Atomic structure · EduSite Academy

▶ Protons, neutrons, and electrons in atoms

Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus, the dense region at the center of an atom. Electrons are found outside the nucleus. Protons are positively charged and have a mass of about 1 u. Neutrons are neutral (have no charge) and also have a mass of about 1 u. Electrons are negatively charged and have a much smaller mass of about 0.0005 u.

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▶ Elements and atomic number

Atoms come in a variety of types, called elements, each with a unique set of physical and chemical properties. An element is defined by the number of protons in its atoms, known as its atomic number. The periodic table organizes the elements by atomic number and provides other key information about them such as their names and chemical symbols.

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📄 Learn and try: Atomic structure

##Zooming in on the atom

If you could keep zooming in on everything around you—your hand, the desk, the air—what would you eventually see?

For centuries, scientists asked the same question. As they looked closer, they discovered that everything in the universe is made of incredibly tiny particles called atoms.

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Atoms are far too small to see directly, so scientists had to get creative to uncover their interior structure. They probed atoms with electricity and bombarded them with other particles to see what happened.

From the clues left behind, they began to build models, or visual representations that described the atom’s internal structure. These models changed over time.

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Early scientists imagined atoms as solid spheres. Later, experiments showed that atoms had smaller parts inside, forcing scientists to rethink their models. Even today, we continue refining these models as new evidence and technology reveal more about the atom.

##The Bohr model of an atom

Let’s begin by describing the basic structure of the smaller parts that make up an atom. We’ll begin with a common analogy, then refine it.

To start, think of an atom as a tiny solar system. At its center is a dense core called the nucleus, like the sun in the middle of our solar system. Around it, much lighter electrons exist in a vast area of mostly empty space, like planets in orbit.

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This visual, or model, helps us understand two key ideas:

Most of an atom’s mass is concentrated in the nucleus in the center.
Most of an atom’s volume comes from the space where electrons move, around the nucleus.

The discovery of the nucleus by Ernest Rutherford first established the solar system analogy for the atom, with the Sun as nucleus and the electrons as orbiting planets. Niels Bohr updated this a few years later with the Bohr model, which more accurately describes the arrangement of electrons in specific energy levels.

This isn’t a perfect picture of how atoms really behave, but it provides a good starting point. Let’s continue to refine our atomic model and zoom in on three [[☃ definition 1]] particles that make up all atoms.

##Parts of an atom

Atoms are composed of three main types of subatomic particles: protons, neutrons, and electrons.

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Protons are positively charged particles found in the nucleus.

The number of protons in an atom’s nucleus determines which element it is. Protons define an atom’s identity.

An atom with $1$ proton is hydrogen. An atom with $6$ protons is carbon. Change the number of protons, and you change the identity of the element.
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Neutrons are neutral particles found in the nucleus. They carry no charge.

Neutrons are about the same size and mass as protons. Together, neutrons and protons make up nearly all of an atom’s mass.

Changing the number of neutrons creates an isotope, or another version of the same element with a different mass.
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Electrons are negatively charged particles. They exist in a region of space around the nucleus called the electron cloud, where their exact position cannot be pinpointed.

Electrons are much smaller and lighter than protons or neutrons.

The number and arrangement of electrons determine how an atom reacts or bonds with other atoms.

##Build an atom with a simulation

The [[☃ definition 1]] below allows us to build a Bohr model of an atom. Drag and drop different subatomic particles into the center and observe the following:

The location and charge of each subatomic particle
The subatomic particle that determines the identity of an element
The subatomic particles that impact the mass of the atom
The subatomic particles that have to be equal for an atom to be neutral

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Tip: Click the $\color{#58ac33}{\text{green}}$ plus sign next to Net Charge and Mass Number to see more visuals to support your analysis.

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##Connecting atomic structure to the periodic table

The Bohr model is useful because it connects the internal structure of the atom with what we see on the periodic table of elements. This table shows every element in a square and includes information about the number of protons, neutrons, and electrons in each one.

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Each element has its own atomic number, which equals the number of protons in the nucleus. This number is written at the top of each square. You can see the periodic table is ordered by atomic number, starting with $1$ at the top left.

An element is defined by its number of protons, so the atomic number can also be used to identify an element. For instance, all hydrogen atoms have one proton, so hydrogen has an atomic number of $1$. The element with an atomic number of $2$ is always helium. All helium atoms have $2$ protons, that’s what defines them as helium.

As you move from left to right, each element's atomic number increases by one, matching the number protons in the nucleus.

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The atomic number is also equal to the number of electrons in a neutral atom. An atom is neutral when the number of protons is equal to the number of electrons. So, a neutral atom of carbon, atomic number $6$, has six protons and six electrons.

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Scientists use another value called the mass number to describe an atom’s total mass. Because electrons have very little mass, only the protons and neutrons in the nucleus contribute to the mass.

$\text{Mass number} = \text{(# of protons)} + \text{(# of neutrons)}$

For example, an atom with six protons and six neutrons has a mass number of $12$. If we know the mass number, the number of neutrons can be found by subtracting the atomic number (the number of protons) from the mass number.

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The discovery of atomic structure unlocked the patterns that organize all matter. When scientists understood this, chemistry became a science of patterns rather than just observation.

##Key points

Atoms are the fundamental building blocks that make up all matter.
The nucleus is the dense center of the atom that contains protons and neutrons, and holds most of the atom’s mass.
Protons are positively charged particles in the nucleus that determine an atom’s identity and contribute to an atom’s mass.
Neutrons are neutral particles in the nucleus that contribute to an atom’s mass. Atoms with different numbers of neutrons are called isotopes.
Electrons are negatively charged particles that move in the space around the nucleus, and control how atoms interact with each other.
The electron cloud is the region around the nucleus where electrons are likely to be found, and makes up most of the atom’s volume.
The atomic number is equal to the number of protons in an atom’s nucleus. It is also equal to the number of electrons in a neutral atom.
The mass number is the total number of protons and neutrons in the nucleus.

[[☃ explanation 1]]