Atomic Theory - Online Tutor, Practice Problems & Exam Prep

John Dalton's atomic theory laid the groundwork for modern chemistry, proposing that all matter consists of indivisible atoms, which are identical for each element. However, modern science recognizes subatomic particles (protons, neutrons, electrons) and isotopes, showing that atoms can differ in mass. Dalton's idea of fixed ratios in compounds is challenged by complex molecules like glucose. Despite some inaccuracies, Dalton's principles, particularly the conservation of mass in chemical reactions, remain foundational in understanding atomic structure and behavior.

Atomic Theory

In our exploration to understand the atom, we now look at the atomic theory. It was Democritus 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. The term "atom" comes from the Greek word "atomos," which means undivided. 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. You're going to compare and contrast Dalton's atomic theory postulates—the five principles that he held in terms of the atom—versus our more modern atomic view. Realize that Dalton in his time didn't have electron microscopes and some of the technology that we have today, 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, agree that matter is composed of atoms, but we can go even further into the atom and look at the 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 now 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. When you subtract the mass number by the atomic number, you get the number of neutrons. So carbon-12 has 6 neutrons, but carbon-13 has 7 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, as we see with the examples of our isotopes. They have the same number of protons but different mass numbers, so their masses are not 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 a 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, 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 2 hydrogens and 1 oxygen. Carbon dioxide, 1 carbon, 2 oxygens. Ammonia, 1 nitrogen, 3 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, has 6 carbons, 12 hydrogens, 6 oxygens. 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 asks 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 considering the modern atomic theory, which chemists today use through the use of further experimentation and instrumentation. So here, for item 1, 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 with his original postulates.

Next, all lead atoms are identical. This is almost word for word one of his postulates. He stated that identical elements or atoms should possess the same mass, size, and properties. So this would be consistent.

Regarding item 2, Barium and chlorine atoms combine in a 1:2 ratio to form barium chloride. So the chemical formula for barium chloride is BaCl2. There is 1 barium to 2 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 5 original postulates.

So, out of the choices, only option 2 could be consistent with Dalton's original atomic theory. Thus, that would give us option 2 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 are the main postulates of John Dalton's atomic theory?

John Dalton's atomic theory consists of five main postulates: (1) All matter is composed of indivisible atoms. (2) Atoms of a given element are identical in mass, size, and properties. (3) Atoms cannot be created or destroyed. (4) Compounds are formed by the combination of two or more different types of atoms in fixed simple whole number ratios. (5) Chemical reactions involve the rearrangement of atoms. These postulates laid the groundwork for modern chemistry, although some have been refined with the discovery of subatomic particles and isotopes.

How does modern atomic theory differ from Dalton's atomic theory?

Modern atomic theory differs from Dalton's atomic theory in several ways: (1) Atoms are composed of subatomic particles: protons, neutrons, and electrons. (2) Atoms of the same element can have different masses due to the presence of isotopes. (3) Atoms can combine in complex ratios, not just simple whole numbers. (4) Atoms are not indivisible; they can be split in nuclear reactions. Despite these differences, Dalton's ideas about the conservation of mass and the rearrangement of atoms in chemical reactions remain foundational.

What are isotopes and how do they challenge Dalton's atomic theory?

Isotopes are variants of a given element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. This challenges Dalton's postulate that all atoms of a given element are identical in mass and properties. For example, carbon has isotopes like carbon-12 and carbon-13, which differ in mass due to the different number of neutrons. This discovery shows that atoms of the same element can have different masses and properties.

Why is Dalton's atomic theory still important despite its inaccuracies?

Dalton's atomic theory is still important because it laid the foundational principles for modern chemistry. His ideas about the conservation of mass in chemical reactions and the concept of atoms as the basic units of matter are still valid. Although some of his postulates have been refined or disproven with the discovery of subatomic particles and isotopes, his work provided a crucial stepping stone for further advancements in atomic theory and chemistry.

How do modern chemists view the concept of atoms combining in fixed ratios?

Modern chemists recognize that while many compounds do form in fixed simple whole number ratios, such as H₂O (water) and CO₂ (carbon dioxide), there are also complex molecules that do not follow this rule. For example, glucose (C₆H₁₂O₆) has a more complex ratio. This understanding has expanded the scope of chemical compounds beyond the simple ratios proposed by Dalton, acknowledging the complexity and diversity of molecular structures.