Atomic Structure and History of Atomic Theory Chemistry 10/6/14

Drill In the standard form of each element, calculate the number of protons, neutrons, and electrons: FeSnHgAuSb Bonus Question: What do these elements have in common? HW: pg. 3 – 3-3 Rev & Rein WS

Objectives IWBAT Find or calculate atomic mass, atomic number, number of protons, neutrons, and electrons, charge, etc. for any atom Trace the development of Atomic Theory from Democritus to the present day. NOTE: Quiz#3 on Atomic Structure on Friday, 10/10

HW Review Let’s go over pg. 2 – 3-3 Prac Prob WS

Democritus’ Theory Democritus was an ancient Greek philosopher—he lived from 460 BC to 370 BC He did NOT agree with Plato’s view of the universe: All things were made of fire, water, air, or earth. Every substance is made up of tiny, indivisible (unbreakable) particles called atoms. A piece of paper is made of paper atoms. A leaf is made of leaf atoms. Skin is made of skin atoms. These atoms cannot be broken down any further. Was there any science involved in this model? NO! Democritus was a philosopher, not a scientist.

Before Dalton Antoine Lavoisier – Law of Conservation of Mass Mass is never gained or lost in a chemical reaction. Joseph Proust – Law of Constant Composition A compound always contains the same elements in the same proportions, by mass. For example: Water is always H2O – 88.9% oxygen, 11.1% hydrogen Hydrogen peroxide is H2O2 – 94.1% oxygen, 5.9% hydrogen

Dalton’s Atomic Model Atoms are indivisible units of matter that are unique for each element, and combine to make various compounds. A lump of gold is made of tiny gold pieces (atoms). Carbon dioxide is made of tiny carbon and oxygen pieces (atoms) combined together. These atoms cannot be broken down further. Was there any science involved in this model? YES! John Dalton did many scientific experiments, especially with gases, to formulate his ideas.

Dalton’s Atomic Theory of Matter Four Postulates: All matter is composed of extremely small particles called atoms. Atoms of the same element are identical, and atoms of different elements are different from those of any other element. Atoms are neither created nor destroyed in any chemical reaction. Atoms combine in definite proportions (the same proportion) to form compounds. Ex. CO vs. CO2

J. J. Thomson – 1898 Thomson took advantage of new technology – the cathode ray tube. Cathode “rays”/particles – negatively charged, very light (but they do have mass— Milliken’s Oil Drop Experiment), came from atoms.

Thomson’s Discovery Thomson knew atoms were usually neutrally charged. So, he realized that if he could remove a negative particle, there must be a positively charged part also.

Thomson’s Conclusions Thomson’s model of the atom is heterogeneous and solid, but NOT indivisible. Electrons can be removed, so it can be divided. The Plum Pudding Model – Solid sphere of positive charge with electrons spread through it like raisins or plums in a plum pudding.

Rutherford’s Gold-Foil Experiment

Rutherford and Radioactivity Ernest Rutherford used alpha particles to investigate the gold atom in We will learn more about alpha particles next week. He conducted a famous experiment that proved that atoms are not solid spheres as proposed by Dalton, or “plum- puddings,” as Thompson said.

The Alpha Particles Experiment The Alpha Particles Experiment In his experiment Rutherford used a very thin sheet of gold as a target, and he shot a beam of alpha particles at the gold. alpha particles (  -particles) are helium nuclei without electrons, and are a form of radioactivity Nearly all of the alpha particles passed through the thin sheet of gold. Rutherford also noticed that a few particles were deflected from their straight- line path, some by 90 degrees or more.

Conclusion The beams were bouncing off the positively charged core of the atoms. Rutherford originally called this a proton, because it was positively charged. Later, it was renamed the nucleus. The nucleus of the atom has almost all the mass of the atom. Therefore, most of an atom is empty space.

Bohr Model (the one you all know)

Niels Bohr Bohr said electrons were in set orbits, like rungs of a ladder. You can only be on one orbit (rung) or the next, not in the middle -- they are quantized Electrons can only move from one to the other by emitting or absorbing a particular wavelength of light (a photon with a certain energy), similar to the metal in the photoelectric effect.

Picture by Atomic Excitation

Schrödinger's/ Modern model of the atom

Particle-Wave Nature of Matter Light has a particle nature (Einstein-1905): photon – quantum of energy that behaves in some ways like a particle Matter has a wave nature (De Broglie-1924): Electrons scatter like X-rays Electron microscopes use this property to see very small things.

Heisenberg’s Uncertainty Principle The position and speed of a moving object cannot BOTH be known exactly at the same time.

Heisenberg, Cont. Why? The act of observing changes an object. To see things, we must bounce light off of them. To observe an electron, we have to bounce light off of it. The light would move the electron so much that we would not be able to tell where it was, or how fast it was going. Think of hot tea and a cold thermometer.

A new idea! Orbitals: A region around the nucleus of the atom where an electron of a given energy is likely to be found. We don’t know exactly where electrons really are, just where they are likely to be. Also called an electron cloud or probability region.

So, what’s an orbital?? Orbit  Orbital ex. Electron is like a bee around a beehive.

What is another metaphor you could use for the Plum Pudding model? How many electrons would Fe 3+ have? Closure