Atomic Structure Chemistry Chapter 4

The Atom The smallest particle of an element that still behaves like that element Ex) The smallest bit of gold that still has the intensive properties of gold. Started with the philosophy of Democritus Atoms are indivisible and indestructible!

Dalton buys the Democritus hype. John Dalton uses experiments to come up with 4 ideas about the atom Elements are composed of indivisible atoms. All atoms of the same elements are identical. Atoms of different elements must be different. Atoms of different elements can be combined together in ratios to form compounds. Chemical reactions occur when atoms are separated, attached, or rearranged. But atoms aren’t changed, only rearranged.

What are atoms made of?

What are atoms made of? Pens!

The Parts of the Atom Three particles have been discovered to be part of the atom Protons Electrons Neutrons PENs! Okay, not really, but hopefully it will help you remember.

http://www.youtube.com/watch?v=IdTxGJjA4Jw

Thomson’s Discovery Thomson used electricity to create a stream of particles (cathode ray tube) He could deflect their flow with positively charged plates The stream was pulled towards positively charged objects Must be negatively charged! Thompson had discovered that atoms MUST have a negative particle. Electrons

http://www.youtube.com/watch?v=XU8nMKkzbT8&feature=related

More about the electron… The mass must be very small (1/1840th of the mass of hydrogen) The charge must always be the same Regardless of the element Must be part of all elements

http://www.youtube.com/watch?v=wzALbzTdnc8&feature=channel&list=UL

The electrons get friends! Rutherford shot positively-charged alpha particles at gold Most went straight through But some bounced straight back, or sideways There must be a solid positively charged “kernel” at the center “The nucleus” made of protons Chadwick discovers that there is also uncharged particles that make it heavier “Neutrons!” Have basically the same mass as protons

The atom gets a “shape” Basically all empty space Nucleus at the center Very, very, very small Positively charged protons Neutrally charged neutrons 99.995% of the mass Electrons revolving around the outside Zooming near the speed of light, with very little mass Require more space than the nucelus

If time permits http://www.youtube.com/watch?v=xqNSQ3OQMGI

Atoms! Dalton was right that different elements would have different atoms. But some different atoms are actually the same elements! What determines the element? THE NUMBER OF PROTONS! Oxygen is oxygen because it has 8 protons. Nitrogen is nitrogen because it has 7 protons.

Atomic Number The number of protons is called the “atomic number” Elements are separated and arranged specifically by their atomic number No two atoms with different atomic numbers can be the same element! EVER! EVVVVVVVVVVVVAAAAR!!!!!! For neutral atoms, the protons and electrons are equal

Mass Number Mass number is how “heavy” the nucleus of an atom is The number of protons plus neutrons The two sub-atomic particles that have a large mass Protons + Neutrons = Mass Neutrons = Mass – Protons The mass number doesn’t affect what element it is EX) Hydrogen can have a mass number of 1, 2 or 3, but it’s atomic number is ALWAYS 1! Mass number

Okay, lets try some! I have 14 protons. I also have 15 neutrons. What element am I? I also have 15 neutrons. What is my mass?

Okay, lets try some! I have 14 protons. I also have 15 neutrons. What element am I? Silicon! I also have 15 neutrons. What is my mass? 29 Therefore I am Silicon-29

How about another one? My mass is 14, my atomic number is 6. What element am I, and how many neutrons do I have?

How about another one? My mass is 14, my atomic number is 6. What element am I, and how many neutrons do I have? I am CARBON-14, and I have 8 neutrons 14-6 = 8!

Isotopes The number of neutrons can be different in an element. Atoms of the same element MUST have the same number of protons The number of neutrons can be different in an element. Ex) Carbon-12 6 protons, 6 neutrons Ex) Carbon-14 6 protons, 8 neutrons Isotopes of elements have different numbers of neutrons, and therefore different numbers of neutrons But Isotopes are still the same element, just different masses

Consider the following… Mass number is just a COUNT of the number of subatomic particles that have mass. Ex) 9 protons + 10 neutrons is 19 masses It gives you a very close, but general idea of the mass of an atom. But it is not exact.

Atomic Masses Units (amu) The mass of protons and neutrons is not always exactly 1 (but really close) The only one that they are is Carbon-12, and everything else is based off that. Neutrons are (usually) a little heavier than 1, protons less So we calculate based on “atomic mass units”, which is based off of Carbon-12=12.000 Then we calculate the mass of all the other atoms (sorry) Mass = 14.003241

So, atomic mass is affected by… The element The number of protons compared to neutrons The number of electrons (barely) But as a general rule it is the protons plus the electrons-ish

(Average) Atomic Mass We can’t fit ALL the isotopes on a periodic table and we don’t need to because the isotopes are usually all mixed together with each other. So, what we do is take the AVERAGE amu, and use that as the mass of an element.

Calculating average AMU You could find every single atom in the universe and get the mass, then divide by the number of atoms. Or you could determine what fraction of the atoms are which isotope, and determine the average. “Weighted average” Average AMU (with isotopes A, B, and C) = (% of A)(mass of A) + (% of B)(mass of B) + (% of C)(mass of C) % MUST BE WRITTEN AS A DECIMAL EX) 65% = 0.65 This is where the significant digits of the AMU come from.

Calculating average AMU practice Carbon has 3 major isotopes. Actually, it has 16 known, but 13 are very, very, very rare Carbon 12 AMU: 12.00 (repeating) 98.928% Carbon 13 AMU: 13.00335483781 1.071% Carbon 14 AMU: 14.0032419894 ~0.000 000 000 001% Don’t forget to turn % into decimal before doing math!