Atomic Theory - Online Tutor, Practice Problems & Exam Prep

John Dalton's atomic theory laid the groundwork for our understanding of atoms, despite limitations in technology at the time. Dalton proposed that matter consists of indivisible atoms, which cannot be created or destroyed, aligning with the law of conservation of mass. He also suggested that all atoms of a given element are identical, a concept modern chemistry refines with the understanding of isotopes, which have the same atomic number but different mass numbers due to varying neutron counts. Dalton's idea that compounds are composed of atoms in fixed, simple ratios has evolved with the recognition of more complex molecular structures. Finally, both Dalton and contemporary chemists agree that chemical reactions involve the rearrangement of atoms, adhering to the principle that matter is neither created nor destroyed. This synthesis of Dalton's postulates with current knowledge highlights the progression from early atomic concepts to a more nuanced view of atomic structure and behavior.

Atomic Theory

In our exploration to understand the atom, we now look at the atomic theory. Now it was democratists in around 400 BC who first discussed the atom itself and John Dalton in 1803 tried to take a more modernized view of the atom. Now atom comes from the Greek word Adamus, which means undivided, and Dalton was able to take portions of Democritus view of the atom as well as Lavoisier's conservation of mass to help formulate his atomic theory.

Now here we're going to compare and contrast Dalton's atomic theory postulates, basically the five principles that he held in terms of the atom versus our modern atomic view. Now realize that Dalton in his time didn't have electron microscopes and some of the technology that we have here, so it's still pretty astonishing that he was able to come up with some of these concepts with the atom. The first postulate Dalton had was that all matter is composed of atoms, and they're the smallest particles of matter. We, on the other hand, in modern atomic theory we agree that matter is composed of atoms. But we can go even further into the atom and look at an atom's three subatomic particles, the protons, neutrons and electrons.

Of course, Dalton didn't have microscopes powerful enough to look inside of the atoms, so it's totally understandable that he didn't know about the presence of these three subatomic particles. Now his second postulate. He said that atoms are indivisible and therefore cannot be created or destroyed. This kind of sounds like Lavoisier's conservation of matter or mass: matter cannot be created nor destroyed. All that happens is it changes forms. Modern chemists agree with this second postulate. Dalton's third postulate is that all atoms of a given element are identical in mass, size and properties. So, for example, all carbon atoms, according to John Dalton, were exactly the same.

Modern chemists don't necessarily agree with that. They say that not all atoms of a given element are identical and exist in the same form. Why? Because we know the presence of isotopes. Remember, there's carbon-12 and then there's carbon-13. So here they have the same atomic number. Yes, but when you subtract the mass number by the atomic number, you get the number of neutrons. So carbon-12 has six neutrons, but carbon-13 has seven neutrons, so they're going to have differences in mass. Their mass numbers are not the same, so the same atoms can have different masses and densities. We see that with the examples of our isotopes. They have the same number of protons but different mass numbers, so their masses are the same.

The same atoms can connect in different ways to give rise to different compounds. For example, the most common form of carbon is graphite. Now, in pencils we no longer use lead in pencils. We use graphite because it's not toxic. But this isn't the only form of carbon that exists. There's also carbon diamond, which is the more valuable form of carbon, as well as other types of carbons that we've created within labs. All of them are carbon, but they come in different forms. Dalton never accounted for these phenomena. Now different atoms can have identical masses. So for example, Argon-40 and Calcium-40, they both have a mass number of 40, but they're not the same element.

Now postulate 4. Postulate 4, John Dalton said that compounds are made up of two or more different types of atoms in fixed simple whole number ratios. For example, water was composed of two hydrogens and one oxygen, carbon dioxide, one carbon two oxygens, ammonia, one nitrogen three hydrogens. But modern chemists say atoms don't always combine in fixed, simple whole number ratios. They can be complex. So a good example of this is glucose, sugar. So it's much more complex than Dalton first postulated: C₆H₁₂O₆.

Then finally, Dalton believed that a chemical reaction is a rearrangement of atoms. Why? Because matter cannot be created nor destroyed. It just changes forms. Modern chemists agree with this idea. Matter cannot be created nor destroyed. It's just a rearrangement of those atoms within chemical reactions. So we can see that although some of Dalton's postulates were proven to be not entirely true or wrong, we can see that a great deal of them helped to establish the basis for modern atomic theory. So now that we've seen the contributions by John Dalton and the more modernized modern atomic theory, let's take a look at some practice questions and example questions.

Atomic Theory Example 1

Here in this example question, it says which of the following statements are consistent with Dalton's atomic theory as it was originally stated. So here we're following just Dalton's first five original postulates. We're not taking a look at the modern atomic theory which chemists today use through the use of further experimentations and instrumentation.

So here for one, nitrogen and phosphorus atoms have the same mass. Remember, Dalton wasn't aware of isotopes. He wasn't aware that different elements could have the same mass number and therefore atomic mass, so this would not be consistent when his original posture was. Next, all lead atoms are identical. This is almost word for word one of its postulates. He said that identical elements or identical atoms should possess the same mass, size and properties, so this would be consistent.

Barium and chlorine atoms combine in a one to two ratio to form barium chloride, so Baron chloride is BaCl2. There's one barium to two chlorines. This is consistent with Dalton's original postulate, where atoms combine in simple whole number ratios. Then finally uranium atoms undergo alpha decay to become thorium atoms. This is really talking about radioactive processes which do occur. But again, in Dalton's time he didn't know that this could happen, so this does not go with one of his five original postulates.

So out of the choices, only option two and three could be consistent with Dalton's original atomic theory. So that would give us option C as the correct answer.

Which of the following is NOT a component of Dalton's Atomic Theory?

• A. A chemical reaction rearranges the grouping of atoms.
• B. Atoms are composed of protons, neutrons, and electrons.
• C. Atoms of a given element are chemical and physically identical.
• D. Atoms of different elements combine in simple, whole number ratios to form compounds.
• E. Matter is composed of atoms that are unable to divided any further.

Dalton used the lightest element as his standard for atomic mass. What is this element?

• A. Helium
• B. Oxygen
• C. Carbon
• D. Hydrogen
• E. Nitrogen

Here’s what students ask on this topic:

What is the atomic theory?

Atomic theory is a scientific concept that describes the nature and behavior of matter. The theory posits that matter is composed of discrete units called atoms, which are the smallest particles that retain the chemical properties of an element. The modern atomic theory, which began with John Dalton in the early 19th century, has evolved through the contributions of many scientists.

Dalton's atomic theory included several postulates: elements are made of extremely small particles called atoms; atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties; atoms cannot be subdivided, created, or destroyed; atoms of different elements combine in simple whole-number ratios to form chemical compounds; and in chemical reactions, atoms are combined, separated, or rearranged.

Over time, discoveries in quantum mechanics and the study of subatomic particles have expanded and refined the atomic theory, revealing that atoms have a dense nucleus made of protons and neutrons, surrounded by a cloud of electrons. These discoveries have led to a deeper understanding of the structure and behavior of atoms and the complex interactions that form the basis of chemistry.

What is Dalton's atomic theory?

Dalton's Atomic Theory, proposed by John Dalton in 1808, is a fundamental concept that laid the groundwork for our understanding of chemistry. The theory posits several postulates:

1. Matter is composed of extremely small particles called atoms, which are indivisible and indestructible.
2. All atoms of a given element are identical in mass and properties, but they differ from the atoms of any other element.
3. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process. A chemical reaction simply changes the way atoms are grouped together.
4. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. For example, water is always composed of two hydrogen atoms and one oxygen atom (H₂O).
5. In chemical reactions, atoms are combined, separated, or rearranged but never created, destroyed, or changed into atoms of another element.

While some aspects of Dalton's theory have been modified due to the discovery of subatomic particles and isotopes, it remains a cornerstone of chemical science, explaining the conservation of mass in reactions and the composition of compounds.

What are the main ideas in Dalton's atomic theory?

Dalton's atomic theory, formulated by John Dalton in the early 19th century, laid the foundation for our modern understanding of matter. The main ideas of Dalton's atomic theory are:

1. Indivisible Particles: Atoms are the smallest unit of an element that can participate in chemical reactions and cannot be created, divided, or destroyed.
2. Identical Atoms: All atoms of a given element are identical in mass and properties, but they differ from the atoms of any other element.
3. Chemical Compounds: Compounds are formed when atoms of more than one element combine in a fixed ratio. The relative numbers and kinds of atoms are constant in a given compound.
4. Chemical Reactions: A chemical reaction involves the rearrangement of atoms. Atoms are neither created nor destroyed during a chemical reaction—they are simply rearranged to form new compounds.

While Dalton's atomic theory has been modified and expanded upon with the discovery of subatomic particles and isotopes, these core principles remain a cornerstone of chemical science.

Why is the atomic theory important?

Atomic theory is fundamental because it provides the basis for our understanding of matter and its interactions. It explains the composition and behavior of everything in the physical world at the most basic level. Atoms are the building blocks of matter; they combine to form molecules, which in turn make up all the materials and substances around us. Atomic theory helps us comprehend chemical reactions, where atoms reorganize into different substances, and it underpins the laws of chemistry and physics. This theory has led to advancements in various fields, from material science to medicine, by allowing scientists to predict the properties of matter and how different substances will react with one another. Moreover, it has paved the way for technologies such as nuclear energy and medical imaging techniques, which have had profound impacts on modern society. Understanding atomic theory is essential for any scientific endeavor that involves the manipulation or analysis of materials at the molecular or atomic level.

What is the modern atomic theory?

The modern atomic theory, often referred to as the quantum mechanical model of the atom, is a fundamental theory in physics that explains the nature of matter at the smallest scale. It is based on the principles of quantum mechanics and incorporates the dual nature of particles and waves. According to this theory, electrons do not orbit the nucleus in neat planet-like orbits but rather exist in fuzzy, probabilistic, cloud-like regions called orbitals. Each orbital can hold a certain number of electrons and has a specific energy level and shape.

The exact position and momentum of an electron within an atom cannot be simultaneously known, a concept encapsulated in Heisenberg's Uncertainty Principle. Instead, the theory provides the probability of finding an electron in a particular location around the nucleus. This model was developed throughout the 20th century, building upon the earlier atomic models and experiments of scientists like Niels Bohr, Erwin Schrödinger, Werner Heisenberg, and many others. It has been successful in explaining and predicting chemical behavior through the interactions of subatomic particles.