What is an Atom?

Hi, I’m Emerald Robinson. In this “What Is” video we’re going to take a closer look at atoms.

All matter is made of elements, substances that cannot be broken down by chemical activity. An atom is the smallest particle of an element that retains the element’s properties. Atoms are the building blocks for everything in the universe, including ourselves.

People often think of atoms as the smallest possible forms of matter, but even smaller particles make up atoms. Each atom has a dense center called a nucleus. The nucleus contains neutrons, which have a negative electric charge, and protons, which are positively charged. The number of protons in an atom determines an element’s physical properties. No two elements have the same number of protons.

Revolving around the nucleus, like planets around the sun, are negatively charged particles called electrons. Electrons and protons attract each other, so atoms usually have equal numbers of the two particles.

Scientists assign atoms an atomic number and atomic mass unit. The atomic number is the number of protons present in the atom. For instance, a helium atom has two protons, so helium’s atomic number is 2.

Add the number of protons and neutrons in an atom’s nucleus together and you get the atomic mass unit, or AMU. A helium atom has two protons and two neutrons, so has a mass of 4 AMU. Electrons have almost no mass, and are not included in AMU calculations.

While an atom’s number of protons remains constant, individual atoms can lose or gain electrons. Atoms with different ratios of electrons to protons are called ions. Atoms of the same element can also have extra neutrons. Scientists call atoms with extra neutrons isotopes. Despite having different neutron counts, isotopes retain their element’s chemical properties because the number of protons remains constant.