All atoms of a particular element have the same number of protons.

The idea

The number of protons is what defines an element. Every carbon atom anywhere has six protons; anything with a different number of protons is simply not carbon. Neutrons may vary, giving isotopes of the same element, and electrons may be lost or gained, giving ions, but neither changes what the element is. Change the proton number and you have changed the element, which is exactly what happens in alpha and beta decay. This is the reason examiners award the mark for identifying two atoms as the same element only when the answer compares proton numbers, or the equivalent atomic numbers, and it is also why electron counts are ignored in those answers.

What earns the marks

• Stating that every atom of an element has the same number of protons.
• Stating that this number is the atomic number of the element.
• Identifying two atoms as the same element by comparing their proton numbers only.
• Stating that a different number of protons means a different element.

Common mistakes and how to put them right

• Identifying an element by its mass number rather than its proton number.

• Concluding that two atoms with different numbers of neutrons must be different elements.

• Using the number of electrons to decide which element an atom is.

• Claiming that an ion of an element is a different element.

Exam tips

• When asked which atoms are the same element, quote the proton numbers and say they match; the mark scheme ignores electrons.
• Say same atomic number as an alternative form of the same answer.
• Remember that this is why alpha and beta decay produce a new element, while gamma emission does not.