History of the Periodic Table
Who discovered phosphorus in 1669?
Hennig Brand
What are horizontal rows called?
Periods
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons.
What is valency?
The combining power of an element when forming compounds.
Who grouped elements into triads in 1829?
Johann Wolfgang Döbereiner
What are vertical columns called?
Groups (families).
What is the chemical formula? give definition
chemical formula is a combination of symbols and numbers that represent compounds.
What is the valency of hydrogen?
1
What was the Law of Octaves?
When elements are arranged by increasing atomic mass, every eighth element has similar properties.
How many groups are in the modern periodic table?
18 groups.
Which isotope is used as the atomic mass standard?
Carbon-12 (¹²C).
Write the formula of hydrogen and sulfur compound.
H₂S.
Who proposed the Law of Octaves?
John Newlands
How many elements are in the first period?
2 elements.
Chlorine has 75% ³⁵Cl and 25% ³⁷Cl. What is its average atomic mass?
35.5 amu
Write the formula of carbon and hydrogen compound (valency of C = 4).
CH₄
What important thing did Mendeleev do that others did not?
He left blank spaces for undiscovered elements and predicted their properties.
Why do chlorine, bromine, and iodine have similar properties?
Because they are located in the same group (Group 17 – halogens).
Define relative atomic mass (Ar).
The average mass of atoms of an element compared to 1/12 of the mass of a carbon-12 atom.
If aluminum has valency 3 and oxygen has valency 2, write the correct formula.
Al₂O₃.