The following atoms of carbon 126C, 136C and 146C can be described as?

Step 1: Analyze the question The three atoms are carbon-12, carbon-13 and carbon-14. All are carbon, but they have different mass numbers (12, 13, 14). We need the term that describes this relationship.

Step 2: Key concept

• Atomic number (Z) = number of protons, which defines the element.
• Mass number (A) = protons + neutrons.

Atoms of the same element with the same atomic number but different mass numbers (because of different numbers of neutrons) are called isotopes.

Step 3: Work it out

| Atom | Protons | Neutrons (A − Z) | |------|---------|------------------| | ¹²₆C | 6 | 6 | | ¹³₆C | 6 | 7 | | ¹⁴₆C | 6 | 8 |

All three have 6 protons, so they are the same element. The number of neutrons differs, so the mass numbers differ. This matches the definition of isotopes.

Step 4: Check the other options

• A. Allotropes: different physical forms of the same element in the same state, such as diamond and graphite, which are both forms of carbon. This is about how the atoms are arranged, not about the atoms themselves.
• B. Isomers: compounds with the same molecular formula but different structures, such as butane and methylpropane. They are not individual atoms.
• D. Isotones: atoms of different elements with the same number of neutrons. That is not the case here, since all three are carbon.

Answer: C. Isotopes

Exam tip: Remember the "p" in isotoPes: same Protons. Isotopes have the same number of protons and different neutrons. Isotones have the same number of neutrons. For allotropes, think "different forms" (diamond, graphite, and fullerene for carbon; oxygen and ozone for oxygen).