Twin atoms

The idea

Isotopes are atoms of the same element with the same atomic number but different numbers of neutrons, and hence different mass numbers.

Are all atoms of an element identical?

Dalton believed that all atoms of an element are identical and have the same mass.

Could two atoms of the same element have different masses?

• Yes, if they have different numbers of neutrons
• No, atoms of an element are identical in every way
• Yes, if they have different numbers of protons

Same element, different neutrons

Choose an atom. Compare the protons, neutrons and electrons.

Isotopes

Atoms of the same element all have the same number of protons, which is the atomic number. But their numbers of neutrons can differ, so their mass numbers differ.

Isotopes are atoms of the same element with the same atomic number but different numbers of neutrons, and hence different mass numbers.

Notes

Isotopes: atoms of the same element (same atomic number) with different numbers of neutrons, hence different mass numbers.

Check yourself

Isotopes of an element have the same …, but different …

¹³₆C is an isotope of carbon with 6 protons. How many neutrons does it have?

Answer: 7

13 − 6 = 7 neutrons.

Tritium, ³₁H, has 1 proton. How many neutrons does it have?

Answer: 2

3 − 1 = 2 neutrons.

Dalton said all atoms of an element are identical. What was found later?

• atomic number, mass number — correct. Yes!
• mass number, atomic number. Isotopes have the same atomic number and different mass numbers.
• number of electrons, number of protons. Isotopes have the same number of protons and of electrons.
• Atoms of the same element can have different numbers of protons. The number of protons is the atomic number, which identifies the element.
• Atoms of the same element can have different numbers of neutrons — correct. Yes!
• Atoms of the same element can have different numbers of electrons in a neutral atom. A neutral atom has electrons equal to its protons, which are the same for the element.