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At a small scale everything is made up of atoms. Then why is it that objects can have different colours? and why are some objects soft and others hard?

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  • $\begingroup$ The question is a basic question that is why things made of atoms have different properties which may be a duplication of a lot of existing questions plus sort of a language/logic trick. $\endgroup$
    – jw_
    Oct 26, 2019 at 11:51
  • $\begingroup$ Idli and dosa are made up of the same thing- rice flour. Do they look the same? $\endgroup$
    – AlphaLife
    Oct 26, 2019 at 14:27

6 Answers 6

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Not all atoms are the same. Uranium is very different to Hydrogen and behaves very differently, because it has many more protons/neutrons in its nucleus and many more electrons. It is a much more complex atom.

Different atoms form different molecules, with different properties. When you compare the behavior of physical materials, you are really comparing the behavior of different molecules, not simply different atoms. Hydrogen gas is gaseous at room temperature and iron is solid, because their atoms form different molecules, with different types of chemical bonds (both atomic and inter-molecular).

In short: molecular behavior is much more complex than simply all being composed of equivalent atoms that look and behave the same.

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    $\begingroup$ To add to this, molecules can contain different sorts of atoms and can have radically diferent properties than either: hydrogen and oxygen are both gases at room temperature, while water, made from hydrogen and oxygen, is a liquid. Carbon is a solid at room temperature, oxygen is a gas, Carbon dioxide is also a gas. And there are quite literally an infinite number of possible molecules! $\endgroup$
    – user107153
    Oct 24, 2019 at 18:31
  • $\begingroup$ @tfb yes, good point. The molecular situation is even more complex than I described. There is a reason why people do PhDs in Chemistry :-) $\endgroup$
    – Time4Tea
    Oct 24, 2019 at 18:40
  • $\begingroup$ @tfb Is there really an infinite amount of possible molecules, assuming you're not counting regular metallic crystals or polymers of slightly different sizes as different molecules? $\endgroup$
    – nick012000
    Oct 25, 2019 at 9:44
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    $\begingroup$ A piece of coal and a diamond are made of the same thing but they look radically different. It's not just the composition of the molecule but also the structure. $\endgroup$
    – JimmyJames
    Oct 25, 2019 at 15:16
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    $\begingroup$ @JimmyJames yes, another good point! $\endgroup$
    – Time4Tea
    Oct 25, 2019 at 15:18
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I wanted to complement the answer by @Time4Tea with a little more specifics. In your question you mention colour and hardness, so I am going to expand a little about those two.

  1. Colour: Most matter we know is made up of chemical compounds (the only monoatomic compound you will normally find in stable form is Helium). The colours we observe come from 1) the absortion and emission of energy ─in the form of photons (corpuscules of light)─ of electrons in atomic/molecular energy levels. The energy of individual photons come from their frequency, every frequency corresponds to a distinct colour. Higher energies mean higher frequencies ($E=h\nu$). 2) the oscillation of atoms in the chemical compounds. The compounds have equilibrium lengths and angles, and the atoms vibrate about these equilibrium points at a given frequency. The color of the light produced by that same frequency (this is exploited in nuclear magnetic resonance which is applied to medical MRIs).

  2. The hardness is a consequence of the bonds between the atoms and molecules in solids. In the case of metals and ceramics the hardness reflects how much energy you need to put in to deform (or break) the bond between atoms. A stronger bond (higher binding energy) will make the material harder. In the case of polymers (i.e., plastics) the bonds are not those of the molecules themselves, but the Van der Waals forces between the chains, but the idea is the same.

So indeed, as @Time4Tea said, not all atoms are the same. This statement hold for compounds as well. This should give you a rough idea of why these properties are what they are.

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  • $\begingroup$ Those are always found in diatomic molecules. Helium is the only one found on its own. $\endgroup$
    – user137661
    Oct 24, 2019 at 19:50
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    $\begingroup$ Please make sure your statements are correct and precise to avoid confusion. $\endgroup$
    – my2cts
    Oct 24, 2019 at 19:57
  • $\begingroup$ You do have a point. I have corrected my answer to reflect your observation. $\endgroup$
    – user137661
    Oct 24, 2019 at 20:03
  • $\begingroup$ Pure elemental metals like iron, copper, gold, silver and mercury aren’t chemical compounds, though they’re not isolated atoms that don’t affect each other either. $\endgroup$
    – Mike Scott
    Oct 26, 2019 at 6:22
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If we ignore molecules for a moment and only look at reasonably pure samples of every element, you'll see that many of them look very similar.

enter image description here

It's a sea of slight variations of 'gray', with only a few solids that have different colors (copper, gold), and a few liquids and gases that add some color. The portraits in this image are elements that have been synthesized in minute quantities so there may not be enough of it to photograph.

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    $\begingroup$ This is a nice chart. Although, I take it that (unlike pets) we can assume that some of these elements don't look like the people they are named after .. ? ;-) $\endgroup$
    – Time4Tea
    Oct 25, 2019 at 17:55
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    $\begingroup$ may I direct you to the last sentence of my answer? $\endgroup$
    – Hobbes
    Oct 25, 2019 at 19:32
  • $\begingroup$ Interesting that the image doesn't have photographs of polonium (grey metal with a half-life of 125 years), technetium (grey metal with a half-life of 420,000 years), or curium (silvery metal with a half-life of 9000 years). Berkelium, californium, and einsteinium have also all been synthesized in macroscopic quantities, so there are probably photographs of them (yes, silvery or grey metals) floating around. $\endgroup$
    – Mark
    Oct 25, 2019 at 22:19
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Here is a way to generalize most of the answers given here by others.

What we experience of atoms in our everyday lives is determined mostly by the outermost electrons that those atoms possess. It is those electrons that determine how the atoms bind themselves to other atoms, and it is the specific nature of those bonds which in turn determine if the resulting solid is hard, soft, brittle, or ductile- or whether it is a solid at all. Furthermore, it is those outermost electrons that are responsible for giving color to some substances and either transparency or opacity to others.

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  • $\begingroup$ Do you mean that the emission spectrum is only defined by the valence electrons and the inner electrons and molecular vibrational and rotational energies do not matter? $\endgroup$ Oct 24, 2019 at 20:30
  • $\begingroup$ No. Only that to a good approximation, the properties of matter that humans experience in everyday life (i.e., electromagnetic interactions) are set by the energy levels available to the valence electrons. $\endgroup$ Oct 25, 2019 at 3:45
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The other answer did not go into the different colors (I am assuming you are only asking about visible light).

Now to our knowledge today, atoms are made up of quarks (and gluons) and electrons, and these are the elementary building blocks.

Now these elementary building blocks can be combined in different ways, building up different atoms.

These different atoms do have the ability to absorb and emit different wavelength photons (in your case different color light). This is about absorption and emission spectra.

The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state. The photon energy of the emitted photon is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element's emission spectrum is unique.

https://en.wikipedia.org/wiki/Emission_spectrum

The main thing is, each emission spectrum is unique. Each of the different atoms (that build up different molecules) has the ability to absorb and emit different light.

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    $\begingroup$ "atoms are made up of quarks (and gluons) and electrons" Atoms are made of nuclei and electrons. Nuclei are made of protons and neutrons. Protons and neutrons are made of quarks and gluons. It all started with the big bang some 14 billion years ago. Not that this is relevant to the question at hand. $\endgroup$
    – my2cts
    Oct 24, 2019 at 19:49
  • $\begingroup$ Almost all the elements are colorless, grey or silvery though periodictable.com/Properties/A/Color.html $\endgroup$
    – user126527
    Oct 25, 2019 at 9:50
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At the risk of being shouted down, I have to point out that OP is in fact correct to a very large degree, the only thing OP has lost track of is scale.

Every one of the relatively few fundamental objects (lets stick with atoms/elements here) has its own colour and hardness (still sticking with OP here). But the 'objects' OP refers to as having such tremendous variety are not fundamental. They are macroscopic aggregations of a very large number of interactions, observed on the macroscopic scale with instruments/senses not designed, or sensitive enough, to detect the fundamental properties OP complains about not being able to see.

Sticking with colour (and ignoring the complexity of how colour actually arises), a 'red' atom and a 'yellow' atom viewed from OP's distance and on the continuous colour scale in OP's brain looks like an 'orange' lump (molecule). Go and stand close enough and OP will see the atoms. Use a quantised method of observation (spectrograph?) and OP will only see the original pure 'red' and 'yellow'. Add another 'red' and everything looks pretty much the 'same' up close but zoom back out and suddenly there is is another colour shade. The apparent complexity of colour exists only on OP's brain colour scale and at his distance of observation.

If the atoms are nailed together with only long/short or rubber/steel nails hammered in to different depths then complexity rapidly arises from simplicity for 'hardness'.

Bottom line. OP perceives apparent complexity on the macroscopic scale because that is where OP lives and it is what the brain was designed to do. OP is not able to see the pure simplicity of the microscopic (ultimately the holy grail of unified field theories, ToE or whatever the flavour is called this week) because OP is not equipped to detect it and is standing too far away anyway.

An aside. If Homo Physicus ever evolved the senses able to detect the absolute fine detail of the universe he would be dead within femto-seconds. Not only because of the boredom of having to observe an almost infinite number of properties to find the one that is interesting; not only due to starvation because there are an infinite number of interesting events to go through in order to find lunch; not only because of the Total Perspective Vortex; but most likely because the Darwinian survival value of knowing that '..it's all pretty much the same, you know...' is not as valuable as avoiding that bit of the universe over there that looks like a bus/sabre-toothed tiger .......

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