Matter is composed of very small ultimate particles, or atoms of matter.

1. Atoms and Atomic
Structure
By
Prof. Liwayway Memije-Cruz
CAS

2. Introduction
Matter is composed of very small ultimate
particles, or atoms of matter.

3. State of the Environment:
Issues and Concerns
Democritus (c. 460 – c. 370 BC) an ancient Greek pre-
Socratic philosopher
 He developed the
first philosophical
statements relating
to an idea similar to
atoms
 He was the one
who coined the
term atomos, which
mean “uncuttable”

4. John Dalton: (6 September 1766 – 27 July 1844) an English
chemist, meteorologist and physicist
 determined the usefulness
of atoms.
 first to realize that the
nature and properties of
atoms could be used to
explain the Law of Definite
Composition of all
substances developed
earlier by Proust and the
way and the proportions in
which substances react
with one another.

5. The atoms are composed of subatomic particles:
 Electrons have a negative
charge and are the least
massive of the three;
 Protons have a positive
charge and are about
1836 times more massive
than electrons; and
 Neutrons have no charge
and are about 1839 times
more massive than
electrons.

6. Assumptions of Dalton’s Atomic Theory
 All matter is made up of minute, discreet, indivisible, and
indestructible particles called atoms.
 Atoms of the same element are chemically alike: atoms of
different elements are chemically different. in particular, the
atoms of one element have a different mass than those of
other elements.
 When atoms combine to form compounds or when such
combination of atoms decomposes, each individual atom
retain its identify.
 When atoms combine they do so in small numbers ratios.

7. Illustration of Dalton’s Theory
Atoms of the same
element are
chemically alike:
atoms of different
elements are
chemically different. in
particular,the atoms of
one element have a
different mass than
those of other
elements.

8. Law of Definite Composition
When atoms combine to form compounds or
when such combination of atoms decomposes,
each individual atom retain its identify.

9. Rutherford’s Experiment
He performed the
experiment on bombarding
gold foil with alpha
particles. They observed
that most (about 99.99%) of
the particles passed
through the film following a
straight path, while some
were deflected at large
angles and few bounced
back.

10. Based on the results, Rutherford proposed the
following
1. That the atom consists of a large empty space that explains
why most of the particles went straight through the film.
2. That the atom consists of a very small region where its
positive electricity is concentrated, hence, heavy. The
particles that bounced back were presumed to have hit this
region.
3. Those, which deflected, approached the positive nucleus;
hence, there was repulsion since the alpha particles were
also positive.
The above reasons describe the central part of the atom, called
nucleus, to be with a very small volume yet a massive one.

11.  known for his foundational
contributions to further
understanding of the atomic
structure and quantum theory.
Niel Bohr (1885 – 1962) a Danish physicist

12. Isotopes
 Atoms of the same element can have the same atomic number but
will differ in their atomic masses and these are called isotopes. The
first isotopes discovered were those of neon by Thomson and Aston
in 1912-1913. The mass spectrograph is a precise instrument used
to determine the atomic masses to 1 part in 10,000.

13. Characteristics of an Atom
 All elements are observed to be electrically neutral, despite the presence of
electrically charged particles in atoms. The number of positive protons in the
nucleus of an atom is equal to the number of electrons surrounding the
nucleus.
 Since elements differ from one another, their atoms must differ structurally.
Each element has an atomic number, the atomic number is equal to the
number of electrons revolving about the nucleus of the atom. Since atoms
are electrically neutral, the atomic number also equals the number of protons
present in the nucleus of an atom.
 Equal number of atoms of different elements weighed under the same
conditions has a different weight. Atoms of different elements have different
atomic weights, the atomic weight of an atom is equal to the sum of the
number of protons and the number of neutrons in the nucleus of an atom.
Thus, all of the weight of an atom comes from its nucleus. Atomic weights
are relative, they do not give the number of grams that an atom weights, but
they merely tell how much heavier or lighter an atom of one element is than
another.

14. Distribution of Electrons
 Electrons revolve around the nucleus of an atom in
a definite pattern. Groups of electrons maintain
definite average distances from the nucleus forming
shell or energy levels of electrons surrounding the
nucleus. Each shell is capable of containing a
definite number of electrons, the number increasing
in distance from the nucleus increases. Letters – k,
l, m, n, o, and p starting with the shell nearest the
nucleus, designates the shells. The maximum
number of electrons in any shell can be calculated
from the relationship:
Number = 2s Where:
 Number = maximum number of electrons possible in
the shell
 S = the number of the shell (K=1, I=2. m=3, etc)

15. Sublevels
The energy levels are further subdivided into
sublevels designated by the letters s, p, d,
f, g … (in alphabetical order), the number
of which corresponds to the number of the
energy level.
 Each sublevel has a set of orbital, which
are of equal energy.
K (n=1) one sublevel: 1s
L (n=2) two sublevels: 2s and 2p
M (n=3) three sublevels: 3s, 3p, and 3d
s = 2 electrons
p = 6 electrons
d = 10 electrons
f = 14 electrons

16. Orbital is the home of the electrons or the region of space where the
probability of finding the electrons is greatest.

17. Rules in building up the electronic configuration
 The number of electrons entering the atom must be equal
to its atomic number, z, and the number of protons. Thus,
the atom is neutral.
 No more than 2 electrons with opposite spins can enter any
single orbital (Pauli’s Exclusion Principle.)
 When there are orbitals of the same kind of energy, the
electrons occupy the equivalent orbitals singly to the
maximum and with their spins parallel (Hund’s Rule).
 The opposite spin may be represented by arrows pointing
upwards and downwards.

18. Wave Mechanics/Quantum Mechanics or Orbital
Theory
Light exhibitsdual wave-particleproperties. Interferenceand
diffraction patternsformed when light passesthrough slitscan only
beexplained by theaddition of waves. Discontinuousemission of
light from hot bodiescan only beexplained by particle-likephotons
of emitted light. Louis deBrogliereasoned that if light can exhibit
waveand particleproperties, then tiny moving particlesof matter
might also exhibit waveproperties.

19. Quantum Numbers
 The Principal Quantum Number (n) is associated with the distance
of the electron from the nucleus and it determines the gross energy of
the electron.
 The Second Quantum Number (Azimuthal Quantum Number) (I)
gives the shape of the orbital. It has integral values from 0 to n-1.
 The Third Quantum Number (Magnetic Quantum Number) (m )
describes the orientation of the orbital in space. The integral values
may be l1, l-1, l-2 down to -l. Positive values of m describe orientation
in the direction of applied magnetic field while negative values refer to
orientation in the opposite direction.
 The Fourth Quantum Number is the electron spin quantum number
(m ). It describes the spinning of the electron on its axis. It can have a
clockwise spin or counter clockwise spin.
 Pauli’s Exclusion Principle states that no two electrons can have
the same set of four quantum numbers.

20. Shapes of Orbitals

22. Exercise: C.HOPKINS CaFe Mighty good in a pinch of salt)
Element At.
no
At.
mass e _
p+ n0 Atomic
Structure
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n
G P Valence Nature

23. Self-Progress Test
 What is the modern atomic theory?
 What are the main parts of an atom? Be able to describe each of the particles.
 What is a shell or energy level?
 How are shells or energy levels designated?
 Define the following: a.orbital b.Quantum Mechanics c. Isotopes d. Atomic Number
e. Atomic Weight
 describe Neil Bohr's Atomic Model
 Explain the Quantum Number Theory.
 Draw diagrams to show the distribution of electrons protons, and neutrons of atoms
of helium, neon, argon, krypton and radon.
 What is the Heisenberg’s Uncertainly Principle? Why is it applicable to electrons
but not a moving vehicle?
 Write the electronic configurations of the following: Carbon, Magnesium,
Potassium, Calcium and iron.

24. Thank you!