Henry Moseley discovered the relationship between an element's atomic number and the wavelength of its x-ray emissions. This led to a better arrangement of the periodic table and allowed scientists to predict and later synthesize new elements in the laboratory. Moseley arranged the elements according to the square root of the x-ray frequency, which more correctly predicted periodic trends. The invention of the cyclotron provided accelerated particles that could bombard heavy nuclear targets to synthesize new elements through nuclear reactions. One of the first artificially made elements was technetium, produced by bombarding molybdenum with deuterons. Many transuranic and superheavy elements have since been synthesized using similar nuclear reaction methods.

1. Henry Moseley, the Atomic Number, and
Synthesis of Elements
By : SIRTHON AZUELA
SHS Teacher
Sabang National High School
Sabang, Calabanga, Camarines Sur
CP no. 09122738106

2. ContentStandards
The learners demonstrate an understanding of :
1. How the concept of the atom evolved from Ancient Greek
to the present
2. How the concept of the element evolved from Ancient
Greek to the present.

3. Performance&Standards& The learners can make a creative
representation of the historical development of the atom or the chemical
element in a timeline. Learning&Competencies& The learners to:
1. Cite the contributions of J.J. Thomson, Ernest Rutherford, Henry
Moseley, and Niels Bohr to the understanding of the structure of the atom
(S11/12PSIIIb-9)
2. Explain how the concept of atomic number led to the synthesis of new
elements in the laboratory (S11/12PS-IIIb-11)
3. Write the nuclear reactions involved in the synthesis of new elements
(S11/12PS-IIIb-12)

4. Specific Learning Outcomes
At the end of the lesson, the learners will be able to:
1. Explain how Moseley discovered the correlation between the
atomic number of an element and the wavelengths of x-rays
emitted by the element
2. Discuss how new elements are synthesized

5. ALCHEMY
A medieval chemical science and
speculative philosophy aiming to
achieve the transmutation of the base
metals into gold. The discovery of a
universal cure for disease, and the
discovery of means of indefinitely
prolonging life.

6. Henry Moseley who was a researcher at
Rutherford’s laboratory.
In 1913 Moseley used Rutherford’s work to
advance the understanding of the elements
and solve the problem with Mendeleev’s
periodic table.

7. Moseley noticed that shooting electrons at elements caused them
to release x-rays at unique frequencies. He also noticed that the
frequency increased by a certain amount when the “positive
charge” of the chosen element was higher.
By arranging the elements according to the square root of the
frequency they emitted, he was able to draw out an arrangement
of elements that more correctly predicted periodic trends.

8. Mention the experimental evidence he gave to an existing
hypothesis: that the elements’ atomic number, or place in the
periodic table, was uniquely tied to their “positive charge”, or
the number of protons they had. This discovery allowed for a
better arrangement of the periodic table, and predicted
elements that were not yet discovered.
His method of identifying elements by shooting electrons and
looking at x-rays became a very useful tool in characterizing
elements, and is now called x-ray spectroscopy.

9. Synthesis of Elements
The invention of the device called cyclotron
paved the way for transmuting one element into
another artificially. The high-energy particles that
are produced from the cyclotron upon hitting
heavy target nuclei produce heavier nuclei.
The bombarding of Mo with deuteron formed
technicium which is the first artificially made
element. Its name is derived from the Greek
word technetos which means artificial.
9642Mo + 21H —> 9743Tc + 10n

10. The Transuranic Elements
Transuranic elements are synthetic elements with atomic
numbers higher than that of Uranium (Z = 92).
Neptunium (Z = 93) – synthesized by E.M. MacMillan in
1940
23892U + 10n —> 23993Np + 0-1ß
Plutonium (Z = 94)
23892U + 21H —> 23893Np + 210n
23893Np —> 23894Np+ 0-1ß

11. The Superheavy Elements
Superheavy elements are elements with atomic
numbers beyond 103. These are produced by
bombarding heavy nuclear targets with accelerated
heavy projectiles.
Bohrium (Z = 107) – projectile used was Cr
20983Bi + 5424Cr —> 261107Bh + 210n

12. Following are the equations of several nuclear reactions that have
important roles in the history of nuclear chemistry:
•The first naturally occurring unstable element that was isolated,
polonium, was discovered by the Polish scientist Marie Curie and
her husband Pierre in 1898. It decays, emitting α particles:
Po84212⟶Pb82208+He

13. The first nuclide to be prepared by artificial
means was an isotope of oxygen, 17O.
It was made by Ernest Rutherford in 1919 by
bombarding nitrogen atoms with α particles:
N714+α24⟶O817+H11

14. James Chadwick discovered the neutron in 1932, as a
previously unknown neutral particle produced along
with 12C by the nuclear reaction between 9Be and
4He:
Be49+He24⟶C612+n01

15. The first element to be prepared that does not
occur naturally on the earth, technetium, was
created by bombardment of molybdenum by
deuterons (heavy hydrogen, H12), by Emilio
Segre and Carlo Perrier in 1937:
H12+Mo4297⟶210n+Tc4397

16. The first controlled nuclear chain reaction was
carried out in a reactor at the University of
Chicago in 1942. One of the many reactions
involved was:
U92235+n01⟶Br3587+La57146+310n

17. Figure 3.1.3 summarizes these types of decay,
along with their equations and changes in
atomic and mass numbers.

18. A synthesis reaction or direct combination reaction is one of the most
common types of chemical reactions. In a synthesis reaction, two or more
chemical species combine to form a more complex product.
A + B → AB
In this form, a synthesis reaction is easy to recognize because you have
more reactants than products. Two or more reactants combine to make
one larger compound.
One way to think of synthesis reactions is that they are the reverse of a
decomposition reaction.

19. Synthesis Reaction Examples
In the simplest synthesis reactions, two elements combine to
form a binary compound (a compound made of two
elements). The combination of iron and sulfur to form iron (II)
sulfide is an example of a synthesis reaction:
8 Fe + S8 → 8 FeS
Another example of a synthesis reaction is the formation of
potassium chloride from potassium and chlorine gas:
2K(s) + Cl2(g) → 2KCl(s)

20. As in these reactions, it's common for a metal to react with a
nonmetal. One typical nonmetal is oxygen, as in the everyday
synthesis reaction of rust formation:
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
Direct combination reactions aren't always just simple
elements reacting to form compounds. Another everyday
example of a synthesis reaction is the reaction that forms
hydrogen sulfate, a component of acid rain. Here, the sulfur
oxide compound reacts with water to form a single product:
SO3 (g) + H2O (l) → H2SO4 (aq)

21. So far, the reactions you have seen have only one product molecule
on the right-hand side of the chemical equation. Be on the lookout
for synthesis reactions with multiple products. A familiar example of a
more complex synthesis reaction is the overall equation for
photosynthesis:
CO2 + H2O → C6H12O6 + O2
The glucose molecule is more complex than either carbon dioxide or
water.
Remember, the key to identifying a synthesis or direct combination
reaction is to recognize two or more reactants form a more complex
product molecule!

22. Predicting Products
Certain synthesis reactions form predictable
products:
Combining two pure elements will form a binary
compound.
The product formed by reacting a metallic oxide and
carbon dioxide will be a carbonate.
Binary salts react with oxygen to form a chlorate

23. Write the nuclear reactions
involved in the synthesis of each of
the following new elements :

24. a) Curium (Z = 96) was formed
by reacting Pu – 239 with alpha
particles 42He. It has a half-life
of 162 days.

25. a) 23994Pu + 42He —> 24296Cm
+ 10n Cm is named after Marie
and Pierre Curie who had done
extensive research on natural
radioactivity.

26. b) Mendelevium (Z = 101)
was formed by reacting En –
253 with alpha particles.

27. b) 25399 En + 42He —>
256101Mv + 10n
Mv is named after Dmitri
Mendeleev.

28. c) Meitnerium (Z = 109) was formed
by cold fusion which involves the
combination of Bi and Fe nuclides
at ordinary temperature

29. c) 20983Bi + 5826Fe
—> 266109Mt + 10n

30. Nuclear Chemistry (Radioactivity) sample of Problems with Solutions
1. Find whether90
231Th is stable or not.
Solution:
n+p=Mass Number
90+n=231
n=141
where n is number of neutrons and p is number of protons.Thus ratio between n and p is;
n0/p+=141/90=1,56
Since ratio is greater than 1, 90
231Th has unstable nucleus and it is radioactive element.

31. 2. A, B, C and D elements form compounds AC, A2D and BD. If AC and A2D are radioactive
and BD is not radioactive compound, find whether the following compounds are
radioactive or not.
I. A2
II. A2C
III. C2D
IV. BC
Solution:
If a compound is radioactive, at least one of the elements of this compound must be
radioactive. Since BD is not radioactive, B and D are not radioactive elements. If AC
and A2D are radioactive then A must be radioactive element C can be radioactive or
not we can not say anything about it.
A2 and A2C are radioactive compounds because of radioactive element A but we can
not say whether C2D and BC are radioactive or not.

32. 2. A, B, C and D elements form compounds AC, A2D and BD. If AC and A2D are radioactive
and BD is not radioactive compound, find whether the following compounds are
radioactive or not.
I. A2
II. A2C
III. C2D
IV. BC
Solution:
If a compound is radioactive, at least one of the elements of this compound must be
radioactive. Since BD is not radioactive, B and D are not radioactive elements. If AC
and A2D are radioactive then A must be radioactive element C can be radioactive or
not we can not say anything about it.
A2 and A2C are radioactive compounds because of radioactive element A but we can
not say whether C2D and BC are radioactive or not.

33. 3. Find number of protons and mass number of Y in given reaction below.
92
234X + β- + α → Y + γ + 2β+
Solution:
Number of protons in left side of reaction is;
92 +(-1) + 2 =93
Thus, number of protons in right side of reaction must be 89.
Y+1.(0) + 2.(+1) = 93
Y=91
Y=91 number of protons
Mass number of reactants must be equal to mass numbers of products.
234+4=238 mass number of reactants
Y+ 1.(0) + 2.(0) =238
Y =238
Y=238 mass number of Y
91
238Y

34. 4. Find X and Y in given reactions.
I. 19
38K → 18
38Ar + X
II. 80
197Hg + Y → 79
197Au
Solution:
I. 19
38K → 18
38Ar + a
bX
mass number and atomic numbers must be equal;
38=38+b
b=0
19=18+a
a=1 thus, +1
0X or +1
0β

35. II. 80
197Hg + c
dY → 79
197Au
80 + c = 79
c=-1
197 + d = 197
d=0 So, Y = -1
0β

36. 5. Which ones of the following statements are true for atom having
following reaction in its nucleus?
1
1p → 0
1n + +1
0β
I. Its mass number increases by 1.
II. Its isotope is formed.
III. Its neutron number decreases by 1.
IV. Its atomic number decreases by 1.
V. Its number of protons increases by 1.
Solution:
In given reaction one proton is converted into one neutron. Thus, atomic
number decreases by 1. IV is true.

37. Assignment :
A. Short Essay (maximum of 3 sentences)
1) Dmitri Mendeleev is often regarded as the Father of the Periodic
Table. Would you say that Henry Moseley deserves the
recognition more than him?
2) Explain why the atomic number is called the “fingerprint” of
elements.
3) How would you relate alchemy to synthesis of new elements?
B. Choose the letter of the correct answer, and then write the nuclear
reaction. Identify the element.
An isotope of element 102, with a mass number of 253 was produced
by bombarding 24096Cm with a projectile, which was absorbed by the
target nucleus. This projectile should be ___ a) 52He b) 136C
c) 126C d) 168O

38. Answer Key
B. b
240
96 Cm+ 136C —> 253102 No Nobelium

39. Thank you!
.Just remember :
“Whatever the problem, be part of the solution. Don’t just sit around raising
questions and pointing out obstacles.”