The idea of a ‘smallest piece of matter’ has been around since the Greek philosopher Democritus proposed the idea of the atom way back around 600 B.C. His idea was that if you cut an object in half enough times you would eventually find that you came up with something that could not be cut anymore. Atom in Greek means ‘uncuttable’.
Another Greek philosopher, Aristotle, shot down the idea, saying that matter could be cut forever without ever finding a ‘smallest piece of matter’. People trusted Aristotle right up until nearly modern times (around the middle of the 1700s). But now we know that Aristotle was wrong and that Democritus was on the right track.
All ordinary matter is composed of atoms. Atoms are the smallest pieces of something called an element that still have the properties of that element. Elements are substances that cannot be chemically broken down into component parts. The modern definition of an element depends on a piece of matter that is smaller than an atom: the proton. Protons (symbolized with p+) are tiny masses which have a single unit of positive electrical charge (+1). You can tell different elements apart by knowing how many protons the atoms of that element have. For example, hydrogen (H) has one proton, oxygen (O) has 8 protons, and carbon (C) has 6 protons. The number of protons is called the atomic number and is symbolized with the letter Z. The atomic number determines the identity and charge of what is called the nucleus of an atom. The nucleus is a very tiny part of an atom when considered by volume but is the largest part when considered by mass. Protons are found in the nuclei of atoms. To sum up, Z stands for the number of protons and the charge of the nucleus of an atom of any element. So the nucleus of a carbon atom has six protons and has a nuclear charge of +6.
The nucleus also contains another small particle called a neutron. Neutrons (symbolized with n0) are particles that have the same mass as a proton but zero electric charge (0). Both protons and neutrons have a mass of 1 atomic mass unit (1 amu). This is a relative mass unit based on a standard mass. The standard mass is the mass of a particular kind of carbon atom called ‘carbon-12’ which is symbolized with 12C. You already know that a carbon atom has six protons in its nucleus. Carbon-12 also has six neutrons in its nucleus, for a total of 12 nuclear particles. The mass of 12C has been defined by scientists as exactly 12 amu. Although the mass of a proton is not exactly the same as the mass of a neutron they are very close. In terms of 12C, both a p+ and a n0 have a mass of 1/12 the mass of a 12C atom, or 1 amu.
Atoms of an element always have the same number of protons. There is some flexibility, though, in how many neutrons an atom of an element may have. For example, carbon has three common variations: carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C). These names and symbols refer to variations on an atom of carbon which are chemically the same but have different masses. These variations are called isotopes. The numbers given in the superscript for each symbol are called mass numbers. The mass number is symbolized with an A. The atomic number (Z) plus the number of neutrons in an atom give the mass number (A): Z + (# of n0) = A or (# of p+) + (# of n0) = A
The symbols below show the atomic symbol of molybdenum (Mo) with its atomic number (Z=42) in a subscript position and the mass number of molybdenum-96 (A=96) in a superscript position, both to the left of the atomic symbol. When you write these symbols always put A and Z in the positions shown here. Sometimes the atomic number is not written because it is implied by the atomic symbol.
Atoms of an element are defined by their nucleus: the number of protons (Z) determines what element it is and the number of neutrons plus the number of protons (A) determines what isotope of an element an atom is. But atoms are not complete without electrons. Electrons (e–) are particles that have a very small mass and one unit of negative electric charge (–1). Electrons are much smaller than protons and neutrons: they have a mass of 1/1840 amu. But even though they are small, they are important. All the chemical reactions that atoms undergo depend on their electrons. Electrons are traded back and forth between atoms to make compounds and are the basic unit of all electric power.
Atoms of an element that are electrically neutral have an overall charge of zero. That means that for an electrically neutral atom, the number of electrons equals the number of protons. For example, a neutral atom of carbon has six electrons. Atoms are not always neutral and when they are not, they are called ions. Atoms can become ions only by gaining or losing electrons. In chemical reactions atoms never gain or lose protons or neutrons. If an atom gains or loses an electron you symbolize this by using the atomic symbol followed by a superscript showing the charge. Positive ions: Na+, Mg2+, Ca2+, Fe3+. Negative ions: O2–, F–, Cl–, C–. The number is shown first, then the charge. To calculate the charge on an atom you can use the following formula: (# of p+) – (# of e–) = overall charge
So if a neutral carbon-12 atom loses one electron this equation would read 6 – 5 = +1 and its symbol becomes12 6C +. If a12 6C atom gains four electrons the equation would read 6 – 10 = –4 and its symbol becomes12 6C 4–. The key to understanding how to find the charge on an ion it to always remember that an atom of any element always has the same number of protons.
But what if you need to know how many protons an element has but I haven’t written it down? That is when you use the periodic table included with this packet. Here is how to read it:
Just remember that a neutral atom of any element has exactly the same number of electrons as atoms of that element have protons. For example, molybdenum has 42 protons in its nucleus. A neutral Mo atom has 42 e– spinning around that nucleus.
To find the mass number, just round the average atomic mass to the nearest whole number. Chances are good that there is an isotope with that mass number. Chances are also good that there are other isotopes of that element with a different number of neutrons. To find out you would have to look up a table of isotopes.
If you have trouble finding the symbol and atomic number for an element based on its name you can use your text book. Nearly every chemistry text ever published has an alphabetic list of the elements which shows the atomic symbol for each one.
|Name||Symbol||No. of p+||No. of n0||Mass||No. of e–||
|Name||Symbol||No. of p+||No. of n0||Mass||No. of e–||
To give you an idea of the size of the atom, imagine that you had a very sharp pair of tweezers and you could pick up atoms one at a time. If you lined up 1 x 108 (100,000,000) atoms you would have a string of atoms about 1 cm long.
Compared to the size of the atom, the size of the nucleus is very small. How small? Take an atom and imagine it blown up to the size of the Houston Astrodome. The nucleus of an atom at that scale would be the size of a marble. The volume of the nucleus is 1/1013 (1/10,000,000,000,000) of the total volume of the atom. The diameter of an atomic nucleus is about 20,000 times smaller than the diameter of an atom. Atomic diameters are measured based on the greatest distance from the nucleus that an electron belonging to the atom could be found.