Transcript Daltons` Models of Atoms

Daltons’ Models of Atoms
Carbon dioxide, CO2
John Dalton proposed
• all elements were composed of atoms
that were characteristic of that element
• Chemical reactions involved the
rearrangement of these atoms
Water, H2O
Thompson’s Model of an atom
Thompson proposed the plum pudding model
Which describes the atom as consisting of these
negatively charged particles embedded
randomly in positive matter
Rutherford’s Model of an atom
Rutherford model proposed that:
• Most of the mass of an atom and all of
the positive charge, must be located
a tiny central region called the nucleus
• Most of the volume of an atom is empty
space
• The electrons move in circular orbits
around the nucleus
• The force of attraction between the
positive protons and negative electrons
is electrostatic
Chadwick’s Model of an atom
James Chadwick discovered the
uncharged particle we know as the
neutron.
He reasoned the nucleus of atoms must
contain neutrons as well as protons
Characteristics of Atoms
• All atoms are neutral
• Same number of positive and negatively
charged particles
• Therefore same number of protons and
electrons
Characteristics of Atoms
• The number of protons in the nucleus of
an atom is the atomic number
• The mass number of an atom is the total
number of protons and neutrons
• The mass of an electron is so small it is
insignificant
Mass number
12
Atomic number
6
C
How many protons?
How many neutrons?
How many electrons?
What about Sodium
11
23
Na
Atomic number is 11 – therefore there are 11 protons
Mass number is 23 – therefore the number of neutrons
23 – 11 = 12
The number of electrons equals the number of protons
So there are 11 electrons
Isotopes
• All atoms of elements have the same
number of protons in the nucleus but the
number of neutrons can vary
• Atoms of the same element with different
numbers of neutrons are called isotopes
• Isotopes have the same atomic number
but different mass numbers
Isotopes
• An isotope can be represented by the
symbol for the atom together with the
atomic and mass numbers
Mass number
Atomic number
12
6
C
13
or
How many neutrons are present now?
6
C
14
or
6
C
Ions
• When an atom loses or gains an electron I
produces a charged particle known as an
ion
• An ion with fewer electrons that protons is
a positive ion or cation
• An ion with more electrons that protons is
a negative ion or anion
Ions
• We know sodium has:
11 protons 12 neutrons 11 electrons
11
We represent this as 23 Na
• If we remove an electron from sodium we would
be left with an ion that had:
11 protons 12 neutrons 10 electrons
Sodium now has a positive
charge
11
+
This is represented as 23 Na
Homework Questions
• Q 3, 7, 9, 12, 21, 23
The Periodic Table
Figure 3.2 This form of the periodic table is in common use.
Colour is used to distinguish the blocks of elements.
What’s are the limitations of
Rutherford’s model
• The laws of physics indicate that electrons
moving in circular orbits should continually emit
electromagnetic radiation (or light).
• As radiation is emitted, electrons should lose
energy and spiral into the nucleus, causing the
atom to cease to exist.
• When the element is heated only light of certain
specific energies, and not light of every energy,
is emitted. When the light is passed through a
prism onto a screen an emission spectrum is
obtained
Emission Spectrum
Calcium
Sodium
Refining Rutherford’s Model
In 1913 a Danish physicist, Niels Bohr suggested that
the laws of physics that scientists used to explain
the behaviours of large objects such as cars did not
apply to the motion of very small objects such as
electrons.
He proposed that electrons in atoms:
• Circled the nucleus without losing energy
• Could move only in certain fixed orbits of particular
energies
Arranging electrons around a nucleus
Niels Bohr also stated:
• The orbit in which an electron moved
depended on the energy of the electron
• Electrons with low energy were in orbits
close to the nucleus
• High-energy electrons were in outer orbits
Arranging electrons around a nucleus
• Heating an element can cause an electron to absorb
energy and jump to a higher energy state
• When the energy returns to a lower energy state it
releases a fixed amount of energy which can be seen as
light
• The electron can return in a number of different ways.
Each possible path produces light of a particular colour
in the emission spectrum
Successive Ionisation Energies
• If an atom has six electrons, then each
can be removed in turn. The electron that
is least strongly attracted to the nucleus
will be most easily removed. The amount
of energy to remove this electron is the
first ionisation energy of the element.
• Each of the remaining five electrons will
have a specific ionisation energy higher
than the first ionisation energy.
Ionisation energy – what can it tell us?
•
Figure 2.17 (a) Graph of the ionisation energies of a sodium atom. (The
logarithm of ionisation energy is used to provide a more convenient
vertical scale.)
(b) The closer an electron to the nucleus, the greater the amount of
energy required to remove it from the atom.
Shells
Each shell is a different energy level
Electrons in the same shell
– are about the same distance from the
nucleus
– Have about the same energy
Shells
• Different shells (energy levels) can hold different
numbers of electrons
• The lowest energy shell closest to the nucleus, known as
the first shell can hold two electrons
• The second shell can hold eight
• The third shell eighteen
Shell
Number
Maximum number
of electrons
1
2
2
8
3
18
4
32
n
2n2 (where n is the
shell number)
Electron Configuration
• The way in which electrons are arranged around
the nucleus is known as the electron
configuration of an atom.
• In a stable atom all electrons are as close to the
nucleus as possible.
• Electrons will occupy inner shells before outer
shells
• The electrons in the outermost shell are called
valence electrons.
• It is valence electrons that are involved in
chemical reactions
However …
After the second shell the electron shells fill in
stages.
– The outer most shell can never have more than eight
electrons regardless of how many the shell can hold.
– Therefore the third shell which can contain 18
electrons will not contain more than eight electrons
until there are at least two electrons in the fourth
shell.
Lets look at some examples
Questions: 10-13 pg 27
Quantum Mechanics
•
Figure 2.19 The electron in a
hydrogen atom acts like a
cloud of negative charge
around the nucleus. The
density of the cloud at any
point gives an indication of the
amount of time an electron
spends there.
German scientist Erwin Schrodinger proposed that
electrons behave as waves around the nucleus. There
are no definite orbits or shells for electrons in this model.
Electrons are thought to move in regions of space
surrounding the nucleus called orbitals.
Shells and Subshells
• Within an atom there are major energy
levels which we call shells
• Within the shells are energy levels of
similar energy called subshells; s,p,d and f
• In atoms containing more than one
electron, the energies of the subshells
increase in order s < p < d < f
Shells and Subshells
• Subshells are made up of orbitals
(the regions in which electrons move)
• The orbitals within a particular subshell are of
equal energy
Pauli exclusion principle
States that:
An atomic orbital may hold a maximum of
two electrons, i.e. it may hold 0, 1 or 2
electrons.
Therefore the maximum number of electrons
in an s-subshell, with its one orbital, is two.
Pauli exclusion principle
• The order of the energies of the subshells is 1s
< 2s < 2p < 3s < 3p < 4s < 3d < 4p
< 5s < 4d < 5p < 6s < 4f < 5d …
Why is 3d after 4s??
Some examples
•
•
•
•
Sodium
Iron
Question 14 pg 31
Topic Questions