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I made an earlier video where I talked about why adding or subtracting just a single proton results in a completely different element. The highly simplified answer is because the number of protons determines the number of electrons, and it is the number and configuration of these electrons that determines the chemical properties of an element. There are a lot of details to this, such as why we don't classify elements based on electrons and why there are electron configurations to begin with. That's all answered in my earlier video. If you haven't seen it, I encourage you to check it out. I'll leave a link in the description.
The question that I did not answer in that video is how and why these elements have different physical properties, such as why some of them are gases, some are liquids, or why some are solids. Why are they different colors and so on? Why are physical properties so drastically different among these elements, even when the proton number just differs by one? What I want to get to the bottom of in this video is what is the core reason for these physical differences. Stay tuned because that's coming up right now.
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So, the simple answer to the question is that it's the element's nuclear properties and behavior of its electrons that determines not only its chemical properties but also its physical properties. But this by itself is of course not a satisfying answer. We want to know how the behavior of electrons results in the different physical properties of any given element.
First, I think you should keep in mind that typically when we're talking about the physical state of anything, such as why there are gases, liquids, or solids, we're really asking what their state is at room temperature and atmospheric pressure. We'll define this "quote unquote" standard condition to be around 20°C or, for our American viewers, around 68°F, and one atmosphere pressure at sea level, which is about 101 kilopascals or 14.7 PSI (pounds per square inch). So, if I'm talking about why helium is a colorless gas, why mercury is a silvery white liquid, or why iron is a shiny gray metal, they are only this way at the standard temperature and pressure I defined. If the temperature were above 1538°C, iron would be a glowing white liquid. On the other hand, if the temperature were below -39°C, mercury would be a silvery solid. And if the temperature were below -269°C, helium would be a transparent liquid. And likewise, the temperatures of these transition states would be different at different pressures as well. So, elements can be in different phases depending on the combination of temperature and pressure.
Let's now use the examples of these same elements to look at why their physical properties are this way at the standard conditions we defined. In general, the principles governing the properties of these three elements can be extrapolated to cover most of the other elements in the periodic table. So, what you learn by understanding what I'm about to talk about can generally be applied to any other element that you might be curious about.
So, to understand what these principles are, we have to start with the electron configuration of these three elements. In the case of helium, it has two protons and thus two electrons in its neutral state. The first two electrons of any element occupy the first electron shell. This makes the shell complete because two is the maximum number of electrons that can fit in this first shell. To understand why two is the maximum, see my first video. The link is in the description. The simplified answer is because of the rules of quantum mechanics, as determined by the Schrödinger equation and the Pauli Exclusion Principle.
Because the shell has a complete set of electrons, helium does not have the propensity to take on any extra electrons or share its electrons with other elements or with other helium atoms. In other words, it doesn't react with anything. It is extremely stable, meaning it is in a low energy state. Sharing one of its electrons with another element or taking on an extra electron would put helium in a higher energy state, which is not favorable. In physics, a core concept in physics says that all systems tend toward their lowest energy state at any given temperature and pressure. So, helium atoms at room temperature, since they are not interacting with anything, simply float around as individual atoms. Their non-reactivity with other atoms prevents them from forming a condensed state like a liquid or metal. The same principle generally applies to all the other noble elements. They're all gases at our standard conditions because, in general, they don't interact with anything.
You might say, "Well, what about the other non-noble gases, such as oxygen and nitrogen? Why are they gases?" That's a great question, and there's a good explanation for it. But before we get into that, I want to say that although we do extensive research to get our facts correct and make sure everything is fact-checked by experts, not everything you read online is necessarily accurate, nor free of bias. This is where today's sponsor, Ground News, can be an invaluable resource. Take, for example, a controversial subject like climate change. It can be difficult to sort through biased perspectives, but Ground News shows you what those biases are likely to be. This recent news discussing how 2023 was the hottest summer in 2000 years is a great example of that. Although 107 news sources reported this, only six right-leaning media outlets were writing about it. This kind of political breakdown is unique to Ground News and provides better context for the news you consume. Ground News was created by a former NASA engineer to help us navigate the rapid spread of information that's often hard to verify. My favorite feature of Ground News is this section called "Blind Spot." It presents information that you may be missing because it's contrary to your political leanings. No matter how impartial we think we are, we all have biases, and I think it's very important to understand opposing perspectives. In my opinion, Ground News is doing really important work, and I hope you'll check them out. Go to ground.news/arvinash to give it a try. Sign up through my link so you can get 40% off the Vantage plan, which is what I use to get unlimited access to all the features. Click the link in the description to get started.
Now, back to the question of why non-noble elements like oxygen and nitrogen exist as gases. Too, these elements exist as gases not because they don't interact, but because they prefer to interact with themselves to form stable molecules. So, an oxygen atom interacts with other oxygen atoms to form O2 because each needs two more electrons in its outer shell in order to have a full shell and be more energetically stable. So, they share two electrons with each other. Once this O2 molecule forms by creating a double bond, it's quite stable. They interact very weakly with other O2 molecules, so the molecules do not form a condensed state like a liquid or solid, but stay in the gas state at our standard conditions. The same general concept applies to nitrogen. It also exists as N2 gas at room temperature because it is more energetically stable as a molecule than as a single atom. Nitrogen has five atoms in its outermost shell. By sharing three electrons with another nitrogen atom, both atoms can achieve a full outer shell and have a stable electron configuration. This sharing results in a rather strong triple bond, and because the molecule is so stable, it interacts weakly with other N2 molecules, and so, just like oxygen, does not form a condensed state at room temperature.
Now, why are all these three gases colorless instead of having a color? This has to do with the frequency of photons that are emitted or absorbed from the gas. The reason we don't see any color for helium gas or oxygen or nitrogen is because they don't absorb or emit any light at standard conditions. Since light simply passes right through these gases, they appear colorless and transparent. But not all gases are colorless. Fluorine is pale yellow, for example, and iodine is dark violet. Why is that the case? Why are they colored? Because of their electron structure. Both these gases have an unpaired electron that is present in the outermost shell of the atom. When photons, that is light particles, of suitable energy hit the atom, the electron gets excited and moves to a higher energy state in the atom. So, light of a certain energy or frequency gets absorbed. What we then see as the emitted light is the complementary color of the absorbed light because that's the light that passes through unabsorbed. Different electron structures absorb different frequencies of light because the amount of energy required for exciting the electron varies depending on the atom. Fluorine, for example, is a smaller element. The force of attraction between its nucleus and the outer electrons is very strong. As a result, it requires a large excitation energy, and so it absorbs high-energy violet light, and so the light that's emitted or light that we see is the complement of violet, which is about a pale yellow. On the other hand, iodine needs significantly less excitation energy and absorbs yellow light of low energy. So, its complimentary emitted light is dark violet, which is what we see as the color of iodine gas.
But of course, not all elements are gases. Why is mercury a liquid but not a solid? In general, an element is liquid when the constituent atoms are able to slide over each other and move. In other words, the interatomic force of attraction is relatively weak, still stronger than a gas, but only strong enough for the atoms to stick to each other, but not strong enough to stop movement. So, its atoms can still slide over each other and do not retain a definite shape. A substance is only solid when its atoms are held together by stronger interatomic forces such that the atoms, for the most part, stay put and can't slide past each other.
Both mercury and iron are metals, but mercury is very unusual in that its melting point is lower than any other metal, and so it's a liquid at room temperature. Why? The short answer is because it doesn't like to share its outermost electrons. This can be explained again because of the electron configuration. What you'll see is that its subshells are filled up to their max capacity, and in its outermost shell, the 6s shell, lie two electrons. These two make a full orbital, which tends to make the elements stable, so they resist removal. Since they resist removal, they're not so readily available to be shared with other mercury atoms, which would be needed to make a solid. The consequence of this is that there is a relatively weak bonding between mercury atoms, and so they can move around like making a liquid instead of being stationary like in a solid.
Now, some textbooks mention a relativistic effect that also plays a role in making the electrons of the outermost shell of mercury less available for sharing. They say the 6s shell electrons have a high orbital speed, about 58% the speed of light, resulting in a gamma factor of 1.23. This means they gain a relativistic mass, which adds about 23% to the rest mass of the electrons. Since the orbital radius is inversely proportional to mass, this higher mass results in a smaller radius, bringing the electrons closer and more strongly bound to the nucleus and making them less available for sharing.
Now, if you've seen my other videos in quantum mechanics, you might object to this description because you know that electrons are not like little balls that orbit the nucleus like a planet. The picture of electrons being excitations in a quantum field and existing in clouds is more accurate. But there is a real relativistic effect that happens in some large atoms like mercury and gold, depending on their electron configuration. You can think of this as a particularly high energy carried by these electrons, and since energy and mass are equivalent, this adds a relativistic mass to the electrons, keeping them closer to the nucleus. The end result is the same as thinking of the electrons as traveling fast in an orbit, but the quantum description is more accurate.
Iron atoms, on the other hand, form strong metallic bonds with each other. Consequently, the atoms don't move much, so it's a solid. We can see why it forms strong bonds by looking at its atomic structure. The electrons in its outermost shell are not strongly bound to their nuclei and become delocalized, meaning that they are free to move in the structure and be shared among many iron atoms. This is what a metallic bond is. In a metallic bond, such as in iron, each atom is surrounded by many other atoms, and they all share their valence electrons. The shared electrons move around, hopping from atom to atom, lowering the energy of the entire system. This also gives metals their typical properties, such as the ability to conduct electricity, since all the electrons are free to move around and carry electrical energy. They're also generally malleable because the interatomic bonds can bend and move slightly. The strength of these metallic bonds is different depending on the size of the metal ions, the charge of the ions, and the number of valence electrons in the atom. These factors are responsible for the different physical properties of the different metals. So, for example, iron has more valence electrons per atom than sodium, and so it has a stronger metallic bonding, resulting in iron being a harder metal than sodium.
What about their color? Why are most metals gray with a few exceptions? This is a direct result of photon absorption by electrons in the D orbital of most metals. This energy absorption results in the D electrons making a jump to the higher S orbitals. This typically requires very high energy levels to happen, so only high-energy ultraviolet photons enable this transition. Photons with lower frequencies in the visible spectrum are not absorbed and simply reflected. This reflected light is what we see, which is typically a silvery gray, which is just a shade of white. Any mirror-like metallic structure we see is due to the metal surface features. A smoother polished surface can result in a shinier color.
Now, some metals, such as gold, have a different color because their D electrons require a lower energy to move to an S orbital. So, gold, for example, absorbs energy in the visible blue range of the spectrum, so it reflects the rest of the visible spectrum, which is the yellow hue we call golden. An interesting note is that a similar relativistic effect as what I mentioned earlier for mercury plays a role in the color of gold too, by keeping the S orbital closer to the nucleus, resulting in lower energy being needed for the D to S transition. This is also why gold has a lower melting point compared to many of the other metals and is softer.
If you enjoyed this video, be sure to check out part one of this video where I talked about how a single proton results in different elements. The link is up here. I'll see you in the next video, my friend.
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