3.1 Structure  (Page 2/2)

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 .

Atomic number (Z)

The number of protons in an atom

You can find the atomic number on the periodic table. The atomic number is an integer and ranges from 1 to about 118.

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 .

Atomic mass number (A)

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

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.

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 $\mathrm{C}-12$ 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
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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
   $\mathrm{Mg}$	24	12
   $\mathrm{O}$			8
   		17
   $\mathrm{Ni}$				28
   	40				20
   $\mathrm{Zn}$
   					0
   $\mathrm{C}$	12			6

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3. Use standard notation to represent the following elements:
   1. potassium
   2. copper
   3. chlorine
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4. For the element ${}_{17}^{35}\mathrm{Cl}$ , give the number of ...
   1. protons
   2. neutrons
   3. electrons
   ... in the atom.
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5. Which of the following atoms has 7 electrons?
   1. ${}_2^5\mathrm{He}$
   2. ${}_6^{13}\mathrm{C}$
   3. ${}_3^7\mathrm{Li}$
   4. ${}_7^{15}\mathrm{N}$
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6. In each of the following cases, give the number or the element symbol represented by 'X'.
   1. ${}_{18}^{40}\mathrm{X}$
   2. ${}_{20}^x\mathrm{Ca}$
   3. ${}_x^{31}\mathrm{P}$
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7. Complete the following table:

   	A	Z	N
   ${}_{92}^{235}\mathrm{U}$
   ${}_{92}^{238}\mathrm{U}$

   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