Atoms and Molecules
Dalton's Atomic Theory
Around 1805, John Dalton proposed a theory to explain the nature of matter that is remarkably similar to modern atomic theory. While this theory is stated in a variety of ways, the key points of this theory are:
- All matter is composed of indivisible particles called atoms.
- All atoms of an element are identical, while atoms of different elements are different.
- Molecules are a combination of any two or more atoms. Compounds are a combination of atoms from two or more different elements.
- Chemical reactions involve the rearrangement of atoms. The atoms themselves do not change.
The Law of Conservation of Mass is a direct consequence of this last postulate. The Law of Constant Composition is a result of the fact that molecules generally contain fixed ratios of atoms.
Classification of Matter
- * Atom: * The smallest possible piece of an element.
- * Element: * A substance in which all of the atoms are (almost) identical
- * Molecule: * A combination of two or more atoms connected in a fixed ratio that act as a single unit.
- * Compound: * A molecule containing two or more elements.
Chemical vs. Physical Changes
Using simple atomic theory, the difference between chemical and physical changes can easily be understood.
A physical change my alter the bulk appearance of a material and/or change its physical state (solid ↔ liquid ↔ gas), but a physical change does not change the arrangement of atoms in molecules. For example, when water boils, the H2O molecules in water do NOT change. The steam that is formed still contains the same H2O molecules.
In a chemical change, the arrangement of or connection between atoms changes. This changes the properties of the material, often quite significantly. For example, hydrogen gas and oxygen gas can combine to form water molecules. While the starting materials are both gases at room temperature, the water that is formed is a liquid under these conditions.
Examples of physical changes include creating or separating mixtures (such as making lemonade or orange juice) and changing the physical state of a substance (such as boiling water or melting iron).
Examples of chemical changes include any burning (combustion) process, rusting of metal, and almost all baking.
Subatomic Particles
| Name | Relative Mass | Relative Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | ≈0 | -1 | Outside nucleus |
* Definitions *
- * Atomic Number: * The number of protons. The identity of an element is determined by this number
- * Mass Number: * The number of protons + number of neutrons
- * Isotopes: * Atoms containing the same number of protons, but a different number of neutrons
Atomic Weight
The atomic weight of an element is defined as the weighted average of mass values of all isotopes of an element. For example, for Chlorine, consists of two isotopes. 35Cl has a mass of 34.9689 and a natural abundance of 75.78%. 37Cl has a mass of 36.9659 and an abudance of 24.22%. The atomic weight of chlorine is calculated to be:
| Mass * Abundance | Average | |
|---|---|---|
| 34.9689 * 0.7578 | = | 26.499 |
| 36.9659 * 0.2422 | = | 8.953 |
| 35.452 | ||
Periodic Table
Dmitri Mendeleev is typically given credit for creating the first version of the periodic table around 1869. The periodic table is arranged with elements in the same column having similar chemical properties, with the lighter elements near the top.
In the modern version of the periodic table, some of Mendeleev's columns are separated out to give a wider table which can be easily related to the arrangement of electrons in the atoms.
In our current table, * metals are located on the lower-left side of the table, and non-metals * are located on the upper-right. (Hydrogen is an exception to this, and is classified as a non-metal).
Periodic Trends
The utility of the periodic table lies in the fact that it can be used to predict the properties of elements. The following periodic trends illustrate this.
- * Atomic Size: * The size of atoms is determined by the number of
electrons.
- Going down a column, the size increases.
- Going across a row, the size decreases slightly.
- * Ionization Energy: * This is a measure of the amount of energy required to remove an electron from an atom. The ionization energy is larger for smaller atoms.
- * Electronegativity: * This is a measure of the ability of an atom to attract and hold onto electrons. As with ionization energy, the electronegativity is generally larger for smaller atoms.