Atoms - Where all matter begins
The basic building block of all matter
All matter in the world begins with atoms - they form elements like oxygen, hydrogen, and carbon.
An atom consists of a nucleus - made up of protons and neutrons - and electrons that orbit the nucleus. The nucleus carries a positive charge - protons are positively charged, and neutrons don't carry a charge. The electrons, which carry a negative charge, move around the nucleus in clouds (or shells). The negative electrons are attracted to the positive nucleus with an electrical force similar to magnets. This is how the atom stays together.
It is the number of protons in the nucleus - the atomic number - that distinguishes each element. The number of protons is unique to each element. There are six protons in carbon; therefore, its atomic number is 6 on the periodic table.
Atoms are stable when the number of neutrons and protons in the nucleus are balanced. When there is a significant imbalance between the number of neutrons and protons in a nucleus, the atom becomes unstable and in order to achieve stability, the atom may undergo a transformation or radioactive decay.
Atoms from one or more elements combine to form larger compounds, which are called molecules. A molecule of water, for example, is formed of two atoms of hydrogen combined with one atom of oxygen (H2O).
A nuclide is a specific type of atom characterized by the number of protons and neutrons in the nucleus, which approximates the mass of the nuclide. The number that is sometimes given with the name of the nuclide is called its mass number (the total number of protons and neutrons in the nucleus). For example, carbon-12 is a nuclide of carbon with 6 protons and 6 neutrons.
An isotope is a variant on a basic element - where the element has the same number of protons, but a different number of neutrons. For example, there are three isotopes (or variants) of hydrogen: hydrogen-1 (one proton and no neutrons), hydrogen-2, or deuterium (one proton and one neutron), and hydrogen-3, which is called tritium (one proton and two neutrons). Another example is uranium-235, which has 92 protons and 143 neutrons.
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