Help Window: Atoms
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- are the smallest unit of matter exhibiting unique, characteristic chemical and physical properties.
- are composed of protons, neutrons, and electrons.
- have equal numbers of protons and electrons.
- have the protons and neutrons packed tightly together in a small volume of space -- the nucleus.
- have the electrons widely dispersed over a very large volume of space relative to the size of the nucleus. To put this in perspective: if the nucleus of an atom were a sphere 1 inch in diameter, the outer reaches of the atom would extend out slightly more than 1.5 miles!!!
Two dimensional sketch of an atom
Imagine a very small sphere (the nucleus, shown in yellow) at the center of another sphere about 100,000 times larger in diameter. The nucleus contains the protons and neutrons in a very small volume. The electrons are distributed throughout the much larger volume of space, shown in gray. |  |
A chemical element is a bunch or collection of atoms --all of the same type-- having different chemical and physical properties from any other collection of like atoms. There are about 109 known elements at present; some occur in nature, while others are created in nuclear reactors and exist but a short time. The naturally existing elements also vary widely in their abundance or occurrence in nature. Hydrogen, oxygen, silicon, and carbon are very common on earth, but gold, platinum, and palladium are much rarer (and hence more expensive!)
Each element has a name and is represented by an abbreviated symbol. Usually the symbol is the first or first and second letter of the name. In a few cases, the symbol does not match the English language name, because the element was named long ago in Latin (or some other language). Examples of some common element names and symbols are:
| Element Name | Element Symbol |
|---|
| Hydrogen | H |
| Helium | He |
| Oxygen | O |
| Sodium | Na |
| Potassium | K |
The table below compares the charge, mass and location of the subatomic particles of interest to chemists.
Because the mass of each particle is so small, a new unit of mass is defined. An atomic mass unit or amu = mass of 1 proton, which is also = mass of 1 neutron.
Notice how small the mass of an electron is. It would take 1835 electrons to weigh the same as 1 proton!
| Particle Name | Charge | Mass in kg | Mass in amu | Location |
|---|
| Proton | +1 | 1.67x10-27 | 1 | in nucleus |
| Neutron | 0 | 1.67x10-27 | 1 | in nucleus |
| Electron | -1 | 9.1x10-31 | 1/1835 | outside nucleus |
The two heavy particles, the proton and neutron, are found in the nucleus only. For practical purposes, this means the entire mass of the atom is concentrated in the very small volume occupied by the protons and neutrons. The lighter electrons are outside the nucleus, and contribute essentially no mass to the total weight of the atom, but occupy an enormously large volume of space.
Since like-charged particles repel each other the repulsive forces among the protons are very large. You can think of the neutrons as "spacers" added to the nucleus to reduce how close the + charged protons get to each other. This helps lower the total repulsion energy.
The atomic number of any element = number of protons in the nucleus. Since an atom is electrically neutral, the number of protons must equal the number of electrons. So we can also say that an element's atomic number = number of electrons. The atomic number uniquely characterizes each element. Any two atoms of the same element have the same atomic number.
Protons and neutrons have practically the same mass, and each one is almost 2000 times heavier than an electron. Hence, an element's atomic mass in amu = the number of protons + the number of neutrons.
The number of neutrons a particular atom possesses can vary, and is not readily predicted. Two atoms of the same atomic number that have different numbers of neutrons are called isotopes. Some elements occur only as a single isotope, while others may have several. For example, all atoms of Lithium have atomic number 3 (= 3 protons in nucleus, 3 electrons outside nucleus). However, if we examine a sample of Lithium, out of every 100 Lithium atoms we find about 93 atoms have 4 neutrons and about 7 of 100 have 3 neutrons. Both Lithium isotopes have the identical chemical properties of Lithium. They differ only in atomic mass: one has a mass of 7 amu (= 3 protons + 3 neutrons), while the other has atomic mass of 6 amu (= 3 protons + 3 neutrons).
| Isotope Symbol | Atomic Number | Number of Neutrons | Atomic Mass |
|---|
| 6Li | 3 | 3 | 6 |
| 7Li | 3 | 4 | 7 |
Notice that the atomic symbol is modified by the leading supercript to indicate which particular isotope is being referred to. When no superscript is given, the naturally occurring mixture of all Lithium isotopes is implied.
Based on what we have just seen, the number of neutrons present in a particular isotope can be readily calculated as:
mass of isotope - atomic number.
For example, one isotope of Tin (symbol is Sn) has atomic mass = 120. Every atom of tin has atomic number 50. So, this particular Sn isotope has 120 - 50 = 70 neutrons.
The protons and neutrons of an atom do not participate in ordinary chemical reactions -- they remain uninvolved unless nuclear fission or fusion processes occur.
Instead, the reactions of interest to most chemists leave the nucleus unchanged and involve electrons only.
An atom is electrically neutral because it possesses equal numbers of protons (+ charge) and electrons (- charge). Atoms may lose or gain electrons to form ions. An ion has an unequal number of protons and electrons.
Cations are positively charged ions.
Anions are negatively charged ions.
If an atom loses electrons, it becomes a cation, since its nucleus now has more + charged protons than there are - charged electrons outside the nucleus. Each electron lost increases the + charge 1 unit.Thus,
Atom ---> Cation+1 + electron-.
For example,
Li ---> Li+ + electron-.
Mg ---> Mg+2 + 2 electron-.
Al ---> Al+3 + 3 electron-.
As we will see later, it is possible to predict how many electrons an element may lose (or gain). Some elements can lose a variable number of electrons as well, forming differently charged cations. Iron (symbol Fe) can lose either 2 or 3 electrons, thus becoming Fe+2 or Fe+3, respectively.
If an atom gains electrons, it becomes an anion, since its nucleus now has fewer + charged protons than there are - charged electrons outside the nucleus. Each electron gained increases the - charge 1 unit.Thus,
Atom + electron----> Anion-1.
For example,
Cl + electron- ---> Cl-.
O + 2 electron----> O-2.
N + 3 electron----> N-3.