Atomic Number, Isotopes, and Ions
Atoms may all look like tiny specks, but three simple counts tell you almost everything about one: how many protons, how many neutrons, and how many electrons it has. Those counts are the key to atomic number, isotopes, and ions, a cluster of ideas that comes up again and again on science tests.
This lesson keeps the counting straightforward and shows how each number changes what an atom is or how it behaves.
The atomic number is the number of protons in an atom, and it decides which element the atom is. The mass number is the total of protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons. Ions are atoms that have gained or lost electrons and now carry an electric charge.
What does the atomic number tell you?
The atomic number is the count of protons in the nucleus, and it is an element’s fingerprint. Every carbon atom has six protons; change that number and you no longer have carbon. In a neutral atom, the number of electrons equals the number of protons, so the atomic number also tells you the electron count when the atom has no charge.
The mass number is protons plus neutrons, because those two particles hold almost all of an atom’s mass. Electrons are so light they barely count. If you know the atomic number and the mass number, you can find the neutrons by subtracting: neutrons equals mass number minus atomic number.
What are isotopes?
Isotopes are atoms of the same element that have different numbers of neutrons. They have the same atomic number but different mass numbers. Carbon-12 and carbon-14 are both carbon, with six protons each, but carbon-12 has six neutrons and carbon-14 has eight. Isotopes behave the same chemically because chemistry depends on electrons, but some isotopes are radioactive, which makes them useful for dating fossils and for medical imaging.
| Atom | Protons | Neutrons | Mass number |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 |
| Carbon-14 | 6 | 8 | 14 |
What is an ion?
An ion is an atom that has gained or lost one or more electrons, so it is no longer electrically neutral. Lose electrons and you have more protons than electrons, giving a positive charge (a cation). Gain electrons and you have more electrons than protons, giving a negative charge (an anion). Sodium easily loses one electron to become a positive ion, and chlorine easily gains one to become a negative ion, which is exactly why they attract and form table salt.
Notice the pattern: protons set the element, neutrons set the isotope, and electrons set the charge. Keep those three jobs separate and these questions become simple bookkeeping.
Watch: A Short Video Lesson
Khan Academy walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for atom-counting questions
- Read the atomic number to get protons, and the element.
- For a neutral atom, set electrons equal to protons.
- Subtract atomic number from mass number to get neutrons.
- Same protons but different neutrons means isotopes.
- A charge means electrons were gained (negative) or lost (positive).
Practice questions
- An atom has 11 protons. What element is it, and what is its atomic number?
- An atom has an atomic number of 8 and a mass number of 16. How many neutrons does it have?
- What do two isotopes of the same element share, and what differs?
- An atom gains two electrons. What is its charge?
- Carbon-13 has how many neutrons? (Carbon’s atomic number is 6.)
- True or false: changing the number of neutrons changes which element an atom is.
Answers:
- Sodium, atomic number 11.
- 8 neutrons (16 minus 8).
- They share the same number of protons (atomic number); they differ in neutrons (mass number).
- A charge of 2 minus, a negative ion.
- 7 neutrons (13 minus 6).
- False. Protons decide the element; changing neutrons only makes a different isotope.
Where this fits
Counting protons, neutrons, and electrons builds directly on the basics of atoms and their parts, and it prepares you to read the periodic table where elements are arranged by atomic number. From here, the way atoms gain and lose electrons leads naturally into chemical bonding. Find every topic on the ASVAB General Science Learning Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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