Fundamental Subatomic Particles
Particle | Symbol | Charge | Mass | |
electron | e- | -1 | 0.0005486 amu | |
proton | p+ | +1 | 1.007276 amu | |
neutron | no | 0 | 1.008665 amu |
The number of protons, neutrons, and electrons in an atom can be determined from a set of simple rules.
- The number of protons in the nucleus of the atom is equal to the atomic number (Z).
- The number of electrons in a neutral atom is equal to the number of protons.
- The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus.
- The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).
Examples: Let's determine the number of protons, neutrons, and electrons in the following isotopes.
12C | 13C | 14C | 14N |
The different isotopes of an element are identified by writing the mass number of the atom in the upper left corner of the symbol for the element. 12C, 13C, and 14C are isotopes of carbon (Z = 6) and therefore contain six protons. If the atoms are neutral, they also must contain six electrons. The only difference between these isotopes is the number of neutrons in the nucleus.
12C: 6 electrons, 6 protons, and 6 neutrons
13C: 6 electrons, 6 protons, and 7 neutrons
14C: 6 electrons, 6 protons, and 8 neutrons
Any substance that contains only one kind of an atom is known as an element. Because atoms cannot be created or destroyed in a chemical reaction, elements such as phosphorus (P4) or sulfur (S8) cannot be broken down into simpler substances by these reactions.
Example: Water decomposes into a mixture of hydrogen and oxygen when an electric current is passed through the liquid. Hydrogen and oxygen, on the other hand, cannot be decomposed into simpler substances. They are therefore the elementary, or simplest, chemical substances - elements.
Each element is represented by a unique symbol. The notation for each element can be found on the periodic table of elements.
The elements can be divided into three categories that have characteristic properties: metals, nonmetals, and semimetals. Most elements are metals, which are found on the left and toward the bottom of the periodic table. A handful of nonmetals are clustered in the upper right corner of the periodic table. The semimetals can be found along the dividing line between the metals and the nonmetals.
The term mole literally means a small mass. It is used as the bridge between chemistry on the atomic and macroscopic scale. If the mass of a single 12C atom is 12.000 amu, then one mole of these atoms would have a mass of 12.000 grams. By definition, aa mole of any substance contains the same number of elementary particles as there are atoms in exactly 12 grams of the 12C isotope of carbon.
Example: A single 12C atom has a mass of 12 amu, and a mole of these atoms would have a mass of 12 grams.
A mole of any atoms has a mass in grams equal to the atomic weight of the element. The term mole can be applied to any particle: atoms, a mole of atoms, a mole of ions, a mole of electrons, or a mole of molecules. Each time we use the term, we refer to a number of particles equal to the number of atoms in exactly 12 grams of the 12C isotope of carbon.
Elements combine to form chemical compounds that are often divided into two categories.
Metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules.
Nonmetals combine with each other to form covalent compounds, which exist as neutral molecules.
The shorthand notation for a compound describes the number of atoms of each element, which is indicated by a subscript written after the symbol for the element. By convention, no subscript is written when a molecule contains only one atom of an element. Thus, water is H2O and carbon dioxide is CO2.
A molecule is the smallest particle that has any of the properties of a compound. The formula for a molecule must be neutral. When writing the formula for an ionic compound, the charges on the ions must balance, the number of postive charges must equal the number of negative charges.
Examples:
CaCl2 | Balanced formula has 2 positive charges (1 calcium ion with +2 charge) and 2 negative charges (2 chloride ions with a -1 charge) |
Al2(SO4)3 | Balanced formula has 6 positive charges (2 aluminum ions with a +3 charge) and 6 negative charges (3 sulfate ions with -2 charge) |