Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Neutron starts by naming the element, charge, and relevant protons, neutrons, or electrons.
Common stuck point:Students often know a formula related to neutron but skip the recognition step: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion? That leads to a correct-looking substitution attached to the wrong chemical model.
Sense of Study hint:Ask: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?
Worked Examples
Example 1
easy
An atom of sodium has a mass number of 23 and an atomic number of 11. How many neutrons does it contain?
Answer
12 neutrons
First step
1
Recall that the mass number A equals the total number of protons plus neutrons: A=Z+N.
Full solution
2
Rearrange to find the neutron count: N=A−Z.
3
Substitute: N=23−11=12 neutrons.
Neutrons contribute to the mass of an atom but not its charge. The mass number is the total count of protons plus neutrons in the nucleus.
Example 2
medium
Explain why neutrons are important for nuclear stability. Why do heavier elements tend to have more neutrons than protons?
Example 3
medium
An ion 2040Ca2+ has lost 2 electrons. How many neutrons does it have?
Example 4
medium
Chlorine has natural isotopes Cl-35 (75.77%) and Cl-37 (24.23%). Estimate the average atomic mass.
Example 5
medium
Iodine-131 is used in medicine. Its atomic number is 53. How many neutrons does it have, and how many more than the stable I-127?
Example 6
hard
An atom has neutron-to-proton ratio 1.52 and 25 protons. Find the mass number.
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easy
Calculate the number of neutrons in 2656Fe.
Example 2
medium
Two atoms each have 17 protons. One has 18 neutrons and the other has 20 neutrons. Are they different elements or isotopes?
Example 3
easy
What is the electric charge of a neutron?
Example 4
easy
Where in the atom are neutrons located?
Example 5
easy
An atom has mass number 14 and atomic number 6. How many neutrons does it have?
Example 6
easy
Do neutrons determine the chemical properties of an atom?
Example 7
easy
How many neutrons does hydrogen-1 (the most common hydrogen) have?
Example 8
easy
Which two particles in the nucleus contribute most of an atom's mass?
Example 9
easy
Compared to a proton, how heavy is a neutron?
Example 10
easy
How are neutrons different from electrons in location and mass?
Example 11
medium
An atom of an element has 17 protons and 18 neutrons. What is its mass number, and name the element?
Example 12
medium
Carbon-12 and carbon-14 are both carbon. How many more neutrons does carbon-14 have?
Example 13
medium
Why can two atoms with the same number of protons have different masses?
Example 14
medium
An atom of iron has mass number 56 and 26 protons. How many neutrons, and would adding a neutron change which element it is?
Example 15
medium
An ion O2− has 8 protons and 10 electrons. How many neutrons does the most common oxygen-16 have?
Example 16
medium
Two isotopes of an element have mass numbers 35 and 37 and atomic number 17. How many neutrons does each have?
Example 17
medium
Why does adding neutrons eventually make some nuclei unstable (radioactive)?
Example 18
medium
An atom has 9 protons and 10 neutrons. What is its mass number?
Example 19
medium
Why does adding a neutron to a carbon-12 nucleus not change it into a different element?
Example 20
challenge
An atom has mass number 40 and 22 neutrons. Give the proton count, the element, and whether it shares chemistry with an atom of 18 protons.
Example 21
challenge
Element Z has two isotopes: A=63 (69%) and A=65 (31%), both with 29 protons. Find the neutrons in each and the approximate average atomic mass.
Example 22
challenge
Why do carbon-12 and carbon-14 react identically in chemical reactions yet differ in nuclear behavior?
Example 23
easy
An atom of aluminum-27 has atomic number 13. How many neutrons are in its nucleus?
Example 24
easy
Does the number of neutrons change when a neutral atom becomes a positive ion by losing electrons?
Example 25
easy
Uranium-238 has 92 protons. How many neutrons does it contain?
Example 26
easy
True or false: two isotopes of the same element have the same number of neutrons.
Example 27
easy
Which subatomic particle was discovered last: proton, neutron, or electron?
Example 28
medium
Silver-109 has 47 protons. State the number of neutrons and the number of nucleons.
Example 29
medium
Deuterium is hydrogen-2. How many protons, neutrons, and electrons are in a neutral deuterium atom?
Example 30
medium
Tritium has 1 proton and 2 neutrons. Write its isotope notation.
Example 31
medium
A neutral atom has 19 protons, 20 neutrons, and 19 electrons. Identify the element and its mass number.
Example 32
medium
Why are neutrons sometimes called the 'glue' of heavy nuclei?
Example 33
medium
An isotope of zinc has 30 protons and 36 neutrons. Write its isotope notation.
Example 34
medium
Phosphorus-31 has 15 protons. Find its neutron count and explain whether changing the neutron count would create a different element.
Example 35
medium
A neutral atom has 7 protons and 8 neutrons. State its mass number and chemical symbol.
Example 36
medium
If a free neutron decays, it produces a proton, an electron, and an antineutrino. What happens to the total charge?
Example 37
hard
An ion 1632S2− has gained 2 electrons. List the number of protons, neutrons, and electrons.
Example 38
hard
An element has only two natural isotopes with mass numbers 10 and 11. The average atomic mass is 10.81. Estimate the percent abundance of the heavier isotope.
Example 39
hard
Two isotopes of magnesium have mass numbers 24 and 26, with abundances 78.99% and 11.01% respectively. A third isotope (mass 25) makes up the rest. Find that third abundance and the neutron count for each isotope (Mg has Z=12).
Example 40
hard
In a nuclear equation, 714N+01n→614C+X. Identify X and explain why neutron count rises by 1 in the carbon product.
Example 41
challenge
U-235 absorbs a neutron and splits to give Ba-141, Kr-92, and several free neutrons. Use conservation of nucleons to find how many free neutrons are released.