Heavier atoms, more neutrons

The idea

Lighter atoms often have equal numbers of protons and neutrons, but as atoms get heavier their nuclei have many more neutrons than protons.

Counting in the nucleus

Lighter atoms often have as many neutrons as protons. What happens as atoms get heavier?

In a heavy atom like uranium (92 protons), how do the neutrons compare with the protons?

• There are many more neutrons than protons
• There are fewer neutrons than protons
• There are exactly as many neutrons as protons

Compare four nuclei

Choose each atom to see its protons and neutrons (numbers from the book).

More neutrons as atoms get heavier

Lighter atoms often have equal numbers of protons and neutrons. For heavier atoms, the neutrons pull ahead.

Lighter atoms often have equal numbers of protons and neutrons, but as atoms get heavier their nuclei have many more neutrons than protons.

Notes

Lighter atoms often have equal protons and neutrons; heavier atoms have many more neutrons than protons.

Check yourself

Uranium has 92 protons. How many neutrons does its nucleus have, according to the book?

Answer: 146

The book says the uranium nucleus has 92 protons and 146 neutrons.

Which atom has equal numbers of protons and neutrons?

As atoms get heavier, their nuclei have …

Iron has 26 protons and 30 neutrons. How many more neutrons than protons does it have?

Answer: 4

30 − 26 = 4.

• Oxygen, with 8 and 8 — correct. Yes!
• Iron, with 26 and 30. Iron has 4 more neutrons than protons.
• Uranium, with 92 and 146. Uranium has many more neutrons than protons.
• fewer neutrons than protons. Heavier nuclei have many more neutrons than protons.
• many more neutrons than protons — correct. Yes!
• the same number of neutrons and protons, without exception. Lighter atoms often have equal numbers, but heavier ones do not.