mass
'no magic to the mole'
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
Now try the following question.
CALCULATING ON THE SIDE OF SUCCESS – Module B(2019): ISOTOPES & RELATIVE ATOMIC MASS
B. ISOTOPES & THE RELATIVE MASSES OF ATOMS
B1. BACKGROUND TO ATOMIC MASS & QUANTITATIVE CHEMISTRY
By the late 18th century the early chemists needed to account for the relationships between the masses of reacting substances that they observed in their experiments, e.g., whenever two elements combined together in a chemical reaction they did so in a definite, fixed proportion by mass: Fe:S ratio 7 g:4 g and Mg:O ratio 3 g:2 g, for instance. At this point accurate and reproducible measurements of mass had only become recently available with the advent of reliable balances. Up until then the Science of Chemistry arguably did not exist.
So, the birth of modern chemistry occurred around this time, when many new elements were being discovered by Priestley, Scheele, Davy and others. In 1808, however, John Dalton was the first scientist to base an atomic theory on experiment, and he was responsible for introducing explanations of experimental results in terms of atoms – the word atom is derived from the
Greek atomos (a = not; tomos = I cut). The term originated with Ancient Greek philosophers Aristotle and Democritus, who held opposing views on whether matter was continuous
- infinitely divisible - or finite - comprising a fundamental, indivisible building block.
Dalton postulated that all matter consists of atoms, minute particles which could not be created, or destroyed or split. He theorized that all atoms of an element were identical in every respect, e.g., mass. Chemists of the c.19th were quick to accept Dalton's ideas but his theory had weaknesses. He thought that water was HO, he did not know about isotopes, and could not reasonably have foreseen the "splitting of the atom".
By the middle of the c.19th, confusion still reigned supreme regarding the chemical formulae of many substances that we now regard as relatively simple. Although it was clear to chemists that atoms of different elements possess differing masses, they were not able fully to understand the reasons why, for example, in compound formation one atom of sodium combines with one atom of chlorine whereas one atom of carbon combines with four atoms of chlorine.
The early development of the Periodic Table occurred without any knowledge of the internal structure of atoms and focused on the so-called atomic weights of the known elements.
Not until the early c.20th could physicists offer a detailed inventory accounting for the internal structure of atoms. A phenomenally dense nucleus provided virtually all of an atom’s mass but, understanding the behaviour of its extra-nuclear electrons, ultimately the particles governing the chemistry of any element, could not be rationalized before the development of a brand new field of mathematics, around 1925, called wave mechanics.