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Atomic structure

As a result of the work done by previous scientists on atomic models (that we discussed in "Models of the Atom"), scientists now have a good idea of what an atom looks like. This knowledge is important because it helps us to understand why materials have different properties and why some materials bond with others. Let us now take a closer look at the microscopic structure of the atom.

So far, we have discussed that atoms are made up of a positively charged nucleus surrounded by one or more negatively charged electrons. These electrons orbit the nucleus.

The Electron

The electron is a very light particle. It has a mass of 9.11 x 10-3110-31 kg. Scientists believe that the electron can be treated as a point particle or elementary particle meaning that it can't be broken down into anything smaller. The electron also carries one unit of negative electric charge which is the same as 1.6 x 10-1910-19 C (Coulombs).

The Nucleus

Unlike the electron, the nucleus can be broken up into smaller building blocks called protons and neutrons. Together, the protons and neutrons are called nucleons.

The Proton

Each proton carries one unit of positive electric charge. Since we know that atoms are electrically neutral, i.e. do not carry any extra charge, then the number of protons in an atom has to be the same as the number of electrons to balance out the positive and negative charge to zero. The total positive charge of a nucleus is equal to the number of protons in the nucleus. The proton is much heavier than the electron (10 000 times heavier!) and has a mass of 1.6726 x 10-2710-27 kg. When we talk about the atomic mass of an atom, we are mostly referring to the combined mass of the protons and neutrons, i.e. the nucleons.

The Neutron

The neutron is electrically neutral i.e. it carries no charge at all. Like the proton, it is much heavier than the electron and its mass is 1.6749 x 10-2710-27 kg (slightly heavier than the proton).

Note: Interesting Fact :

Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. He predicted this because if there were only positively charged protons in the nucleus, then it should break into bits because of the repulsive forces between the like-charged protons! Also, if protons were the only particles in the nucleus, then a helium nucleus (atomic number 2) would have two protons and therefore only twice the mass of hydrogen. However, it is actually four times heavier than hydrogen. This suggested that there must be something else inside the nucleus as well as the protons. To make sure that the atom stays electrically neutral, this particle would have to be neutral itself. In 1932 James Chadwick discovered the neutron and measured its mass.

Table 1: Summary of the particles inside the atom
  proton neutron electron
Mass (kg) 1.6726 x 10-2710-27 1.6749 x 10-2710-27 9.11 x 10-3110-31
Units of charge +1 0 -1
Charge (C) 1.6 x 10-1910-19 0 -1.6 x 10-1910-19
Note: Interesting Fact :

Unlike the electron which is thought to be a point particle and unable to be broken up into smaller pieces, the proton and neutron can be divided. Protons and neutrons are built up of smaller particles called quarks. The proton and neutron are made up of 3 quarks each.

Atomic number and atomic mass number

The chemical properties of an element are determined by the charge of its nucleus, i.e. by the number of protons. This number is called the atomic number and is denoted by the letter Z.

Definition 1: Atomic number (Z)

The number of protons in an atom

The mass of an atom depends on how many nucleons its nucleus contains. The number of nucleons, i.e. the total number of protons plus neutrons, is called the atomic mass number and is denoted by the letter A.

Definition 2: Atomic mass number (A)

The number of protons and neutrons in the nucleus of an atom

Standard notation shows the chemical symbol, the atomic mass number and the atomic number of an element as follows:

Figure 1
Figure 1 (atom_sym.png)

For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as:

26 56 Fe 26 56 Fe
(1)

where the atomic number is Z=26Z=26

 
and the mass number A=56A=56. The number of neutrons is simply the difference N=A-ZN=A-Z.

Tip:

Don't confuse the notation we have used above with the way this information appears on the Periodic Table. On the Periodic Table, the atomic number usually appears in the top lefthand corner of the block or immediately above the element's symbol. The number below the element's symbol is its relative atomic mass. This is not exactly the same as the atomic mass number. This will be explained in "Isotopes". The example of iron is shown below.

Figure 2
Figure 2 (CG10C3_004.png)

You will notice in the example of iron that the atomic mass number is more or less the same as its atomic mass. Generally, an atom that contains n nucleons (protons and neutrons), will have a mass approximately equal to nnu. For example the mass of a C-12 atom which has 6 protons, 6 neutrons and 6 electrons is 12u, where the protons and neutrons have about the same mass and the electron mass is negligible.

The structure of the atom

  1. Explain the meaning of each of the following terms:
    1. nucleus
    2. electron
    3. atomic mass
    Click here for the solution
  2. Complete the following table: (Note: You will see that the atomic masses on the Periodic Table are not whole numbers. This will be explained later. For now, you can round off to the nearest whole number.)
    Table 2
    ElementAtomic massAtomic numberNumber of protonsNumber of electronsNumber of neutrons
    Mg2412   
    O  8  
      17   
    Ni   28 
     40   20
    Zn     
         0
    C12  6 
    Click here for the solution
  3. Use standard notation to represent the following elements:
    1. potassium
    2. copper
    3. chlorine
    Click here for the solution
  4. For the element 17351735Cl, give the number of ...
    1. protons
    2. neutrons
    3. electrons
    ... in the atom.
     
    Click here for the solution
  5. Which of the following atoms has 7 electrons?
    1. 2525He
    2. 613613C
    3. 3737Li
    4. 715715N
    Click here for the solution
  6. In each of the following cases, give the number or the element symbol represented by 'X'.
    1. 18401840X
    2. 20x20xCa
    3. x31x31P
    Click here for the solution
  7. Complete the following table:
    Table 3
     AZN
    9223592235U   
    9223892238U   
    In these two different forms of Uranium...
    1. What is the same?
    2. What is different?
    Uranium can occur in different forms, called isotopes. You will learn more about isotopes in "Isotopes".
     
    Click here for the solution

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