Never trust an atom, they make up everything! But, what is an atom and what is an atom made of?
Never trust an atom, they make up everything! But, what is an atom and what is an atom made of? Episode 2 reviews the history of the discovery of the atomic structure. It starts with Democritus who established the term “atom” (1:03), then fast forwards to Dalton, who provided experimental proof for Democritus idea (2:00). J.J. Thomson discovered the electrons (2:35), Ernest Rutherford protons (3:40) and Chadwick neutrons (4:30). The arrangement of these subatomic particles is described by Bohr’s Atomic Model (5:08).
Why was it significantly harder to prove the existence of neutrons? (6:22)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Our first few episodes will review some introductory chemistry content that is essential knowledge for the AP Course. Today’s episode will recap Atomic Structure and the history of atomic theory.
An old chemistry joke goes like this: Never trust an atom, they make up everything! Haha! But let’s get serious: If atoms make up everything, what is an atom and what is an atom made off? You are not the first to ask this question. We actually have to go waaaaay back in time to start looking for an answer. Therefore, this episode will recap the history of atomic theory and atomic structure as you will need it in your chemistry class.
Let’s zoom in!
We’ll start our journey about 450 BC - history-wise we are in classical greece - and that is when we find Democritus who first established the idea of atoms. In his thought process he hypothesized that when you keep splitting material you will eventually get to a point when you can no longer do so. Your particles are “indivisible” - or in greek “atomos”. Not very creative when it came to naming it, if you ask me… But… let’s think of making that paper plane again. Your frustration about it not flying very far doesn’t stop at ripping it once, not twice, but you keep going until you can no longer divide it - it is indivisible - at least for you. Democritus was a philosopher who influenced western culture in many ways, but his idea had one huge flaw: He didn’t have any scientific proof! Tststs... it was, however, a really good hypothesis, because it took more than 2000 years until scientists actually gathered evidence.
Fast forward in time and let’s travel to England to meet John Dalton, a chemist, physicist and meteorologist. Using improved balances, working with gases and establishing atomic weights, Dalton was able to provide some scientific proof for Democritus' hypothesis: all matter is composed of extremely small particles, atoms, which can not be subdivided, created or destroyed. Dalton’s work goes way beyond atomic theory and we will actually meet him again in another episode, when we talk about Dalton’s law of partial pressure!
We are now entering a few very exciting decades of discovery with scientists that actually knew each other! First, meet J.J. Thomson - a real “wunderkind”, who was admitted to the University of Manchester at the age of 14. 14! Can you imagine? Think about what you did when you were 14 - and let’s compare it to J.J. - or better not… Thomson set out to investigate electricity and performed a cathode ray tube experiment. The cathode ray was deflected by a negatively charged object - he therefore concluded that the ray itself has to be made up of negatively charged particles: electrons! Because their size was smaller than atoms and the knowledge that atoms itself do not repel each other, he proposed his “plum-pudding model” (And for those who didn’t live in medieval times - plum-pudding is a Christmas dish of boiled pudding with dried fruit - Yum) Let’s use something a bit more contemporary: a chocolate-chip cookie. The chocolate chips are the negatively charged electrons embedded in a dough canceling this negative charge.
Next, meet Ernest Rutherford! His studies focused on radioactivity, working closely with Hans Geiger and Ernest Marsden. You might have heard about the “Geiger counter” measuring ionizing radiation and named in Hans Geiger’s honor. These three set up the famous “gold-foil experiment” in which they bombarded gold foil with positively charged alpha-particles. If Thomson’s model was correct, the particles should move, more or less, undisturbed through the gold foil. But things didn’t go as planned - but when do they ever? - especially in science! A small portion of particles was deflected in wide angles, some even thrown back to the source! To explain these results Rutherford proposed that a very small part of the atom has to be made up by densely packed positive particles - welcome to the stage: protons and the atomic nucleus! Rutherford further hypothesized that the nucleus has to contain some type of “glue” so the like charged particles don’t repel each other. This was proven by his associate James Chadwick twelve years later.
Is there more? YES! We know now that Democritus and Dalton’s atomic theory has to be modified: atoms are actually divisible: we have positively charged protons as well as neutrons in the nucleus and negatively charged electrons surrounding them. But where are those electrons? That is a MUCH harder question to answer than you can imagine right now… but let’s introduce Niels Bohr, a danish physicist and student of Ernest Rutherford’s. To understand his atomic model, think about an onion - hey, there is no reason to cry! Bohr’s atomic model shows the atomic nucleus with protons and neutrons in the center of the atom. The electrons travel around the dense nucleus in defined circular orbits - like our onion rings!
And this is, where most physics and chemists students wish history had taken another 2000 year break - it didn’t! Bohr’s work laid the foundation for quantum mechanics, a field that made a cat very famous - even more than “grumpy cat”. But, in this recap, we are not going there - phew!
To recap……
Atomic theory was developed over the course of more than two thousand years. Democritus, a greek philosopher, hypothesized almost 500 years BC that everything is made of indivisible particles - atoms. In the 19th and 20th century this idea was challenged by scientific experiments showing that atoms are made up of positively charged protons as well as neutrons in the nucleus and negatively charged electrons surrounding them, travelling in fixed orbits.
Coming up next on the Apsolute RecAP Chemistry Edition: the Periodic Table
Today’s Question of the day is about Atomic Structure.
Question: Why was it significantly harder to prove the existence of neutrons?