The missing mass
Neutrons and the mass of an atom.
Helium is too heavy
A hydrogen atom has one proton. A helium atom has two protons. Yet a helium atom is about four times as heavy as a hydrogen atom, not double.
What could explain the extra mass?
What this lesson covers
The idea
A neutron (n) has nearly a proton's mass and no charge, and is in the nucleus of all atoms except hydrogen; an atom's mass comes mainly from its protons and neutrons, electrons being so light their mass can be ignored.
Helium is too heavy
A hydrogen atom has one proton. A helium atom has two protons. Yet a helium atom is about four times as heavy as a hydrogen atom, not double.
What could explain the extra mass?
- A particle in the nucleus with mass but no charge
- More electrons in the helium atom
- A heavier kind of proton
The missing mass
Press Next step to follow the puzzle.
A particle with no charge
The nucleus holds protons and also neutrons. Neutrons add mass but no charge. Neutrons are found in the nucleus of all atoms except hydrogen.
A neutron (symbol n) has a mass nearly equal to a proton's and no charge, and is found in the nucleus of all atoms except hydrogen. An atom's mass comes mainly from its protons and neutrons; electrons are so light that their mass can be ignored.
Notes
A neutron has nearly a proton's mass and no charge, and is in the nucleus of all atoms except hydrogen. An atom's mass comes mainly from its protons and neutrons.
Check yourself
A neutron has …
According to Table 8.3 of the book, which atom has no neutron in its nucleus?
The mass of an atom comes mainly from …
Helium has 2 protons. Its mass is about 4 times that of hydrogen, which has one proton. If a neutron has about the same mass as a proton, how many neutrons does helium have?
Answer: 2
The mass is about 4 units. Two units come from protons, so 4 − 2 = 2 neutrons.
- almost no mass and a negative charge. That describes an electron, not a neutron.
- the mass of an electron and a positive charge. A neutron has no charge and a mass nearly equal to a proton's.
- nearly the mass of a proton and no electrical charge — correct. Yes!
- Hydrogen — correct. Yes!
- Helium. Table 8.3 shows helium with 2 neutrons.
- Lithium. Table 8.3 shows lithium with 4 neutrons.
- electrons revolving around the nucleus. Electrons are so light that their mass can be ignored.
- protons and neutrons in the nucleus — correct. Yes!
- the empty space in the atom. Most of an atom is empty space, which has no mass to add.