Chapter 2 - Wits Structural Chemistry

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Transcript Chapter 2 - Wits Structural Chemistry

Atoms, Molecules and Ions
Chapter 2
The Modern View of Atomic Structure
Properties of the Electron, Proton & Neutron
amu = atomic mass units
1 amu = 1.66x10-24 g
Mass of carbon-12 = 12 amu
The nucleus of an atom has a structure
– It is composed of protons and neutrons
The proton
 a nuclear particle
 positive charge equal to that of the
 mass more that 1800 times that of the
The neutron
 a nuclear particle
 having a mass almost identical to that
of the proton
 but no electric charge.
Isotopes, Atomic Numbers
and Mass Numbers
Atomic number (Z) = number of protons in the
nucleus of an atom.
An element is a substance whose atoms all
have the same atomic number.
Mass number (A) = the total number of protons
and neutrons in a nucleus.
Isotopes have
the same
atomic number
(p+), but
different mass
Nuclide symbols
A nuclide is an atom characterised by a
definite atomic and mass number.
You write the nuclide symbol for the naturally
occurring sodium nuclide as follows:
Mass number  23
Atomic number  11
Alternate Example
Write the nuclide symbol for the nucleus that
has 19 protons and 20 neutrons.
Periodic table of the elements
A tabular arrangement of
elements in rows and columns,
highlighting the regular
repetition of properties of the
Molecules and Molecular Compounds
A molecule is a definite group of atoms that
are chemically bonded together - that is
tightly connected by attractive forces.
Molecular compounds  compounds that
consist of molecules
The chemical formula of a substance is a
notation that uses atomic symbols with
numeric subscripts to convey the relative
proportions of atoms of the different elements
in the substance.
eg. Alumina = Al2O3
Salt = NaCl
Ethanol = C2H6O
A molecular formula gives the exact number
of different atoms of an element in a molecule
Ethanol = C2H6O
Some molecules
Elements that can exist as molecules
H2, O2, N2, F2, Cl2, Br2, I2,
Polymers are very large molecules that are
made up of a number of smaller molecules
repeatedly linked together.
Monomers are the small molecules that are
linked together to form the polymer
e.g. ethylene to polyethylene
... ...
H3C (CH2 CH2)n CH3
Ions and Ionic Compounds
Not all substances are molecular, some are made
up of ions.
An ion is an electrically charged particle
obtained from an atom or chemically bonded
group of atoms by adding or removing electrons.
Anion - a negatively charged ion
Cation - a positively charged ion
An ionic compound is a compound composed of
cations and anions, eg NaCl
NaCl - Lattice
The empirical formula of a substance is the
relative number of atoms or ions symbolized
in the formula.
 Use the smallest set of integers to express
the correct ratios of atoms present.
Eg. Consider hydrogen peroxide:
Molecular formula = H2O2
Empirical formula =
Writing an ionic formula
What is the formula of magnesium nitride?