Three particles, three clues
Protons have a positive charge and sit in the nucleus. Neutrons have no net charge and also sit in the nucleus. Electrons have a negative charge and occupy the space around the nucleus.
The number of protons is the atomic number, and it defines the element. Changing the neutron count makes a different isotope of the same element. Changing the electron count makes an ion.
Read a symbol as information
An element symbol tells you which element you are working with. Its atomic number gives the proton count. For a neutral atom, the electron count matches the proton count.
Mass number is the total number of protons and neutrons in one atom. Subtract the atomic number from the mass number to find the neutron count. Net charge depends on the difference between the proton and electron counts: more protons gives a positive charge, while more electrons gives a negative charge.
Remember these ideas
Key ideas
- The proton count defines the element.
- A neutral atom has equal numbers of protons and electrons.
- A positive ion has fewer electrons than protons; a negative ion has more electrons than protons.
- Isotopes have the same proton count but different neutron counts.
- Mass number equals protons plus neutrons.
Make the reasoning visible
Read carbon-14
- 1
Carbon’s atomic number is 6, so it has 6 protons.
- 2
The mass number is 14, so protons + neutrons = 14.
- 3
Subtract: 14 − 6 = 8 neutrons.
- 4
If the atom is neutral, it also has 6 electrons.
Try the next step
Read a new ion
A sodium ion has atomic number 11, mass number 23, and a 1+ charge. How many protons, neutrons, and electrons does it have?
What happens when the neutron count changes, and what happens when the electron count changes while the proton count stays fixed?