Isotopes: Same Element, Different Weight
Every carbon atom in your body has 6 protons -- that's what makes it carbon. But not every carbon atom has the same number of neutrons. Some carbon atoms have 6 neutrons, some have 7, and a rare few have 8. Scientists call these different versions of carbon 'isotopes,' and one of them, Carbon-14, is used by archaeologists to figure out how old a 5,000-year-old skeleton is.
What You'll Learn
You will learn what protons, neutrons, and atomic number mean. You will learn what makes two atoms isotopes of the same element. You will learn about real isotopes like Carbon-12, Carbon-14, and the three forms of hydrogen. You will learn how scientists use one isotope to find the age of ancient objects.
Protons Decide the Element, Neutrons Can Change
Every atom has a nucleus in its center made of two kinds of particles: protons and neutrons. The number of protons is what decides which element an atom is -- every atom with exactly 6 protons is carbon, no matter what. Every atom with exactly 1 proton is hydrogen. But the number of neutrons in that same nucleus can be different from one atom to the next, even though the number of protons stays the same. Atoms of the same element with different numbers of neutrons are called isotopes.
Meet the Carbon Isotopes
Carbon-12 is the most common isotope of carbon: it has 6 protons and 6 neutrons, adding up to a mass number of 12 (that's where the '12' in its name comes from). Carbon-13 has 6 protons and 7 neutrons. Carbon-14 has 6 protons and 8 neutrons. Carbon-14 is special because it's radioactive -- it slowly breaks down over thousands of years at a steady, predictable rate. Every living thing absorbs a small, steady amount of Carbon-14 while it's alive. Once it dies, the Carbon-14 inside it starts disappearing at that same steady rate, which lets scientists calculate how long ago something died -- a method called radiocarbon dating.
Hydrogen Has Isotopes Too
Hydrogen, the simplest element with just 1 proton, has three well-known isotopes. Regular hydrogen (called protium) has 0 neutrons. Deuterium has 1 neutron. Tritium has 2 neutrons. Deuterium is used to make 'heavy water,' which is used in some types of nuclear reactors. Tritium is radioactive and is used to make the glowing numbers on some watch faces and emergency exit signs that glow without needing a battery.
Because isotopes have different numbers of neutrons, they have different masses even though they're the same element. Scientists often write an isotope's name with its mass number, like Carbon-14 or Uranium-235, so you instantly know how many protons plus neutrons are packed into that atom's nucleus.
Match each isotope to its correct number of neutrons.
Terms
Definitions
Drag terms onto their definitions, or click a term then click a definition to match.
What makes two atoms isotopes of the same element?
Why can scientists use Carbon-14 to estimate the age of an ancient skeleton?
Build an Isotope Model
Using two colors of small balls, beads, or balls of clay (one color for protons, one for neutrons), build a model of Carbon-12 (6 protons + 6 neutrons) and a model of Carbon-14 (6 protons + 8 neutrons) side by side. Label each model with its name and total mass number (protons + neutrons). Write one sentence explaining what is different between your two models and what stayed the same.
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