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# Atomic Number

Atomic Number The nucleus of an atom has protons and neutrons in it. Each element (like carbon or oxygen or gold) has a different number of protons in its atoms. Scientists have a special name for the number of protons in an atom. They call it the “atomic number”.

Why is the atomic numbemr important? Normal atoms have the same number of electrons as protons. The number of electrons is what makes each element behave a certain way in chemical reactions. So the atomic numbe,r, which is the number of protons and thus of electrons, is what makes one element different from another.

Hydrogen atoms have 1 proton, and thus an atomic numbe,r of 1. Carbon has 6 protons and an atomic number of 6; oxygen has 8 protons and thus and atomic numbe,r of 8. The atomic number of uranium is 92!

Atoms of the same element and same atomic number can have different numbers of neutrons. All carbon atoms have 6 protons. Most carbon atoms also have 6 neutrons, but some carbon atoms have 7 or even 8 neutrons. Scientists call these different kinds of carbon atoms “isotopes” of carbon.

Scientists also talk about the “atomic mass” of an atom. The nucleus of an atom contains nearly all (more than 99%) of an atom’s mass. Neutrons and protons have almost exactly the same mass. So, to calculate atomic mass, we just add up the number of protons plus the number of neutrons. A carbon atom with 6 protons and 8 neutrons has an atomic mass of 14 ( = 6 + 8). Sometimes scientists use the letter “Z” to stand for atomic number and the letter “A” to stand for atomic mass.