Download Bohr´s atomic model (1913)

st
1
unit of Chemistry
Atomic structure
Dalton´s atomic theory
(1803)
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Matter is made of indivisible particles called atoms
(we now know that atoms are not indivisible).
Elements consist of the same kind of atoms, atoms
that are exactly the same (we now know that there
are atoms of the same element that are different,
those are the isotopes).
Atoms of different elements are different (they have
different mass and properties).
Chemical compounds are made up of molecules, in
which atoms are combined according to a fix
whole-number ratio.
Chemical reaction
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It was soon discovered that atoms are
formed by smaller particles.
The first particle that was discovered was the
electron, a particle with a negative charge.
The atom was not indivisible, so new models
of the structure of an atom were needed.
Thomson´s atomic model (1904)
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Atoms can be considered as uniform positively charged
spheres in which electrons are embedded.
The amount of positive and negative charge is always the
same so that the atom is electrically balanced.
Rutherford´s experiment
(1911)
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It consisted on shooting alpha particles (which are positively
charged) at a thin gold foil.
Some of the particles went through the gold foil without
diverting, some of them were slightly diverted and some
were strongly diverted.
This experiment showed that the atom was not
homogeneous.
Rutherford´s experiment
Rutherford´s atomic model
Rutherford explained the results of his experiment proposing a
new atom model:
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Atoms are divided into a tiny core, called nucleus, and an
electron shell.
Between the nucleus and the electron shell there is empty
space. Most of the atom is empty space.
The nucleus of an atom is positively charged and contains
almost all the mass of the atom. We know now that it is
composed of protons and neutrons.
Electrons orbit around the nucleus in circular orbits. The
nucleus holds the electrons by means of an electric force
between positive and negative charges.
Rutherford´s atomic model
How the model of Rutherford explained
his experiment
Soon, new particles were
discovered: protons and neutrons
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Particles of the atom:
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Protons: they have a positive charge.
Neutrons: they don´t have charge. Their mass is the same
as protons.
Electrons: they have a negative charge. Their mass is
negligible compared with the mass of protons.
Atomic number, Z (número atómico)
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The number of protons in an atom is
called the atomic number.
Z = number of protons
All the atoms of the same element have
the same atomic number. Therefore, it is
the atomic number that defines an
element.
Mass number, A (número másico)
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It is the number of protons and neutrons in an
atom.
A = Z + number of neutrons
The mass number is not necessarily the same in all
the atoms of an element, because the number of
neutrons can change in an element.
Atoms with the same Z (therefore of the same
element) and with different A (therefore with
different number of neutrons) are called isotopes.
An example of isotopes: isotopes
of hydrogen
Periodic table
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Elements are ordered in the periodic table in increasing
atomic number.
Cations and anions
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The number of electrons and protons in a neutral atom is the
same.
An atom can lose electrons and become positively charged,
a cation: K → K+ + 1eAn atom can gain electrons and become negatively
charged, an anion: O + 2e- → O2-
Bohr´s atomic model (1913)
Electrons orbit around the nucleus (which, as we know now, is
formed by protons and neutrons) in different layers.
In each layer there is a maximum number of electrons: In the first
layer there are 2 electrons at most, 8 in the second layer, 18 in the
third layer... In the layer n there are 2n2 electrons at most.
The energy and the radius of each orbit is quantized, that means it
has an specific value (it can´t be any number).
Each layer has different energy. Inner layers have lower energy and
outer layers have higher energy.
An electron can absorb energy and go to a higher layer. Or it can go
to a lower layer and release energy.
Bohr´s atomic model (1913)
Bohr´s atomic model (1913)
Bohr´s atomic model (1913)
As atoms absorb or release energy in the form of light,
absorption and emission spectra of the elements are
explained.
The spectrum of each element is specific and unique for that
element.