Atomic Structure

The humble atom, the basic building block of the universe. In this lesson, you will learn about its structure and defining characteristics.

A diagram of the carbon atom, an essential element to all life on Earth.

Definition of an atom:

"The smallest part of a substance that cannot be broken down chemically"

National Cancer Institute

Atomic structure

At the top level, an atom is made up of a nucleus and electron cloud. The nucleus is the very small, dense center of the atom, while the electron cloud is where the electrons orbit the nucleus in orbitals or shells (which will be talked about later). You can think about this in terms of our solar system: the sun represents the nucleus, while the orbiting planets represent electrons. An atom is made of three subatomic particles: the proton, the neutron, and the electron. Each of these subatomic particles has its own characteristics, listed below:

Additional info

Charge

Location

Proton

Electron

Neutron

-1

In the nucleus

Orbit around the nucleus

A very important part of atomic structure is atomic mass. The atomic mass unit, or amu, is a way of measuring the mass of subatomic particles. 1 amu is defined as the mass of one proton. A neutron has a similar mass as a proton, so it is also 1 amu (at such small scales, these tiny differences are negligible). Because an electron has so little mass, it is often simply considered to be equal to 0 amu. However, if you want to be specific, know that 1 electron is equal to roughly 1/1840th of an amu.

So what makes atoms different from each other? The most defining factor is the atomic number (denoted Z), which is defined as the number of protons in that atom. and determines the identity of the atom (the element). The periodic table, which we will touch on later, is organized by atomic number. The mass number of an atom is defined as the total mass of the protons and neutrons (again, the mass of electrons is ignored in this case). It is denoted as A.

Example:

Given an atom has 8 protons and 8 neutrons determine the atomic number and mass number.

Solution:

Atomic number = number of protons = 8

Mass number = mass of protons + mass of neutrons = 8 protons * (1 amu/proton) + 8 neutrons * (1 amu/ neutron) = 16 amu

Isotopes

Isotopes of atoms are atoms of the same element with different numbers of neutrons. This means that they have different atomic masses, as neutrons are 1 amu. Note that isotopes of an element all have the same number of protons and electrons, only the number of neutrons changes.

Different numbers of neutrons means different physical properties, such as mass, density, boiling point, and others. Note that the chemical properties (such as how it reacts with other atoms/elements) does not change.

Types of isotopes

There are two types of isotopes: stable and radioactive. As the name suggests, stable isotopes have nuclei that do not change over time. Radioactive isotopes have unstable nuclei, meaning that the element of the atom can change (changing number of protons) and the atom releases energy.

Example:

The element carbon (C) has three common isotopes:

Carbon-12 (6 protons, 6 neutrons)

Carbon-13 (6 protons, 7 neutrons)

Carbon-14 (6 protons, 8 neutrons)

Atomic shells, subshells, and orbitals

Electrons orbit the nucleus so fast that, at any point in time, we cannot pinpoint exactly where an electron is in space. We can, however, guess at where the electron is most likely to be located. This is where shells, subshells, and orbitals come in. The hierarchy is: shells -> subshells -> orbitals -> electrons. You can think of these in the context of an apartment building: atom = apartment building, shells = floors, subshells = different apartments on each floor, orbitals = individual rooms within each apartment, atoms = apartment residents.

Shells

Shells, sometimes referred to as energy levels, are the highest and most general level of organization for electrons. They are labeled by the principle quantum number, or n. So, energy level 1 or shell 1 would be written as n = 1, energy level 2 would be written as n = 2, and so on. Shells are the main energy levels that electrons occupy. As the energy level, or shell level, increases, you get farther and farther away from the nucleus. This means that higher shells have higher energy levels (hence the name "energy level"). The number of shells can technically equal infinity, for example, nitrogen has an infinite number of shells, though most of these are empty (not occupied by electrons).

Subshells

Each shell is divided into one or more subshells. These can be classified into four main types based on their shape: s, p, d, and f subshells. The number of subshells in each shell equals the shell number: shell 1 has 1 subshell, shell 2 has 2 subshells, and so on.