Download Grade 9 Chemistry I. Dalton`s Atomic Theory II. Atomicity III

Grade 9 Chemistry
I. Dalton’s Atomic Theory
John Dalton presented the world with his atomic theory which states that all matter is made of
atoms. The basic proposals of his atomic theory were:
1.
The simplest substances found in nature are called elements.
Each element consists of identical tiny particles called atoms.
Examples of elements are gold, oxygen, diamond (carbon), iron, copper, etc.
2.
Atoms of one element are all identical, and they are different in properties and mass from
the atoms of other elements.
3.
Elements combine to form compounds.
Atom combine to form molecules, the smaller particle of a compound is a molecule.
4.
Atoms cannot be created or destroyed, so in any reaction atoms are always “conserved”:
the number you start with is the number you end up with.
II. Atomicity
Atomicity is a term that refers to the number of atoms in a molecule.
Thus the atomicity of He is 1, that of hydrogen gas, H2, is 2, and that of ammonia, NH3, is 4.
Oxygen exists in nature as diatomic molecules, water as triatomic molecules and helium as
monatomic.
Give applications.
a) Give the atomicity of each of the following particles:
Ne(neon), NO, HCl, NH3, CH4, CO2
b) Which of the above particles is monatomic? Diatomic? Tri-atomic? Tetratomic?
III. Chemical Representations
More than 100 elements were discovered during the last two hundred years. The periodic
table, and the table of atomic masses (sometimes incorrectly called atomic weights), shows most
of these elements. Some of these elements exist naturally and some are man made. Some are
stable and others “live” for a very short time before they decompose to other elements and
particles. In time we will know more about many of them. All that is important at the moment is
to know that everything on this earth is made of these 100 or so elements.
1
Each element is given a name and a symbol. Some of the symbols are single letters, like S for
sulfur, and some are two letters, like Si for silicon. Some symbols come from the English origin,
like C for carbon, some from Latin, like Na for sodium (natrium in Latin), some are named after
scientists (Einsteinium) and some after countries or places e.g. Yttrium, Americium, named after
a town, Ytterby, in Sweden and America, respectively.
Give applications.
1. From the periodic table, find what element each of the following symbols represents. H, He,
O, N, C, Ca, Co, Cl, Cs, Cu, Pb, Fe, K, Hg, S, Si
2. Write the symbols for the following elements: gold, silver, titanium, chromium, lithium and
uranium.
IV. Chemical formula
Nitrogen monoxide is made of nitrogen and oxygen in the ratio of one atom of nitrogen to one
atom of oxygen. We write its chemical formula as NO.
The chemical formula (of a molecular compound) gives the number and the kind of atoms of
each element in a molecule of that compound.
The compound nitrogen dioxide is made of NO2 molecules, each consisting of one atom of
nitrogen and two atoms of oxygen. Although NO and NO2 are made of the same two elements,
the ratios in which the atoms of the elements are combined are different; and so are the
properties of these two compounds, both physical and chemical.
When more than one atom of an element is found in a molecule of a compound, the number of
atoms is written as a subscript. Thus NO2 tells us that each molecule of nitrogen dioxide is made
of one atom of nitrogen and two atoms of oxygen, “bonded” together in such a way that they
always move together as one particle, in the same way three table tennis balls glued or “bonded”
together can move only as one particle.
The symbol we use to represent two molecules of nitrogen dioxide, in the “same place” is 2NO2.
The 2 written before the symbol NO2 refers to the number of molecules. These molecules are not
tied together, but are free to move as two separate molecules. If however the two molecules of
NO2 were tied together to become a single molecule, its symbol becomes N2O4. N2O4 molecules
actually exist.
Give applications.
a) Explain the meaning of “2” in N2O4.
b) What does 3 in 3 N2O4 mean?
2
V.Molecular formula
The molecular formula is the chemical formula in molecular compounds, i.e.,compounds where
the smallest particle is a molecule. As examples, the molecular formulae for nitrogen dioxide and
a compound called ethylene are the same as the chemical formulae:NO2 and C2H4.
In gases, like chlorine or ammonia, Cl2 is diatomic (made of diatomic molecules) and NH3 is
tetratomic (made of tetratomic molecules). In this case it makes sense to talk about the molecular
formula.
Some solids, like sugar, have molecules, so they have molecular formulae. Other solids are not
molecular; they have no molecules, so we can only talk about their chemical formula.
For example, in ionic compounds, like NaCl and all salts, there are no molecules, just ions, so it
is meaningless to talk of the molecular formula. For ordinary salt, NaCl is the chemical formula,
not the molecular formula.
In metals, like Na, Fe (iron), Ag (silver) etc, again there are no molecules. There are atoms that
are all bonded together. In certain solids like pure carbon in diamond or pure silica, SiO2, the
main constituent of sand, atoms are all networked together, so there are no molecules.
In short, the term molecular formula applies only to compounds where the smallest particle is a
molecule, and we should remember that there are certain compounds that do not have molecules,
like NaCl and SiO2.
Give applications.
Give the meaning of each of these symbols in terms of atoms and molecules:
C, H2, O3 (Ozone), NH3 (ammonia), Cl2, 2 Cl2, 5 NH3, 6 CO2 (carbon dioxide)
VI. Simplest formula
The simplest formula gives the simplest ratio in which the atoms (or ions) are found together in
the compound.
Similarly,CaCl2 is the chemical formula or simplest formula for the ionic solid calcium chloride.
The simplest formula is also called the empirical formula.
Even molecular compounds have simplest formulae. For example, the molecular compound
N2O4 has a simplest formula of NO2. N2O4 and NO2 have the same simplest formulae but
different molecular formulae.
Give applications.
Give the simplest formula of each of the following: H2O2, C12H22O11, C8H18, CaCO3
3
VII. Avogadro’s Number and the Mole Concept
32.0g of oxygen gas, 17.0g of ammonia gas and 2.02g of hydrogen gas all contain the same
number of molecules, x, because every quantity occupies 22.4 dm3 at the same temperature (0ºC)
and pressure (1 atmosphere pressure). This number x was determined experimentally (the details
of the experiment are complicated and will not be studied in this course), and found to be about
6.02 x 1023.
This number is called Avogadro’s number, N, in honor of Amadeo Avogadro.
Often we give names to numbers. The number “12” is called a “dozen”, and the number 1.0 x
106, is called a million. Both atoms and molecules are so small that it is more convenient to
define a new unit called a mole, which is Avogadro’s number 6.02 x 1023
VIII. The Atomic Mass Unit
Scientists have ways (beyond the scope of this book) to find the relative masses of atoms.
Masses of atoms are compared to the mass of a particular variety of carbon atoms called carbon12 (the technical name is isotope of carbon, a term which we will study later on). Since the gram
is too large a unit in which to give masses of atoms, a new unit of mass, called the atomic mass
unit, symbol amu, is defined such that 1 amu is exactly 1/12 of the mass of a carbon-12 atom, or
such that 1 atom of cabon-12 has a mass of 12 amu:
Mass of 1 atom of carbon-12 = 12 amu
To establish the relation between the gram and the amu,we know that 12 g of C-12 have 6.023 ×
1023 atoms. These also have a mass of 6.023 × 1023 × 12 amu.
Equate the two:
6.023 × 1023 × 12 amu = 12 g
We get:
6.023 × 1023 amu = 1.0000 g
From this relation, we can write two “conversion factors” between the gram and the amu.
The first is:
And the second is:
Give applications.
4
1. What is a mole?
2. Given that one atom of C-12 has a mass of 12 amu, find its mass in grams
IX. Meaning of Atomic Mass
Examine the table of atomic masses. In this table, the elements are listed alphabetically. You can
see their symbols, a quantity called the atomic number, and the atomic mass. The atomic mass
is given as a unit less number because it gives the relative mass of an atom of that element
relative to the isotope of carbon C-12, taken as 12 amu.
Naturally occurring oxygen exists as diatomic molecules, O2. Since the atomic mass of oxygen is
16, the mass of one atom of oxygen is 16 amu, the mass of one molecule of oxygen is 16 + 16 =
32 amu, and the mass of 1 mole of oxygen molecules is 32 grams.
We must learn the following terms:
a)
Molecular mass refers to the sum of the atomic masses of the atoms in the molecule
b)
The mass of one mole refers to the mass - in grams - of one mole particles of the
substance in the form in which it is found naturally
c)
The molar mass is the mass (in grams) per mole of the substance, so it has the same
numerical value as the mass of one mole, except that the units of molar mass are g/mol
(grams per mole).
5
The table illustrates the above:
Give applications.
Complete the following table
X.Number of moles =
Give applications.
1. How many moles are present in 3.0 g of Carbon?
2. How many moles are contained in 30 g of pure H4CO2?
6
7