Chemistry - Atomic theory

1. Atomic Theory
Ir. Dr. Alifdalino Sulaiman

2. Dalton’ s Atomic Theory
Postulates that:
1. Matter is composed of exceedingly small particles called atoms. An atom is
the smallest unit of an element that can participate in a chemical change.
2. An element consists of only one type of atom, which has a mass that is
characteristic of the element and is the same for all atoms of that element. A
macroscopic sample of an element contains an incredibly large number of
atoms, all of which have identical chemical properties.
3. Atoms of one element differ in properties from atoms of all other elements.
4. A compound consists of atoms of two or more elements combined in a small,
whole-number ratio. In a given compound, the numbers of atoms of each of
its elements are always present in the same ratio.
5. Atoms are neither created nor destroyed during a chemical change, but are
instead rearranged to yield substances that are different from those present
before the change.
A pre-1982 copper penny (left) contains approximately 3 × 1022 copper
atoms (several dozen are represented as brown spheres at the right), each
of which has the same chemical properties. (credit: modification of work by
“slgckgc”/Flickr)
Copper(II) oxide, a powdery, black compound, results from the combination
of two types of atoms—copper (brown spheres) and oxygen (red spheres)
—in a 1:1 ratio. (credit: modification of work by
“Chemicalinterest”/Wikimedia Commons)
When the elements copper (a shiny, red-brown solid, shown here
as brown spheres) and oxygen (a clear and colorless gas, shown
here as red spheres) react, their atoms rearrange to form a
compound containing copper and oxygen (a powdery, black solid).
(credit copper: modification of work by http://imagesof-
lements.com/copper.php)

3. Atomic structure
• Made of electrons, protons and
neutrons.
• Atomic nucleus consists of positively
charged protons and uncharged
neutrons.
• Negatively charged electrons “orbit” the
nucleus forming electron cloud
surrounding the nucleus.
• The nucleus makes up the most of the
mass of the atom while the electrons
make up most of the volume of the
atom.
To remember: electrons controls the chemical behavior of an element.
Added neurons only add mass but does not affect the chemistry.

4. Characterization of Atom
• atomic number (Z) = number of protons
• atomic mass (A) = number of protons + number of neutrons
• A Z = number of
− neutrons
• Atomic charge = number of protons − number of electrons
ANion (-
ve)
CaTion (+ve)

5. Atomic symbol
• Atomic symbol is used the abbreviate the
atomic structures of each element.
• X is the symbol of the element.
• Z is the atomic number of the element.
• A is the mass number.
 Mass number (A) is the sum of the number of protons
and neutrons of the nucleus.
 Atomic number (Z) is the number of protons in the
nucleus of the atom.
 The number of electrons equals to the number of
protons for a neutral atom.

6. Atomic mass
• At atomic scale, we use the atomic mass unit (amu) (also called
Dalton) to define an element’s mass.
• 1 amu = 1.66054 x 10-24
g
• Represents an average of all the naturally occurring isotopes.
• Sum of the masses of each isotope of the element multiplied by
the fraction of its natural abundances.
• Atomic mass C = (mass 12
C) (0.99) + (mass 13
C) (0.01)

7. Atom model (classical mechanics)
• Niels Bohr’s model
• Atom is pictured to have a dense,
positively charged nucleus and
negatively charged electrons in
orbits (also called energy levels
or shells).

8. Atom model (Quantum mechanics)
• Electrons can be found inside orbitals.
• Orbital is the region of space where the electron is most likely
to be found.
• The orbital is characterised by four quantum numbers.

9. Principal quantum number
• Electron shells were designated by the principal quantum
number, n.
• Indicates the main energy level occupied by the electron.
• Each level is designated by a number: 1, 2, 3…
• The orbit closest to the nucleus is of lowest energy, while the one
farthest from the nucleus is of highest energy.

10. Azimuthal quantum number
• Subshells are also designated by a quantum number
called the azimuthal quantum number, l.
• Indicates the shape of the orbital.
• Each orbital can contain a maximum of two
electrons.
• Value of l for different subshells in an
increasing positive integer beginning with 0 until n-1.
Subshell l Shape
s 0 Spherical
p 1 Dumb bell ( 2 lobes)
d 2 Double-dumb bell ( 4
lobes)
f 3 Complex (6-8 lobes)

11. Magnetic quantum number,Ml
• Tells the orientation of the orbital.
• Values depend on l.
• Ml goes from –l to l.

12. Orientation of the orbitals

13. Spin quantum number
• An electron is 'spinning' around an axis.
• Spin up (½) and spin down (-½)

14. Example: Quantum numbers for the first shells
n orbitals (n2
) No of electrons in
orbital
Total no of
electrons in shell

15. Electron configuration
• The assignment of electrons to the electronic shells,
subshells and orbitals is called the electronic configuration
of the atom.
• Represented by the subshell notation.
• Lists the principal quantum number of the electron number,
followed by the subshell type, and the number of electrons.
• Can also be represented by the orbital diagram that
provides information on electronic orbitals/ electron spin.
• Upward pointing arrow represents spin of + ½ and
downward spin - ½

16. Electron configuration
• Electrons of any element are assigned to the shells, subshells
and orbitals in the order of increasing energy.
• Each electron will occupy the orbital of lowest energy
available.
• They will enter an orbital of higher energy only when the
orbital of lower energy are all filled.
• Assigning electrons to shells, subshells and orbitals in the order
of increasing energy is known as the Aufbau principle.

17. Aufbau principle (‘building up’)
Governed by:
1. Pauli Exclusion principle: no more than 2 electrons can occupy
any one orbital
2. Hund’s Rule: within a subshell, electrons will occupy the
orbitals individually (with parallel spins) before filling them in
pairs (with opposite spins)

18. Pauli Exclusion Principle
- The energy level of any electron within an
atom requires all four quantum numbers (n, l,
m and s)
- No two electrons in the same atom can have
the same four quantum numbers.

19. The Periodic Table Of The Elements
• Periodic law: The physical and chemical properties of the elements
recur periodically in a systematic and predictable way when the
elements are arranged in order of increasing atomic number.
• Vertical columns are called groups. Each group share the same
chemical properties with trends associated with increasing atomic
number going down a group.
• Horizontal rows are called periods. Number of each period
corresponds to the principal quantum number of the outermost
electron shell containing electrons for all elements in the period.

20. • There are 18 groups in the periodic table numbered 1-18.
• The elements are also arranged in 7 horizontal rows called periods.
• Increasing the atomic number by one corresponds to adding one electron to the
electronic configuration of the previous element in the period.
• All elements in the same group have the same electron configurations in their
outermost occupied electron shell called the valence shell.
• The electrons in the valence shell are called the valence electrons.

21. Electron configuration for the 1st
period

22. Electron configuration for the 2nd
period

23. Electron configuration for the 3rd
period

24. Electron assignments
• For atomic numbers above 18, the Aufbau principle
is modified where:
– Electron subshells are filled in the order of increasing
n+1
– Where two subshells have the same n + 1, the subshell
with the lowest n is filled first.
– No more than two electrons can occupy any one orbital.
– Electrons will fill all orbitals in a subshell individually
(with parallel spins) before filling them in pairs (with
opposite spins)

25. Electron configuration for the 4th
period

26. • Some areas of the periodic table are called blocks.
• The blocks indicate the filling sequence of the electron
shells and each block is named according to the subshell
in which the outermost electrons reside for elements in
that block.

27. Periodic trends
• Electronic configurations of elements change within the
periods or groups.
• Some important periodic trends are: atomic radius, ionization
energy, electron affinity and electronegativity.

29. Isotopes
• Atoms with the same number of protons and
electrons (so same chemical properties), but
with different numbers of neutrons (have
different masses).
• Hydrogen is the only element whose isotopes
have different names.
• The mass number (A) is the atomic symbol is
used to distinguish between the different
isotopes of an element.

30. Radioisotopes
• Unstable isotopes are called radioisotopes.
• Radioisotopes are atoms that have excess nuclear energy,
causing them to be unstable.
• The unstable nucleus undergo a number of nuclear reactions
or processes that lead to the formation of a more stable
nucleus.
• These nuclear reactions are called radioactive decay, which
releases the excess nuclear energy as ionizing radiation.

31. Radioactive decay
• Radioactive decay processes follow first order kinetics.
• The decay rate of a radioisotope, also called the activity, is
directly proportional to the concentration of the radioisotope,
which is directly proportional to the number of radioactive
particles in the sample (N).

32. Radioactive decay
• The activity of a radioisotope (-dN/dt) is measured in becquerels (Bq) or curies
(CI)
• The specific activity (SA) is the activity for one unit of a radioisotope in Bq/g or
Bq/mol.
• The unit for the decay constant (λ) is 1/s.
The integrated rate law for radioactive decay processes is given by:
So, the half life (t1/2) is the amount of time, measured in seconds, required for the
amount of radioisotope at time zero (No) to decay by one half (No/2). The half life is
related to the decay constant as:

33. Example: Determining decay time of a radioisotope from the
half-life
• How long will it take for a 40.0 g sample of iodine-131, with a
half life of 8.040 days, to decay to 1/100 of its original mass?