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Atoms and
theories of matter
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What are atoms?
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The atomic theory
The atom is the smallest particle of
an element that preserves its
chemical properties.
Molecules are made up of combined
atoms.
Ions are atoms or molecules that
have a positive or negative electrical
charge.
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Dalton’s law of multiple proportions
The law of multiple proportions states that when two
elements combine to give more than one compound, the
masses of one element that combine with the fixed mass of
the other are in a ratio of small whole numbers.
Compound
Oxygen mass
Carbon mass
Ratio
Carbon monoxide
4g
3g
1.33 : 1
Carbon dioxide
8g
3g
2.66 : 1
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The atomic theory /1
Dalton developed the first atomic theory in order to explain
the law of conservation of mass, the law of definite and
constant proportions and the law of multiple proportions.
According to the first atomic theory, matter is made up of
small atoms that cannot be created, divided or destroyed.
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The atomic theory /2
All atoms of an element are identical and have the same mass.
Atoms of different elements combine in simple number to form
compounds.
Carbon monoxide
1 g carbon +
1.33 g oxygen
Carbon dioxide
1 g carbon +
2.66 g oxygen
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Electrical properties of matter
+
+
+
-
Bodies can have a positive (+) or negative (-) charge or
they can be neutral, which means they have the same
number of positive and negative charges.
Objects with opposite charges attract each other; objects
with the same charge repel each other.
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Subatomic particles
Atoms are made up of subatomic particles:
• protons (p), with charge +1 and mass equal to 1.673  10-27
kg;
• electrons (e–), with charge -1 and negligible mass;
• neutrons (n), with neutral charge and mass equal to
1.675  10-27 kg.
Particle
Mass
Charge
Electron
9.109  10-31
– 1.602  10-19
Proton
1.673  10-27
1.602  10-19
Neutron
1.675  10-27
0
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Atomic number and mass number
Each atom is identified by an atomic number (Z) and a mass
number (A).
Mass number = number of p + n
Atomic number = number of p
A
Z
E
Z is the number of protons and it also indicates the number
of electrons in a neutral atom. A is the sum of protons and
neutrons.
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The structure
of the atom
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The atomic nucleus
Atoms have a tiny, dense
nucleus with a positive charge.
The nucleus is made up of
protons and neutrons and is
surrounded by an empty space
in which electrons move.
Nuclear force keeps protons and neutrons together inside
the nucleus, overcoming the mutual repulsion caused by
the positive charges.
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Isotopes
Isotopes are atoms
that have the same
atomic number but
different atomic mass.
They have the same nuclear charge and chemical properties,
but they differ in their mass and in some physical properties.
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From atoms to ions
E
n+
E
n-
Number of positive charges =
number of lost electrons
Number of negative charges =
number of acquired electrons
Na – e–
Na+
Cl + e–
Cl–
Atoms can lose or acquire electrons, becoming positive ions
(or cations, when they lose one or more electrons) or
negative ions (or anions, when they acquire one or more
electrons).
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Electronic structure
The number and disposition of an atom’s
electrons influence its tendency to join other
atoms.
+
In an atom, only certain energy levels are
allowed. The first level can have two
electrons, the second and third levels can
have eight electrons each.
An atom is stable when its outermost energy level is complete.
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Chemical bonds
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Chemical bonds
With a chemical bond, atoms try to reach the most stable
electronic configuration; by sharing, losing or gaining one or
more electrons they can complete an energy level.
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Covalent bond
When two atoms share a couple of electrons, they form a
covalent bond. Each atom provides one electron in the
couple.
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Ionic bond
–
+
Na
+
Cl
Na
+
Cl
When two atoms have very different properties, with one atom
attracting electrons more strongly than the other, the former
gains one or more electrons, while the latter loses them.
The atoms become ions, connected by electrostatic attraction,
called ionic bond.
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Chemical formulae,
equations
and reactions
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Chemical formulae
CO2
It indicates the
element carbon
It indicates the number of
atoms of oxygen
It indicates the
element oxygen
Elements and compounds are represented by chemical
formulae: the formula indicates the type and number of
atoms or ions in the smallest particle that has all the chemical
properties of the substance.
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Formulae of the elements
S
S
S
S
Al
Chemical formula: Al
Cl
Cl
Chemical formula: Cl2
S
S
S
S
Chemical formula: S8
In nature, seven elements are made of diatomic molecules,
four elements contain more than two atoms, while the others
are formed by single atoms.
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Formulae of compounds
H
H
N
H
Chemical formula
of ammonia : NH3
H
H
H
C
H
Chemical formula
of methane: CH4
The formula of a molecular compound describes the
composition of a single molecule, while the formula of an ionic
compound indicates the numerical ratio between ions, without
showing their charges.
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Atomic mass units
The relative mass of atoms and molecules is expressed in
atomic mass units (u). It is a relative mass:
1u = 1/12 of the mass of a 12C.
The atomic mass unit, also called dalton, is very small:
1 u = 1.6605  10-24 g
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Atomic mass and molecular mass
The atomic mass of an element is the average mass of the
atoms present in a natural sample. It is expressed in u.
The molecular mass is the relative mass of a molecule and
it is calculated by summing up the mass in u of the atoms of
the molecule’s formula.
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The mole
The mole (mol) is the quantity of substance that contains
6.02  1023 particles.
The molar mass is the mass of a mole in grams; numerically
it is equal to the mass of one particle of the substance in u.
Mass (g)
Quantity of
substance (mol)
m
__
n=
M
Molar mass (g/mol)
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Chemical equations
Chemical equations describe reactions, indicating changes
in composition and the ratio between particles of the reagents
and products.
2H2O
The coefficient 2
indicates 2
molecules of water.
O2 + 2H2
The coefficient 2
indicates 2
molecules of
hydrogen.
There is no coefficient before
the oxygen: this means that
there is only one molecule.
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Balancing chemical equations
A chemical equation is balanced if the total number of atoms
of each element in the reagents and in the products is equal.
2H2O
O2 + 2H2
Reagents:
4 atoms of H, 2 atoms of O
Products:
4 atoms of H, 2 atoms of O
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Balancing chemical equations:
three practical rules
In order to balance a chemical equation it is a good practice
to follow these procedures:
1.balance the atoms of metals and non metals first;
2.even when they are present in compounds, oxygen and
hydrogen should be balanced after the others, because they
often appear in many formulae;
3.balance water and pure elements last.
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