The document summarizes the key people involved in the discovery and development of the periodic table of elements. It discusses Johann Dobereiner who discovered triads of elements, de Chancourtois who arranged elements in a helix, Newlands who proposed periodicity based on atomic mass, and Mendeleev who created one of the first recognizable periodic tables. It also mentions the contributions of Meyer, Ramsay, Moseley, and Seaborg in refining the table and adding new elements. The periodic table organizes elements based on electron configuration in their outermost shells and exhibits trends in properties from atomic radius to metallic character across the table.

2. PEOPLE BEHIND THE DISCOVERY OF
PERIODIC TABLE OF ELEMENTS

3. Johann Wolfgang
Dobereiner
(1780-1849)
- group the elements in
threes and found that the
atomic mass of the middle
element is the average of
the other two.
- Instance: Ca – 40 amu and
Ba – 137 amu, the average
of the two is equal to 88
which is the atomic mass of
Sr (element between the
two)
- Law of triads

4. Alexander Beguyer de
Chancourtois
(1820-1886)
- prepared a helical graph
containing a list of all known
elements at that time
wrapped around a cylinder.
- The elements with similar
properties fall under the
same column.

5. John Newlands
(1837-1898)
- arranged the elements
according to increasing
atomic weights and
observed that the properties
of the elements are
repeated after an interval of
eight elements.
- Hence, proposed the idea of
periodicity among the
elements.
- Later wrote a paper
proposing the Law of
Octaves.

6. Dmitri Mendeleev
(1834-1907)
- arranged the elements in
increasing atomic mass.
- His periodic table consisted
of 17 columns.
- In 1871, he revised into 8
columns with the transition
metals occupying the 8th
column.
- Lothar Meyer also had
similar results.

7. Lord Reyleigh (1842-1919)
William Ramsay (1852-1916)
- Lord Reyleigh discovered
the noble/inert gases whose
properties did not fit all the
other elements. It was him
who first discovered argon
and this was followed by
Ramsay who discovered the
other inert gases and placed
them in the periodic table.

8. Henry Moseley
(1887-1915)
- arranged the elements in
the periodic table according
to increasing atomic
number.
- This arrangement eliminated
some of the inconsistencies
in the work of Mendeleev.
- The modern periodic table is
based on his work.

9. Glen Seaborg
(1912-1999)
- Discovered the
transuranium elements with
atomic numbers 92-102.
- also he added the
lanthanide and actinide
series projected
perpendicular to the plane
of the periodic table.

10. - electron found in the outermost energy level.
- Ex:
Element: Lithium
Symbol: Li
Atomic number: 3
electron: 3
Electron configuration: 1s2 2s1
2s1 – outermost energy level
2s1 – valence electron

11. Element Ne S Cl
Outermost energy Level 2s2 2p6 3s2 3p4 3s2 3p5
Valence electrons 8 6 7
Complete the table below.
Ne – Neon
A = 10
e- = 10
Electron
configuration:
1s2 2s2 2p6
1s2 2s2 2p6
1s2 2s2 2p6
S – Sulfur
A = 16
e- = 16
Electron
configuration:
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p4
1s2 2s2 2p6 3s2 3p4
Cl – Chlorine
A = 17
e- = 17
Electron
configuration:
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p6 3s2 3p5

13. Periodic table and classification based on orbital being filled:

14. • Periodic table is a very systematic tool. It is not just systematic but it is also
user-friendly.
• Trends of some properties of the elements: atomic radius, ionization energy,
electron affinity, electronegativity, and metallic character.

15. ATOMIC RADIUS
- Size of an atom is defined by its
atomic radius.
- Atomic radius is one-half the
distance from the two nuclei of
two adjacent atoms.
- In the periodic table, the atomic
radii of the elements decrease
from the left to right and
increase from top to bottom.

16. IONIZATION ENERGY
- energy that is needed to
remove the valence electron.
- Low ionization energy would
be mean that removing the
valence electron is relatively
easy, and requires less energy.
- Ionization energy increases
from left to right and decrease
from top to bottom.
- Elements in the leftmost part
and bottom portion of the
periodic table have the greater
tendency to lose electrons and
form cations.

17. ELECTRON AFFINITY
- when electron is added to an
atom in gaseous phase, energy
is released.
- A large negative value for
electron affinity indicates that
the formation of the ion is
favorable while a more positive
value indicates that the atom
tends to accept an extra
electron.
- From left to right, the general
trend for electron affinity
increases in general it is
decr4easing from top to
bottom.

18. ELECTRONEGATIVIT
Y
- Developed by Linus Pauling.
- a value indicate the ability of an
atom to attract an electron pair
towards itself in a chemical
bond.
- Difference in electronegativity
between to atoms will dictate
what type of bond will be
formed – either ionic bond or
covalent bond.
- Increases from left to right and
decreases from top to bottom.

19. METALLIC PROPERTY
- Metals are in the left portion and
the nonmetals are in the right
portion of it.
- An increase, in metallic property
can be observed as you go across
of it (left to right) and as you go
down a group, an incr4ease is
noted.
- Characteristic of metals: ability to
lose electrons to form cations.
- Higher ionization energy will have
difficulty to form cations, thus, less
metallic.
- While those with lower ionization
energy will tend to form cations
and thus are metallic in nature.