The mass of a nucleus
Definition
Nucleon
A proton or a neutron, the particles that make up a nucleus.
Definition
Mass number
The total number of protons and neutrons in the nucleus of an atom.
- Protons and neutrons together are called nucleons.
- A proton and a neutron have almost exactly the same mass, so each is given a relative mass of 111.
- An electron has a relative mass of only about 12000\frac{1}{2000}20001, so electrons add almost nothing to the mass of an atom.
- The mass of a nucleus therefore depends on the total number of nucleons it contains.
- The total number of protons and neutrons in a nucleus is called the mass number, AAA.
- A nucleus with a mass number of 141414 has a relative mass of about 141414, and it is roughly 141414 times heavier than the simplest hydrogen nucleus.
Isotopes
Definition
Isotope
One of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons.
- All atoms of an element have the same number of protons, but they do not all have the same number of neutrons.
- Atoms of the same element with different numbers of neutrons are called isotopes of that element.
- Isotopes of an element have the same atomic number and the same nuclear charge, because their number of protons is the same.
- Isotopes of an element have different mass numbers and different nuclear masses, because their number of neutrons is different.
- Each extra neutron increases the mass number by 111 without changing the element.
- An isotope is named using the element followed by its mass number, so carbon-12 is the isotope of carbon with a mass number of 121212.
Examples of isotopes
- Hydrogen has three isotopes, and every one of them has 111 proton.
- Hydrogen-1 has no neutrons, so its mass number is 111.
- Hydrogen-2, also called deuterium, has 111 neutron, so its mass number is 222.
- Hydrogen-3, also called tritium, has 222 neutrons, so its mass number is 333.
- Carbon has the isotopes carbon-12, carbon-13 and carbon-14, which all have 666 protons but contain 666, 777 and 888 neutrons.
- Uranium has the isotopes uranium-235 and uranium-238, which both have 929292 protons but differ by three neutrons.
- Isotopes of the same element have the same number of electrons in their neutral atoms, so they behave the same way in chemical reactions.
- Some isotopes have nuclei that are unstable, such as carbon-14, while others, such as carbon-12, are stable.
Example
Comparing the masses of isotopes
- A lithium nucleus contains 333 protons and 444 neutrons.
- A second lithium nucleus contains 333 protons and 333 neutrons.
- Both nuclei have 333 protons, so they are isotopes of lithium with the same charge of +3+3+3.
- The lithium-7 nucleus has one more neutron, so its nuclear mass is greater by a relative mass of 111.
Common Mistake
- Do not say that isotopes have different numbers of protons, because a different number of protons would make a different element.
- Do not say that isotopes differ in their number of electrons, because the difference is in the nucleus.
- Do not forget the neutrons when finding the mass number, because AAA counts both protons and neutrons.
Exam technique
Defining isotopes
- A full definition of isotopes needs both parts: the same number of protons and a different number of neutrons.
- Examiners allow "same element" in place of "same number of protons", but the neutron part must always be stated.
- When asked why isotopes have different masses, link the extra neutrons to the extra mass, because each neutron has a relative mass of 111.
Self review
- What is the relative mass of a proton and of a neutron?
- What does the mass number of a nucleus tell you?
- What is an isotope?
- Why do two isotopes of the same element have the same nuclear charge?
- How many neutrons does a hydrogen-3 nucleus contain?
