence electrons as dots
Ionic substances
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High melting and boiling point solids
Hard and brittle
Do not conduct electricity as solid
Conduct electricity in solution or as the melt
Covalent molecule
substances
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May be gases, liquids or low melting point solids at
room temperature
When solid they are soft
Do not conduct electricity as solid or melt
Only conduct electricity in solution if they ionise
Intermolecular forces
Forces between molecules (generally weak). The stronger
the intermolecular forces the high the melting and boiling
points
Covalent network
(lattice) solids
Solids in which the covalent bonding extends indefinitely
throughout the whole crystal
Metallic bonding
Force of attraction between the positively charged ions in
the orderly three-dimensional array and the mobile,
delocalised, valence electrons.
Properties of metals
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Usually solids at room temperature but have a wide
range of melting and boiling points
Conduct electricity as the solid and the melt
Malleable and ductile with variable hardness