| Most materials consist of more than one
type of substances. For example, even pristine sources, water has minerals
and gases dissolved in it.
Heterogeneous (multiphase, lumpy) Vs. Homogeneous (single phase, smooth) Homogeneous Materials
Classes of Substances
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Characteristics of Matter
Chemical Properties
Physical Properties
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Atoms and Molecules
So far, we’ve been using our particle model with energy to explain stuff like density, conservation of mass, diffusion, and change of state. Our model is essentially the model proposed by Democratus in ancient Greece. According to this model, different materials like gold or water were made up of water or gold particles. By the 17th and 18th centuries, scientists were performing experiments that provided evidence that some substances were made up of different particles combined in definite ratios. Joseph Gay-Lussac discovered that gases react in simple whole number ratios. For example 2 volumes of hydrogen gas react with one volume of oxygen gas to produce 2 volumes of water vapor. We verified this in class with the electrolysis of water demonstration. In the Ring of Truth video, we saw how hydrogen and oxygen could only combine in a 2:1 ratio to produce water. These results led John Dalton to modify Democratus’s model. He proposed four principles about the nature of matter: 1) All matter is made up of tiny indivisible particles called atoms. 2) An element is made up of atoms that are all identical. 3) Different matter consists of different kinds of atoms. 4) Elements combine in simple whole number ratios to form compounds (Law of Definite Proportions). When compounds are broken down, the combined elements are recovered. |
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Adding Detail to the Model: JJ Thomson
Matter and electricity were originally thought of as being two separate materials. The first connection between the two was made by JJ Thomson in the 1880s. Thomson studied cathode rays made from a cathode ray tube. The cathode rays originated from a hot plate exposed to a negative charge. These rays were actually extremely small particles. These particles were deflected by a magnetic field in such a way as to confirm that they were negatively charged. These particles were named electrons. |
| JJ Thomson’s model of the atom was called the plum pudding model. He proposed that electrons were embedded in a positively charged “pudding”. |
| Our results from the
sticky tape experiment are consistent with Thomson’s model. Matter and
electrical charge are related. Atoms are divisible into positive and negative
charges.
In an atom, the positive and negative charges balance, leaving it neutral. Sometimes an atom can accept or release electrons and become a charged particle called an ion.
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Rutherford’s Experiment:
The Nuclear Atom Thomson’s discovery of
electrons and protons added much new detail to the model of the atom. Ernest
Rutherford wanted to know more about the internal structure of the atom. In
about 1900, he devised an experiment that he hope would give him more detail
about the internal structure of the atom. The results of his experiment were
very important in developing the modern day model of the atom.  If the gold atoms were like
Thomson’s model, the alpha particles would have blasted through without any
deflection or reflection. The small amount of deflection and reflection
suggested that the alpha particles were on occasion, slamming into a very dense
positive charge. This led 1) The atom is mostly empty space. (The majority of
alpha particles passed through unhindered.) 2) The atom has a very small, dense, positively charged
nucleus surrounded by electrons in a planetary orbit. The nucleus contained the
protons. (The deflection was caused by the repulsion between the positive alpha
particle and positive nucleus.)  A third subatomic particle,
called the neutron, was discovered just a few years later. Up to this point our
model of the atom is a nucleus containing positively charged protons and neutral
neutrons. Nearly all of the mass resides in the nucleus. Negatively charged
electrons surround the nucleus. |
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Representing Matter :
Symbols and Formulas
Symbols represent elements or single atoms of the element. Symbols consist of either one or two letters. Formulas represent compounds or single molecules of the compound. Formulas consist of a combination of symbols and subscripts, which indicate which elements are in the compound and how many of each element is present. |
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How Elements Combine
All elements try to acheive a stable state by either sharing, losing or gaining outer electrons in order to acheive an outer electron configuration that is stable. This usually, but not always, results in an electron that resembles a Noble gas.
Elements that react end up with an oxidation number. When a formula is written, the positive element is usually written first. Oxidation Number - represents the "charge" on an element after it combines with another element.
The oxidation number reflects the number of electrons added or removed from the outer level of electrons in order to acheive a stable electron configuration. In many cases, this stable electron configuration is referred to as a stable octet. For groups IA - VIIIA (1, 2, 13 - 18) the oxidation numbers can be determined from the table below. |
| Group | Number of Outer Electrons | Positive Oxidation Number(s) | Negative Oxidation Number |
| IA | 1 | +1 | --- |
| IIA | 2 | +2 | --- |
| IIIA | 3 | +3 | --- |
| IVA | 4 | +4 (+2) | - 4 |
| VA | 5 | +5 (+3) | - 3 |
| VIA | 6 | +6 (+4) | - 2 |
| VIIA | 7 | +7 (+5) | - 1 |
| VIIIA | 8 | --- | --- |
| When elements combine to form neutral
molecules, the sum of the oxidation numbers is zero.
For example: sodium (Na+1) and oxygen (O-2) ---> Na2O Sometimes elements can combine with a group of atoms that act as one ion which are called polyatomic ions. The combine in the same way as individual elements combine. The sum of the charges is zero. For example: potassium (K+1) and sulfate (SO4-2) --> K2SO4 Nomenclature Binary Compounds are compounds that have only two kinds of elements. When naming them the element with the positive oxidation number is named first. The element with the negative oxidation number is named second with the name modified so that it has a suffix of ide at the end. For example
Compounds with Polyatomic ions are named in a similar manner. The element with the positive oxidation number is named first and the polyatomic ion is named second. For example
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