DEFINING THE ATOM

QUESTIONS TO BE ANSWERED

THE ATOM AND DEMOCRITUS Atom – the smallest part of an element that retains its identity in a chemical reaction Democritus Greek philosopher ( B.C.) Was one of the first to suggest the existence of atoms Believed they were indestructible and indivisible Did not explain chemical behavior Not based on scientific method No experimental data Video segment

JOHN DALTON English School Teacher and Chemist Proved Democritus’ ideas by scientific experimentation He studied the ratios by which elements combined His results supported his hypothesis and theories

DALTON’S THEORIES Dalton’s work was not perfect. His idea of the atom had no internal structure. It was the work of later scientists who brought about this understanding.

LOOKING AT THE ATOM

SIZING UP THE ATOM Dalton believed if you took an element and kept breaking it apart over and over, eventually you would get to a piece of the element you couldn’t break apart anymore. This smallest part of an element is the atom Atoms are extremely small Most atomic radii are between 5* m and 2* m. A copper coin the size of a penny has approximately 2.4*10 22 atoms in it. Scanning Tunneling Electron Microscopes allow us to see things on this scale We even have the ability to move individual atoms around to create new and important new materials

QUESTIONS TO BE ANSWERED

STRUCTURE OF THE NUCLEAR ATOM By PresenterMedia.comPresenterMedia.com

ESSENTIAL QUESTIONS What are the 3 subatomic particles? Item 1 Describe Rutherford’s atomic model. Item 2

Dalton was mostly right. One exception: atoms are divisible There are 3 subatomic particles: 1.Electrons 2.Protons 3.Neutrons SUBATOMIC PARTICLES

Discovered by Sir Joseph Thomson Thomson Video ELECTRONS

Thomson realized a few things Something was attracted to the positively charged plate What did this mean was in the atom? He found that the charge to mass ratio was constant. And that it did not matter what gas was in the electrode. What does this tell us about all atoms? ELECTRONS

A scientist named Millikan furthered Thomson’s research. He discovered the mass of an electron was 1/1840 of a proton And an electron carries 1 unit of electrical charge ELECTRONS

Atoms have no net charge Electrically neutral Electric charges are carried by particles of matter Electric charges exist in whole number ratios Not fractional charges +1,+2, 0, -1, -2 etc. When equal numbers of positively charged particles and negatively charged particles combine, an electrically neutral particle is formed. ATOMS AND THEIR CHARGES

Think about this, if an atom is electrically neutral, what happens when you take away an electron ( a-1 charge)? What charge is left? A Hydrogen atom has one proton and one electron What must the charge of a proton be, if it has to counteract the negative charge of an electron? PROTONS

A scientist name Eugen Goldstein discovered protons in a cathode ray tube running in an opposite direction compared to the electrons. He surmised that these particle must be positive in nature These particles are called protons Protons are 1840 times the size of an electron The carry a positive charge of 1 PROTONS

James Chadwick confirmed another subatomic particle, the neutron They have no charge They are approximately the same size as the proton Scientists believe that these three subatomic particles can be divided even farther into subnuclear particles call quarks NEUTRONS AND QUARKS

ERNEST RUTHERFORD Rutherford was a student of Thomson. Protons (technically a helium nucleus, 2P & 2N) were shot at a thin sheet of gold. It was thought that the protons would go through with a little bit of deflection as the prevailing thought was that positive particles were scattered throughout the atom

ERNEST RUTHERFORD What happened was a surprise to all. Most protons went straight through, but a few were deflected. Rutherford theorized that most of the atom must be empty space because most of the protons went straight through (where the electrons are).

ERNEST RUTHERFORD He also theorized that there must be a positively charged small space at the center of an atom to account for the deflection of the protons. This pocket of protons and neutrons is called the nucleus.

REVIEW Rutherford’s Gold Foil Experiment Today

DISTINGUISHING AMONG ATOMS

ESSENTIAL QUESTIONS

PERIODIC TABLE KEY

All atoms are made up of protons, neutrons, and electrons But, every different element has a different number of protons This # is the atomic number Example: Oxygen and only Oxygen has 8 protons ATOMIC NUMBER So the atomic number is equal to the number of ________ and the number of _______ in each atom.. Since atoms have a neutral charge, how many electrons must be in each atom?.

We know the atomic # tells us the number of H + (protons) in an atom and the number of e - (electrons). Mass Number is more than that. It is how much matter that is in the atom. Protons, electrons, and Neutrons. Since Protons and Neutrons are the majority of the mass, the number is close to the number of protons and neutrons added together. (we will talk about the difference later) MASS NUMBER

The Atomic number tells us more than just the total mass. Since we know that atomic mass = all electrons, protons, and neutrons. What would we get if we subtract the atomic number? Remember, electrons have virtually no mass when compared to protons and neutrons. Mass Number How many neutrons in H? In B? In O? In Ne?

If Uranium has an Mass Number of 235 and 92 e -, what is the atomic #? Sometimes chemists write atomic mass, atomic number, and chemical symbol in a shorthand fashion. For example: Which is the big #, Mass#? Mass Number

Every element has a specific number of protons If there are a different number of protons, we have a different element. What do we have if a substance has the same protons but different neutrons? ISOTOPES How many neutrons In each?

What are the atomic #s of each of these isotopes? What are the Mass #s or each of these isotopes? ATOMIC MASS

Subatomic particles are extremely small. The mass of a proton is about 1.67* g The mass of a electron is about 9.11* g These numbers are really hard to use all the time, so scientists decided to use a standard reference. Carbon-12 (the most common isotope) has 6 protons and 6 neutrons. These scientists decided to make this element equal 12 atomic mass units (AMU)s 1 AMU = 1/12 of the mass of a Carbon-12 atom ATOMIC MASS

You may have noticed that the atomic masses are not whole numbers. A little of this difference comes from the electrons, but most of it comes from the isotopes. ATOMIC MASS AND RELATIVE ABUNDANCE * * Weighted Average

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ESSENTIAL QUESTIONS