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Course: Biology archive>Unit 2

Lesson 1: Elements and atoms

Introduction to the atom

Learn how atoms are made up of protons, neutrons, and electrons. Elements are defined by the atomic number, the number of protons in the nucleus. The mass of an atom is determined by the total number of protons and neutrons.  Created by Sal Khan.

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• Why doesn't light pass through objects if 99.99% is nothing or free space?
• Because light interacts with the electromagnetic fields the objects generate. Some objects (like glass) easily allow light through, some do not.
• what is the free space made up of?
• Free space is made up of absolutely nothing.
• If a piece of string is mostly free space, then how come we can pull it without it breaking?
• The same force which is keeping the string together in the first place.

The chemical bonding in the string between individual atoms forms the string itself, by pulling on the string, you put strain on the bonds and if you pull hard enough, the bonds would break ultimately causing the string to snap in two.
• If you were to use a knife to cut a piece of string, would there be a chance of an atom and its electron to split apart?
• no, as an atom completely splitting apart is nuclear fission.. and that causes chain reactions... thats why its not possible unless atoms collide in like special conditions and a knife is just too broad to cut an atom
• If you were somehow able to, what would you get if you got rid of all the free space in an atom and compressed everything together (The protons, electrons, and neutrons)?
• Its called a singularity, otherwise known as a black hole.
• Aren't atoms made up of smaller pieces? It has to be made up of SOMETHING right? Could someone show me a biggest to smallest chart type thing?
• http://htwins.net/scale2/
That's a link for a scale of the universe. It's quite interesting actually.
• How do we know the average mass number of elements on earth? Is it just an estimate of sorts?
• Yeah, basically. We assume a more or less random distribution of the elements throughout the earth, so in any given sample, we just need to have enough atoms to ensure that our sample average is equivalent to the population (i.e. all atoms of that type) average. Since atoms are so tiny, it doesn't take very much mass of a sample to have enough atoms for that assumption to be pretty good...
• why do we see and feel things like a wall or a table as a solid object when they are really mostly free space
• Basic rundown: The negatively charged atoms in your hand are repelling the ones in the table.
• I had a doubt. Sal says that all of the protons stay inside the nucleus. But if there are protons staying together inside the same nucleus wont the protons repel each other?Then they should move apart and the nucleus must blast. But the atom is quite stable.
• You are quite correct, the protons DO repel each other due to having like electromagnetic charges. However, there is another force at work. It is called the strong force (really, that is its name). Both neutrons and protons carry the strong force (but electrons do not). The strong force is what binds the nucleus together, by overcoming the repulsion between the protons.

But an atom must have just the right balance of protons to neutrons to make a stable nucleus. If there are too few or too many neutrons, the nucleus won't be stable. The details about which combinations of protons and neutrons are stable and which are not is a very advanced topic and is not usually covered in General Chemistry. But, if the ratios are not exactly correct, then the atom will be radioactive and undergo decay.
• Hey did you guys know that:

Silicon is one of the most useful elements to mankind. Most is used to make alloys including aluminium-silicon and ferro-silicon (iron-silicon). These are used to make dynamo and transformer plates, engine blocks, cylinder heads and machine tools and to deoxidise steel. Silicon is also used to make silicones.

and

Chlorine is used to disinfect water and is part of the sanitation process for sewage and industrial waste. During the production of paper and cloth, chlorine is used as a bleaching agent. It is also used in cleaning products, including household bleach which is chlorine dissolved in water.

And

Xenon is used in certain specialised light sources. It produces a beautiful blue glow when excited by an electrical discharge. Xenon lamps have applications as high-speed electronic flash bulbs used by photographers, sun bed lamps and bactericidal lamps used in food preparation and processing.

AND

Germanium is key to fibre optic cables and also used in high-speed computer chips and plastics as well as infrared radiation. The metal and its oxides are used in military applications like night-vision devices as well as satellite imagery sensors. It is also important for low-carbon technologies such as solar cells.