0.2 The atom  (Page 6/14)

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" .
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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 $\mathrm{C}$ (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 $\mathrm{X\; -\; A}$ where the X is the element symbol and the A is the atomic mass of that element. For example, $\mathrm{C-}12$ has an atomic mass of 12 and $\mathrm{Cl-}35$ has an atomic mass of 35 u, while $\mathrm{Cl-}37$ has an atomic mass of 37 u.