The Atomic Nature of Matter Chapter 17

Legal Stuff Parts of this lecture are from a website that I have linked to our class website. It is an excellent resource and should be viewed by all students.

Background Info One of the main quests of science throughout the ages has been to discover what matter is made of - and what holds it together. All matter is made out of many tiny particles called atoms. The study of how these atoms interact is called chemistry. Modern physics has discovered how atoms are made up of smaller particles and how these particles interact to build atoms into molecules and larger objects of matter.

Background Info The philosopher Democritus came very close to our modern understanding when he proposed that everything is made up of very small particles, which he called atoms, from the Greek atomus, for “indivisible”. In the 1600s, Sir Isaac Newton and other scientists experimented with materials, light, and heat, and developed many basic laws of physics. By the early 1800s, the theory of the atom became popular.

Background Info By the 1900s, scientists had discovered that atoms are themselves made of even smaller particles. This century research in particle physics has taken us deep into the heart of the atom, far beyond the limits of the most powerful conventional microscopes.

Elements Atoms are the building blocks of matter. An element is a class of identical atoms – a material whose atoms are all exactly the same.

Elements Hydrogen was the earliest atom, formed when quarks and an electron came together shortly after the Big Bang. 90% of all the atoms in the universe are still Hydrogen. Other heavier atoms were formed by smashing hydrogen atoms together; this took place in the early stars. There are 88 naturally occurring basic types of atoms--called elements. Carbon (C), hydrogen (H), oxygen (O),and nitrogen (N) are the four most abundant elements in living things.

How big is an atom? A typical atom is about one millionth of a millimeter across - a million of them laid in a line would measure one millimeter across. The assigned reading in your textbook has explanations of how small atoms are.

How do we know atoms exist? Have you ever noticed that tiny particles in a fluid move about randomly? Even when the fluid is standing still… why does it do that? This is called Brownian Motion, named after a botanist named Robert Brown. In 1905, Einstein explained that Brownian Motion of dust in a liquid is because atoms are bouncing off of the dust particles. Many leading scientists did not believe in atoms until this paper!

Molecules When atoms stick together to form larger particles, we say they are forming molecules. Most substances that are liquid or gas at room temperature are made of molecules, not just atoms.

Molecules In chemistry and physics, we sometimes draw molecules like this so the formula for making the molecule is easy to see. This molecule, by the way, is caffeine.

Molecules Some molecules are extremely large and complicated.

Compounds A compound is a substance that is made of different elements combined in a specific proportion. Example: water is H2O, not H3O or H4O.

What does an atom look like? Scientists figured out that there are two different kinds of electrical charge present in atoms (+ and -), and that a normal atom has an equal number of each. They assumed there must be a negative particle (the electron) stuck in a positive mushy substance, like pieces of fruit in a pudding.

What does an atom look like? This was known as the “plum pudding model”.

Ernest Rutherford Physicist Ernest Rutherford decided to figure out more about the atom. By the way, Rutherford liked physics a lot but wasn’t impressed with chemistry. He didn’t seem to think the periodic table was real science.

Gold Foil Experiment He fired a stream of positively charged “alpha” particles at a strip of gold foil and measured the angle at which the particles were deflected.

Gold Foil Experiment Based on the unexpected results of his gold foil experiment, Rutherford came up with a new model for the atom.

What does an atom look like? We have improved upon Rutherford’s model, but it is still the basis of how we think of atoms – a positive nucleus with orbiting negative charges.

Elements The number of protons in a nucleus determines what element that atom is. It cannot be changed without changing what the element is (like changing lead to gold). The number of protons in an element is what we call its atomic number. For example, the atomic number of Carbon is 6.

Sub-Atomic Particles With this understanding that the atom is not actually the smallest particle (just the smallest we can get without changing the element), scientists began investigating Sub-Atomic particles. There are three sub-atomic particles: 1. protons 2. neutrons 3. electrons

Sub-Atomic Particles Protons and neutrons are nearly identical. They are the same size and mass. They both reside in the nucleus. What is the difference between a proton and a neutron? With what you know about elements, you can determine what the nucleus of any element looks like.

Table of Elements How many nucleons are there in Carbon? How many of these are protons? How many must be neutrons?

The nucleus Protons and neutrons have roughly the same mass, and each is about 2000 times as massive as the electron. For any given element, all nuclei have the same number of protons, but the number of neutrons can vary from atom to atom. If the nucleus of an element has a different number of neutrons than it usually does, that particular nucleus is called an isotope.

Nuclear Structure Just as we have discovered that atoms are not the smallest particles, we have also discovered that protons and neutrons are made of even smaller particles. These particles are known as “quarks”.

Quarks There are six types of quark. The way they combine determines whether they are making a proton, a neutron, or some other exotic subatomic particle. As far as we know, there is nothing that makes up a quark. At this point, it seems to be the smallest particle there is. What you need to know is that quarks make up protons and neutrons, and are the reason protons are charged while neutrons are not.

Quarks

Electrons The number of protons is the same as the number of electrons, which orbit the nucleus. Electrons can only exist along very specific paths, called “shells”. Each shell can only hold a certain number of electrons before it is full, and a new shell will be needed farther out.

The periodic Table of the Elements is arranged the way it is because of the arrangement of electrons in their shells.

Electrons If a proton is positively charged, and an electron is negatively charged, causing a force of attraction between the two… …why don’t the electrons move into the protons and actually hit them?

States of Matter Matter can arrange itself in different ways depending on how much energy the atoms or molecules have, and how they attract one another. There are four states of matter that you will need to know about.

States of Matter

Homework Read ch 17 and do problems #1-25 on pages 249 and 250. Due 2/6