UNIT 5: THE ATOM AND THE PERIODIC TABLE
PROBLEMS 1
1. ATOMIC MODELS
1. This is the definitive summary to understand the application of the scientific method to explain what
matter is composed of. Complete it with the information gathered from the video:
Around 440 B.C., Democritus first proposed that everything in the world is composed of tiny
particles surrounded by empty space, speculating that they vary in size and shape depending on the
substance. He called these particles "__________" from the Greek adjective "_____________,"
challenging the prevailing ideas of philosophers like Aristotle, who believed matter consisted of four
elements.
Fast forward to 1808 when John _________ challenged Aristotelian theory. While Democritus'
atomism was purely theoretical, Dalton demonstrated that common substances break down into
the same elements in consistent _____________, leading to the conclusion that different
compounds were combinations of atoms with specific sizes and masses, uncreated and
indestructible.
The atomic theory gained acceptance, but the next breakthrough came almost a century later with
the discovery of the electron in ______ by physicist J.J. Thomson. His "________ ___________" atom
model depicted a sphere uniformly coated with positive matter and filled with negatively charged
electrons. Thomson won the Nobel Prize in 1906, though his atomic model was short-lived.
Ernest ________________, one of Thomson's brilliant students, conducted experiments on the
effects of X-rays on gases. By firing positively charged _________ particles at a _________ foil, he
concluded that atoms consisted mostly of empty space, with electrons scattered around and a
dense, positively charged nucleus at the center. This was called the ‘nuclear model’.
In 1913, Niels _________, another student of Thomson, expanded nuclear model. Drawing on the
works of Max ____________ and Albert _____________, he proposed that electrons orbit the nucleus
at specific energy levels and distances, capable of jumping between levels without intermediate
space. This ____________ model garnered attention but faced complications as electrons were
discovered to behave like waves and were not confined to specific points in space.
Werner Heisenberg's principle of uncertainty, formulated in _________, revealed that determining
the exact position and _______ of electrons as they move around an atom was impossible. This led
to the current _______ model, suggesting that electrons do not have a fixed location but operate
within a range of possible positions. This is Schrödinger atomic model, the currently one accepted.
In Schrödinger's model, electrons do not follow sharply defined orbits (like in Bohr's model), but
rather are found in orbitals. Orbitals are regions in which an electron can be found. Inside an orbital,
the probability of finding an electron is described by mathematical equations proposed by
Schrödinger.
UNIT 5: THE ATOM AND THE PERIODIC TABLE
2. An atomic model is a structural representation of the atom. Here is an infographic about the five
atomic models that have been studied; fill it in with the date of its publication, the scientist who
developed it, and the name of the model:
Model
Scientist
Date
3. Associate each of the following statements with the corresponding model:
a) The atom is a compact indivisible sphere.
b) Electrons orbit in specific allowed paths.
c) The nucleus is very small compared to the atom.
d) Electrons are embedded in a positive mass.
e) Proposed a non-solid atomic model composed of a nucleus and a shell.
f) The model known as the "plum pudding" is attributed to...
g) The experiment of bombarding alpha particles onto a gold foil led to the model of...
h) He was the first to introduce a systematic classification of known chemical elements...
i) It is the atomic model that first explains the electrical nature of matter...
j) Associate a mathematical wave function to explain the behaviour of electrons...
4. Explain in your own words the experiment of bombarding alpha particles onto gold foils and what
differences exist, according to this, between the atomic models of Thomson and Rutherford:
5. Complete the following table:
Subatomic Atomic mass unit Discovery
Mass (kg) Charge (C) Charge unit Scientist
particle (amu) date
Electron (e-)
Neutron (n0)
Proton (p+)
