In the previous lesson you learned that both Democritus and Dalton were wrong about the atom being indivisible. The atom is made up of smaller subatomic particles called protons, neutrons, and electrons. Dalton's atomic theory was also incorrect about all atoms of a given element being identical. While it is true that all neutral atoms of a given element have the same number of protons and electrons, the number of neutrons in their nuclei may differ. Study the slideshow to learn more.
This image illustrates the three naturally occurring forms of hydrogen: protium, deuterium, and tritium. They can be written as hydrogen-1, hydrogen-2, and hydrogen-3.
From the image, you can see the characteristics of the three subatomic particles. Recall that protons are positively charged subatomic particles located in the nucleus, neutrons are neutral subatomic particles also located in the nucleus, and electrons are negatively charged subatomic particles that are found in the space outside the nucleus.
Look closely at the subatomic particle composition of each form of hydrogen. How do they compare?
While each form of hydrogen has the same number of protons and electrons, each one has a different number of neutrons.
The image also shows the nuclear notation that represents each form of hydrogen. Recall that the nuclear notation for a given atom shows its chemical symbol with the mass number as a left superscript and atomic number as a left subscript.
Now look closely at the nuclear notation for each form of hydrogen. How do the nuclear notations compare?
While each form of hydrogen has the same atomic number, each one has a different mass number.
These different forms of hydrogen are called isotopes. Isotopes are atoms of the same element with the same number of protons, but different numbers of neutrons. In other words, the isotopes of a given element have the same atomic number but different mass numbers.
Because the atomic number is the same as the number of protons in an atom, isotopes of an element also share the same number of protons. When the atomic number (number of protons) is subtracted from the mass number (number of protons and neutrons), the number of neutrons is the result.
The three isotopes of hydrogen can be written as hydrogen-1, hydrogen-2, and hydrogen-3. What do the numbers “1,” “2,” and “3” stand for, and how do these numbers relate to protons and neutrons?
The numbers “1,” “2,” and “3” give the mass number of each isotope of hydrogen.
Each isotope of hydrogen has 1 proton. Hydrogen-1 has no neutrons because the mass number (1) - the atomic number (1) is zero, hydrogen-2 has one neutron, and hydrogen-3 has two neutrons.
The word isotope comes from a Greek word that means, “same location.” Isotopes are atoms of the same element that share the same atomic number and the same location on the periodic table.
Question
How do two isotopes of the same element differ?
Isotopes of the same element have different numbers of neutrons and different mass numbers.
