Atoms and Sub-Atomic Particles

Atoms are the basic unit of matter. They make up molecules and macromolecules, and basically everything you can imagine. Each type of atom, called an element, has a specific structure of sub-atomic particles which gives it certain properties. The structure of each element is organized in the Periodic Table of the Elements.

Basic Structure: Protons, Neutrons, and Electrons
Atomic Mass, Weight, and Number
Electron Orbitals
Simple Bonding Properties

Atomic Properties and Bonds
Phases of Matter and Properties
Properties of Water
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Basic Structure: Protons, Neutrons, and Electrons

Atoms are made up of three types of particles: Protons have a positive charge, electrons have a negative charge, and neutrons are neutral. Neutrons and protons are basically of the same mass, but electrons are about one two thousandth (1/2000) of that. Protons and neutrons bind together to form the nucleus, and the electrons orbit around it in the electron cloud. The electrons' negative charge is attracted to the protons' positive charge, but the electrons must orbit at a distance because they repel eachother. Most atoms are of neutral charge. This means that the number of protons equals the number of electrons. An atom that has an overall positive or negative charge is called an ion. This is due to the gain or loss of electrons.

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Atomic Mass, Weight, and Number

Atomic mass refers to the number of protons and neutrons in the the nucleus of the atom. Atomic weight refers to the weight, which also refers to the number of protons and neutrons. (This is because each proton and neutron weigh about 1 unit each, while electrons, which are 1/2000, are so tiny that they are not even counted.) Atomic number refers to the number of protons. In a neutral atom, this matches the amount of electrons. Any extra electrons or missing electrons are indicated with a - or + sign, respectively. For example, a carbon atom would with an two extra electrons would be written as C2- and a carbon atom which has lost two electrons would be written C2+. This is because with a gain of electrons, the overall charge becomes negative and with a loss of electrons, the overall charge becomes positive. The periodic table of the elements organizes each element according to atomic number. The atomic number also determines each element's properties, because it indirectly indicates the number of electrons in the atom. (see Simple Bonding Properties)

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Electron Orbitals

Electrons orbit around the nucleus of the atom. This is called the electron cloud. Withine the electron cloud, there are different energy levels, or electron shells. Higher energy electrons take up outer energy levels, and lower energy electrons take up inner energy levels. Within each energy level there are different orbitals.

In the first energy level, there is only one sphere shaped orbital, labeled the s orbital. In the second energy level, there is an s orbital, and three orbitals shaped like dumbells that extend out of the nucleus at right angles. They are labeled the Px, Py, and Pz, orbitals for their corresponding axis. The third energy level has one s orbital, three p orbitals, and five d orbitals, which are too difficult to diagram. The fourth energy levels has one s orbital, three p orbitals, five d orbitals, and seven f orbitals which are also too difficult to diagram. Known elements have as high has seven energy levels. Levels after the fourth add the same types of orbitals.


Each orbital can hold only two electrons of opposite spins. The actual path of an electron is unknown, but according to quantum mechanics it is most probable that any any given moment, the electron is somewhere within that orbital.

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Simple Bonding Properties

Bonding between molecules depends on the number of electrons in the outer shell of each atom. These are called valence electrons. Since each orbital in the outer shell can hold only two electrons, the number of unpaired of electrons is called the atoms valence. The valence number is the number of bonds that that atom is capable of forming. Carbon, which has six electrons, has a valence of four because it has four unpaired electrons. The fact that carbon can form the strongest and most bonds explains why it forms the backbone of most molecules, called organic molecules, and is the basis for life. Organic molecules consist of chains of carbons which then are linked to other combinations of atoms that determine the properties of the molecule. Here is a table of common elements and their valences:

Element            # of Electrons (neutral)            # of Valence Electrons            Valence
Hydrogen (H) 1 1 1
Helium (He) 2 2 0
Carbon (C) 6 4 4
Nitrogen (N) 7 5 3
Oxygen (O) 8 6 2
Flourine (F) 9 7 1


When two atoms bond, they share electrons. A covalent bond is when two atoms share electrons equally, and an ionic bond is when one atom strips an electron completely from another atom, but the difference in charges bonds them together. Ionic bonds are the strongest of the two. In general, ionic compounds are made between a metal and a non-metal. Typically there is a dividing line, which I took the liberty of drawing in on the Periodic Table of the Elements which shows the division between metals on the left and non-metals on the right.
When being written, covalent bonds are displaye with a - sign. For example, carbon bonded to oxygen is written C-O. It is posible for an atom to bond covalently with another more than one time. Carbon double bonded to carbon would be written C=C, and so on.


When atoms bond the electrons still follow the rule that there can only be two electrons to each orbital. They can only bond where there is an unpaired electron in both atoms. In order for a compound to be stable, the atoms must be bonded in such a way that their orbitals are all full. This can be best illustrated with the use of Lewis dot diagrams.
In a Lewis dot diagram, each atom is illustrated with the electrons in its outer shell surrounding it. Bonds are represented by lines. When bonded, each atom must have a total of eight electrons surrounding it. Lines, representing bonds, count as two electrons because, even though only one electron in the bond actually belongs to the atom, they are shared and equally belong to both. The only exception is hydrogen and helium; they only need two electrons to fill their outer shell.

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