# 0.2 The atom  (Page 7/14)

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## Interesting fact

In Greek, “same place” reads as $\stackrel{}{\iota }\sigma o\varsigma$ $\tau \stackrel{}{o}\pi o\varsigma$ (isos topos). This is why atoms which have the same number of protons, but different numbers of neutrons, are called isotopes . They are in the same place on the Periodic Table!

The following worked examples will help you to understand the concept of an isotope better.

For the element ${}_{92}^{234}\mathrm{U}$ (uranium), use standard notation to describe:

1. the isotope with 2 fewer neutrons
2. the isotope with 4 more neutrons
1. We know that isotopes of any element have the same number of protons (same atomic number)in each atom, which means that they have the same chemical symbol. However, they have a different number of neutrons, and therefore a different mass number.

2. Therefore, any isotope of uranium will have the symbol:

$\mathrm{U}$

Also, since the number of protons in uranium isotopes is always the same, we can write down the atomic number:

${}_{92}\mathrm{U}$

Now, if the isotope we want has 2 fewer neutrons than ${}_{92}^{234}\mathrm{U}$ , then we take the original mass number and subtract 2, which gives:

${}_{92}^{232}\mathrm{U}$

Following the steps above, we can write the isotope with 4 more neutrons as:

${}_{92}^{238}\mathrm{U}$

Which of the following are isotopes of ${}_{20}^{40}\mathrm{Ca}$ ?

• ${}_{19}^{40}\mathrm{K}$
• ${}_{20}^{42}\mathrm{Ca}$
• ${}_{18}^{40}\mathrm{Ar}$
1. We know that isotopes have the same atomic number but different mass numbers.

2. You need to look for the element that has the same atomic number but a different atomic mass number. The only element is ${}_{20}^{42}\mathrm{Ca}$ . What is different is that there are 2 more neutrons than in the original element.

For the sulphur isotope ${}_{16}^{33}\mathrm{S}$ , give the number of...

1. protons
2. nucleons
3. electrons
4. neutrons
1. $Z=16$ , therefore the number of protons is 16 (answer to (a)).

2. $A=33$ , therefore the number of nucleons is 33 (answer to (b)).

3. The atom is neutral, and therefore the number of electrons is the same as the number of protons. The number of electrons is 16 (answer to (c)).

4. $\begin{array}{c}\hfill N=A-Z=33-16=17\end{array}$

The number of neutrons is 17 (answer to (d)).

## Isotopes

1. Atom A has 5 protons and 5 neutrons, and atom B has 6 protons and 5 neutrons. These atoms are...
1. allotropes
2. isotopes
3. isomers
4. atoms of different elements
2. For the sulphur isotopes, ${}_{16}^{32}\mathrm{S}$ and ${}_{16}^{34}\mathrm{S}$ , give the number of...
1. protons
2. nucleons
3. electrons
4. neutrons
3. Which of the following are isotopes of ${}_{17}^{35}\mathrm{Cl}$ ?
1. ${}_{35}^{17}\mathrm{Cl}$
2. ${}_{17}^{35}\mathrm{Cl}$
3. ${}_{17}^{37}\mathrm{Cl}$
4. Which of the following are isotopes of $\mathrm{U-}235$ ? (X represents an element symbol)
1. ${}_{92}^{238}\mathrm{X}$
2. ${}_{90}^{238}\mathrm{X}$
3. ${}_{92}^{235}\mathrm{X}$

## Relative atomic mass

It is important to realise that the atomic mass of isotopes of the same element will be different because they have a different number of nucleons. Chlorine, for example, has two common isotopes which are chlorine-35 and chlorine-37. Chlorine-35 has an atomic mass of 35 u, while chlorine-37 has an atomic mass of 37 u. In the world around us, both of these isotopes occur naturally. It doesn't make sense to say that the element chlorine has an atomic mass of 35 u, or that it has an atomic mass of 37 u. Neither of these are absolutely true since the mass varies depending on the form in which the element occurs. We need to look at how much more common one is than the other in order to calculate the relative atomic mass for the element chlorine. This is the number that you find on the Periodic Table.

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