# The Atom

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• 0:05 The Atom
• 0:47 Protons, Neutrons and…
• 1:55 Elements and the…
• 3:11 Lesson Summary

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Lesson Transcript
Instructor: Ori Gold
Tune into this lesson to find out what matters about matter. What exactly is an atom? And, how do the atoms that make up the elements in the periodic table differ from one another?

## The Atom

All things are composed of matter; you, me and everything around us is made up of matter. Matter is a physical substance that takes up space and has mass. An atom is the basic unit of matter, but what does it look like? What is it made of?

Atoms are composed of three subatomic particles: protons, neutrons, and electrons. We can categorize protons, neutrons, and electrons using two basic traits: charge and mass. Charge is the electrical property of the particle - whether it's positive, negative, or neutral. Mass is just a measure of the how much matter the particle has. Let's talk about the protons, neutrons, and electrons in terms of these two characteristics.

## Subatomic Particles and the Nucleus

We'll start with protons. Protons are positive in charge and have a mass of one atomic mass unit. At the opposite end of the spectrum, you have the electron with a negative charge. The mass of an electron isn't zero, but it's very, very small - about .05% of a proton's mass. It's close enough to zero for our purposes that we'll consider it to have essentially negligible mass. And, finally, we have the neutron, which is neutral in charge and also has a mass of one atomic mass unit.

But how are these particles arranged in atoms? The first thing to know is that each atom has an atomic nucleus, which is a collection of protons and neutrons at the center of the atom. This cluster of protons and neutrons is very small, very dense, and very positively charged. That just leaves the electrons, which exist in the electron cloud, or the space in which electrons move within an atom. Electrons move around the nucleus of the atom, kind of like how planets in our solar system orbit the sun. We'll talk more about the electron cloud later.

## Elements and the Periodic Table

We know how atoms are structured, but how do we know whether we're looking at an oxygen atom, a nitrogen atom, or a carbon atom? All we need to do is find the atomic number, the number of protons in the atom's nucleus. This number determines the atom's identity. For example, carbon's atomic number is six, so we know that any atom with six protons in its nucleus is a carbon atom.

An element is a substance that is composed of a single type of atom, such as oxygen, aluminum, and gold. These are all elements because each is only made of one type of atom: oxygen atoms, aluminum atoms, and gold atoms.

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