Question

Section I-5 describes the postulates of Dalton's atomic theory. With some modifications, these postulates hold up very well regarding how we view elements, compounds, and chemical reactions today. Answer the following questions concerning Dalton's atomic theory and the modifications made today. a. The atom can be broken down into smaller parts. What are the smaller parts? b. How are atoms of hydrogen identical to each other and how can they be different from each other? c. How are atoms of hydrogen different from atoms of helium? How can H atoms be similar to He atoms? d. How is water different from hydrogen peroxide \(\left(\mathrm{H}_{2} \mathrm{O}_{2}\right)\) even though both compounds are composed of only hydrogen and oxygen? e. What happens in a chemical reaction and why is mass conserved in a chemical reaction?

Short answer

a. Atoms can be broken down into protons, neutrons, and electrons. b. Hydrogen atoms are identical in their atomic number (1) and basic structure (1 proton, 1 electron), but can differ in the number of neutrons, creating isotopes (e.g. protium, deuterium, tritium). c. Hydrogen and helium differ in atomic number and number of subatomic particles (H: 1 proton, 1 electron; He: 2 protons, 2 electrons). They share similar chemical reactivity due to both being in Group 1 of the periodic table. d. Water (H2O) and hydrogen peroxide (H2O2) have different molecular structures and chemical properties due to varying arrangements of hydrogen and oxygen atoms. e. In a chemical reaction, bonds between atoms are broken and reformed to create new compounds. Mass is conserved because the number of atoms remains the same, aligning with the laws of conservation of matter and energy.

Step-by-step solution

a. Smaller parts of an atom

An atom, according to the modified version of Dalton's atomic theory, can be broken down into even smaller subatomic particles known as protons, neutrons, and electrons. The protons and neutrons form the atom's nucleus, while the electrons move around the nucleus in specific energy levels or orbitals.

b. Identical and differing aspects of hydrogen atoms

All hydrogen atoms share the same basic structure, containing one proton and one electron. This gives them the same atomic number (1). However, hydrogen atoms can differ in the number of neutrons they possess. The variations in the number of neutrons create isotopes of hydrogen, such as protium (no neutrons), deuterium (one neutron), and tritium (two neutrons).

c. Differences and similarities between hydrogen and helium atoms

Hydrogen and helium atoms differ mainly in their atomic number and the number of protons, neutrons, and electrons they possess. Hydrogen has one proton and one electron, while helium has two protons and two electrons. As a result, hydrogen's atomic number is 1, and helium's is 2. In terms of similarities, both hydrogen and helium are located in Group 1 of the periodic table, which means they have similar chemical reactivity.

d. Differences between water and hydrogen peroxide

Water (H2O) and hydrogen peroxide (H2O2) are both compounds composed of hydrogen and oxygen, but they differ in their molecular structure and chemical properties. Water has two hydrogen atoms and one oxygen atom, giving it a bent molecular shape. Hydrogen peroxide, on the other hand, has two hydrogen atoms and two oxygen atoms, resulting in a different, linear molecular structure. Due to these structural differences, water and hydrogen peroxide have distinct chemical and physical properties.

e. Chemical reactions and conservation of mass

In a chemical reaction, the bonds between atoms in reactants are broken and new bonds are formed to create products. The total mass remains conserved during a chemical reaction because the number of atoms in the reactants and the products remains the same. Atoms are neither created nor destroyed in a chemical reaction; they are simply rearranged to form new compounds. This principle of mass conservation aligns with the laws of conservation of matter and energy.

Key concepts

Atomic Structure

Understanding the fundamental building blocks of matter is essential for comprehending various scientific concepts. The atomic structure refers to the composition of an atom, which is the smallest unit of an element that retains its chemical properties. At the center of every atom lies its nucleus, which contains protons and neutrons—the protons are positively charged, and the neutrons carry no charge. Surrounding the nucleus is a cloud of negatively charged electrons.

Each element is defined by its unique number of protons, known as the atomic number. While the protons and neutrons define the mass of the atom, the electrons are responsible for the chemical behavior of the element. This particular arrangement of subatomic particles explains why atoms bond with each other to form molecules, leading to myriad substances we observe.

Chemical Reactions

Chemical reactions are at the heart of all biological and physical processes on Earth. A chemical reaction involves the transformation of reactants into products through the making or breaking of chemical bonds. In this intricate dance of atoms, elements combine or change partners to form new compounds. For instance, when hydrogen gas reacts with oxygen gas, a chemical reaction occurs that produces water.

This process is not just a simple mixing of elements; it's governed by precise rules of chemical behavior, reflecting the desire of atoms to reach a state of maximum stability or lowest energy. These changes often result in observable effects, such as energy release in the form of light or heat, color change, or the formation of gas or precipitate.

Conservation of Mass

The concept of conservation of mass is a fundamental principle in science, particularly in chemistry. It states that during a chemical reaction, the total mass of the products equals the total mass of the reactants. In other words, matter is neither created nor destroyed; it simply changes form. This principle, also known as the law of conservation of matter, was formulated by Antoine Lavoisier in the 18th century.

When atoms rearrange during chemical reactions, their total count remains the same, meaning that if you start with 20 atoms in your reactants, you'll end up with 20 atoms in your products. Even though they may be configured differently, the mass they represent remains constant. This concept is pivotal in chemical equations, as it ensures that the equation is balanced, with an equal number of atoms of each element on both sides.

Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but differ in the number of neutrons. This means that while they have the same atomic number, they have different mass numbers. The varied number of neutrons does not significantly affect the chemical properties of the element; instead, it defines the isotope's mass and stability.

Hydrogen, for example, has three common isotopes: protium with no neutrons, deuterium with one neutron, and tritium with two neutrons. Isotopes can be stable or unstable. Unstable isotopes, also known as radioisotopes, undergo radioactive decay, leading to the emission of radiation. This property of unstable isotopes has applications in fields like medicine, archaeology, and energy production.

Subatomic Particles

Subatomic particles are the smaller constituents of an atom, playing critical roles in determining the properties and behavior of atoms. There are three primary subatomic particles relevant to Dalton's atomic theory: protons, neutrons, and electrons.

Protons are positively charged and contribute to the identity of the element, as the atomic number is determined by the number of protons. Neutrons have no charge and add to the mass of the atom; their number can vary in different isotopes of the same element. Electrons are negatively charged and orbit the nucleus in energy levels or shells, dictating how an atom interacts with others and its resultant chemical properties. Collectively, these subatomic particles form atoms, the fundamental units of matter that participate in chemical reactions to create the diverse materials and substances around us.