Understanding Atomic Structures

Teacher Guide by Amy Roediger

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The atom is the fundamental unit of chemical elements everything around us that has mass and takes up space is made of atoms. For many students, it is incredibly difficult to comprehend this idea, since atoms are so small, that even with very powerful scientific tools, they are not easily seen. Science teachers often rely on analogies and comparisons to help students picture what an atom “looks like”. Understanding the atom is crucial; it serves as the foundation for the rest of chemistry, and plays an important role in many parts of physics.

By the end of this lesson your students will create amazing storyboards like the ones below!

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Teaching Atoms Lesson Plans, Student Activities and Graphic Organizers

History of The Atomic Model

Many lessons on atomic structure begin with a recitation of the historic developments that led to the current theory. It is a valuable experience for students to understand that scientific ideas evolve over time in response to new and better data. Using the timeline layout, students can learn about major atomic developments and demonstrate their knowledge.

  • Democritus is often credited with the first atomic theory, though it was a philosophical idea without evidence. He is also credited with using the word "atomos" (from which we get our word atom) to describe a small unit of matter that was unbreakable.

  • Thousands of years later, John Dalton completed scientific experiments to develop his atomic model with evidence. His theory included the ideas that atoms make up all matter, are indivisible, and that atoms of an element are identical.

  • JJ Thomson is credited with proving the first part of Dalton’s theory incorrect when he discovered the electron, as a result of his cathode ray tube experiment.

  • Ernest Rutherford later showed that most of the mass, and all of the positive charge, of an atom is found in a small dense core of the atom that we call the nucleus.

  • Niels Bohr studied the atomic emission spectra in a quest to better understand the electron arrangements in atoms. He suggested that electrons move on orbits like planets around a sun. This was later found to be too specific a model and was pushed aside for the current theory of quantum mechanics which is based on mathematics and describes a probability of “locating” an electron.

Students can upload pictures of these scientists or, in some cases, of their equipment, using Photos for Class. They can also describe the major advancements in each description box to make as detailed or general a timeline as is required by the level of chemistry they are studying.

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Parts of an Atom

We once thought that atoms are indivisible, but we now know that they are made of three subatomic particles: the proton, the neutron, and the electron. This activity helps to reinforce the location, mass, and charge of these particles and the meaning of two key vocabulary terms: atomic number and mass number.

  • The proton and neutron are found in the nucleus of the atom. The proton has a relative mass of 1 amu and a relative charge of +1. The neutron has a relative mass of 1 amu and a neutral charge. Most of the mass of the atom is found in the small, dense nucleus because these two particles are the more massive of the three.

  • The electron, by comparison, has a mass almost 2000 times less than a proton or neutron. That means that it would take around 2000 electrons to equal the mass of one proton. Because the biggest atoms we know of have only 118 electrons, the electron contributes almost nothing to the atom in terms of mass. Each does, however, have a relative charge of -1.

  • The atomic number is defined as the number of protons in an atom. Because atoms are electrically neutral, the atomic number also tells us how many electrons are in a neutral atom.

  • The mass number is simply the mass of the atom. Since the protons and neutrons contribute the mass to the atom, the mass number tells us the number of protons and neutrons combined.

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What are Isotopes?

Though John Dalton postulated that all atoms of an element are identical, scientists now know that isotopes, or different versions of each element, exist in nature. A Frayer Model is a perfect tool to deepen the understanding of isotopes. Students will define isotopes, show important characteristics, and draw examples and non-examples in this activity.

Isotopes are atoms of an element that differ in their number of neutrons. Because the identity of an atom is defined by its atomic number, as long as the atoms have the same number of protons, the atoms are the same element. Just as apples can be different sizes, atoms can also be heavy or light, even when they are the same element. Boron, for example, is found in nature as atoms with a relative mass of 10 amu (5 protons and 5 neutrons) or 11 amu (5 protons and 6 neutrons). In a sample of boron, around 20% of the atoms would have a mass of 10 amu and 80% would have a mass of 11 amu.

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What is Atomic Mass?

Students will use the atomic mass of an element often throughout any chemistry course. A common misconception is that some atoms have a mass equal to the atomic mass, but that is false in many cases. Furthermore, because the atomic mass is a weighted average that reflects the abundance of isotopes in nature, a model helps students better understand this concept.

In order to calculate the atomic mass of an element, scientists must know what isotopes exist and how abundant they are. Once this is known, the average atomic mass is calculated by taking into account the masses of the isotopes and how prevalent they are. This can be compared to finding the average age of students in a class. When most of the students are 16, but some of the students are 17, the average age can be predicted to be close to 16. Students typically understand this intuitively. The average age is not necessarily 16.5 (the average of 16 and 17) unless their are equal numbers of students who are 16 or 17. This model can be extended to isotopes: the more abundant an isotope, the closer the average will be to the mass of that isotope and the average will not necessarily be the midpoint between the masses of the isotopes.

This storyboard could be used to help students understand this idea or mathematically, where students create examples that illustrate the percent abundances for an element and calculate the atomic mass for an element.

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The Electron Hotel

Perhaps the most difficult to understand part of atomic structure is the current notion of how electrons are arranged outside the nucleus. This can also be the hardest to picture or describe. Students come to chemistry comfortable with the idea of electrons orbiting the nucleus like planets around a sun. Though this idea is known to be incorrect for nearly 100 years, it persists, in part, due to the ease at which people can relate to it. In order to help students grasp the notion of electrons moving about in three-dimensional space, arranged by increasing potential energy and described by clouds of probability, an analogy is helpful.

Electrons with the least potential energy are found near the nucleus. As the distance between the nucleus and the electrons increases, the potential energy also increases. Where students might be tempted to talk about “rings” of electrons, electrons are actually arranged into energy levels and sublevels. The sublevels are sets of orbitals, clouds of space that predict the 90% chance of locating the electron. These probability clouds are determined mathematically.

Though not a perfect analogy, this can be likened to an electron hotel. The floors of the hotel would represent energy levels, each one farther away from the ground floor, or nucleus. Each floor would have wings that are sublevels and within the wings are rooms, or orbitals. A room may contain 0, 1, or 2 electrons. The hotel would populate the rooms, wings, and floors in order of increasing energy, just as the Aufbau Principle describes the build-up of electrons according to energy. Creating a visual representation of this sophisticated idea helps students transition from the imperfect model of a solar system atom to more a complicated (but more accurate) model based on probability.

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•   (English) Understanding Atomic Structures   •   (Español) Descripción de las Estructuras Atómicas   •   (Français) Comprendre les Structures Atomiques   •   (Deutsch) Verstehen Atomstrukturen   •   (Italiana) Capire Strutture Atomiche   •   (Nederlands) Inzicht Atomic Structures   •   (Português) Compreendendo Estruturas Atômicas   •   (עברית) הבנת מבנה אטומי   •   (العَرَبِيَّة) فهم التراكيب الذرية   •   (हिन्दी) परमाणु संरचना को समझना   •   (ру́сский язы́к) Понимание Атомных Структур   •   (Dansk) Forståelse Atomic Structures   •   (Svenska) Förstå Atomic Strukturer   •   (Suomi) Ymmärtäminen Atomirakenteita   •   (Norsk) Forstå Atomstruktur   •   (Türkçe) Atomik Yapıları Anlama   •   (Polski) Zrozumienie Struktur Atomowych   •   (Româna) Înțelegerea Structurilor Atomice   •   (Ceština) Pochopení Atomové Struktury   •   (Slovenský) Pochopenie Atómových Štruktúr   •   (Magyar) Megértése az Atomi Struktúrákat   •   (Hrvatski) Razumijevanje Atomske Strukture   •   (български) Разбиране на Атомните Структури   •   (Lietuvos) Supratimas Atominiai Struktūrų   •   (Slovenščina) Razumevanje Atomske Strukture   •   (Latvijas) Izpratne ATOMIC Struktūru   •   (eesti) Mõistmise Aatomi Ehituse