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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 n u. 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.

Use standard notation to represent sodium and give the number of protons, neutrons and electrons in the element.

  1. Sodium is given by Na
  2. Sodium has 11 protons, so we have: 11 Na
  3. Sodium has 12 neutrons.
  4. A = N + Z = 12 + 11 = 23
  5. In standard notation sodium is given by: 11 23 Na . The number of protons is 11, the number of neutrons is 12 and the number of electrons is 11.

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.)
    Element Atomic mass Atomic number Number of protons Number of electrons Number of neutrons
    Mg 24 12
    O 8
    17
    Ni 28
    40 20
    Zn
    0
    C 12 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 17 35 Cl , 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. 2 5 He
    2. 6 13 C
    3. 3 7 Li
    4. 7 15 N
    Click here for the solution
  6. In each of the following cases, give the number or the element symbol represented by 'X'.
    1. 18 40 X
    2. 20 x Ca
    3. x 31 P
    Click here for the solution
  7. Complete the following table:
    A Z N
    92 235 U
    92 238 U
    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

Isotopes

What is an isotope?

The chemical properties of an element depend on the number of protons and electrons inside the atom. So if a neutron or two is added or removed from the nucleus, then the chemical properties will not change. This means that such an atom would remain in the same place in the Periodic Table. For example, no matter how many neutrons we add or subtract from a nucleus with 6 protons, that element will always be called carbon and have the element symbol C (see the Table of Elements). Atoms which have the same number of protons, but a different number of neutrons, are called isotopes .

Isotope

The isotope of a particular element is made up of atoms which have the same number of protons as the atoms in the original element, but a different number of neutrons.

The different isotopes of an element have the same atomic number Z but different mass numbers A because they have a different number of neutrons N . The chemical properties of the different isotopes of an element are the same, but they might vary in how stable their nucleus is. Note that we can also write elements as X - A where the X is the element symbol and the A is the atomic mass of that element. For example, C- 12 has an atomic mass of 12 and Cl- 35 has an atomic mass of 35 u, while Cl- 37 has an atomic mass of 37 u.

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Source:  OpenStax, Chemistry grade 10 [caps]. OpenStax CNX. Jun 13, 2011 Download for free at http://cnx.org/content/col11303/1.4
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