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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 .
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 but different mass numbers because they have a different number of neutrons . 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.
In Greek, “same place” reads as
The following worked examples will help you to understand the concept of an isotope better.
For the element (uranium), use standard notation to describe:
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.
Therefore, any isotope of uranium will have the symbol:
Also, since the number of protons in uranium isotopes is always the same, we can write down the atomic number:
Now, if the isotope we want has 2 fewer neutrons than , then we take the original mass number and subtract 2, which gives:
Following the steps above, we can write the isotope with 4 more neutrons as:
Which of the following are isotopes of ?
We know that isotopes have the same atomic number but different mass numbers.
You need to look for the element that has the same atomic number but a different atomic mass number. The only element is Ca. What is different is that there are 2 more neutrons than in the original element.
For the sulphur isotope S, give the number of...
Z = 16, therefore the number of protons is 16 (answer to (a)).
A = 33, therefore the number of nucleons is 33 (answer to (b)).
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)).
The number of neutrons is 17 (answer to (d)).
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