ASVAB · General Science · 7 min read
Atoms, Elements and Compounds
- Identify the subatomic particles that make up an atom and explain how atomic number, mass number, isotopes, and ions are determined.
- Describe the organization of the periodic table, including periods, groups, and the distinct physical and chemical properties of metals, nonmetals, and metalloids.
- Differentiate between elements, compounds, and mixtures, and distinguish between physical and chemical changes as well as states of matter.
Atomic Structure, Subatomic Particles, and Ions
Atoms are the basic building blocks of ordinary matter. Every physical object you encounter is constructed from these microscopic units. At the very center of an atom lies the atomic nucleus. The nucleus contains nearly all of an atom's mass concentrated in a very small, dense volume. Inside this dense nucleus, you will find two types of subatomic particles: protons and neutrons. Protons are positively charged subatomic particles, while neutrons are neutral subatomic particles with no electrical charge. Protons and neutrons have approximately equal mass, with each particle measuring roughly one atomic mass unit.
Surrounding the atomic nucleus is a cloud of electrons. Electrons are negatively charged subatomic particles that occupy orbitals surrounding the nucleus. Compared to the mass of a proton or neutron, an electron's mass is negligible. In an electrically neutral atom, the negative charges balance the positive charges exactly because the number of electrons equals the number of protons. However, this balance can change when an atom interacts with other atoms.
The atomic number of an element is equal to the number of protons in its nucleus. This count is critical because the number of protons uniquely determines the identity of an element. If an atom has a different number of protons, it is an entirely different element. Meanwhile, the mass number of an atom is the total count of protons and neutrons combined. While all atoms of a specific element share the exact same number of protons, they can have varying numbers of neutrons. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
When an atom gains or loses electrons, it loses its electrical neutrality. An ion is an atom or group of atoms with a net electrical charge because its number of electrons differs from its number of protons. If a neutral atom loses one or more electrons, it has more protons than electrons, forming a positively charged ion known as a cation. Conversely, if an atom gains one or more electrons, it ends up with more electrons than protons, forming a negatively charged ion called an anion.
Common mistakes
- Confusing atomic number with mass number: The atomic number is solely the count of protons and identifies the element, whereas the mass number is the total combined count of both protons and neutrons.
- Assuming that forming a cation means gaining a particle: A cation is a positively charged ion formed when a neutral atom loses one or more negatively charged electrons, not by gaining positive particles.
- Thinking mixtures require chemical reactions to separate: Mixtures generally can be separated by physical methods, such as filtration, distillation, or evaporation, chosen according to the mixture; breaking a compound into simpler substances requires a chemical reaction.
- Reversing sublimation and deposition: Sublimation is the transition directly from a solid to a gas without becoming a liquid, whereas deposition is the direct transition from a gas to a solid without passing through the liquid state.
Key points
- Atoms have a dense nucleus of protons and neutrons surrounded by an electron cloud; the number of protons is the element's atomic number.
- Isotopes are atoms of the same element with different numbers of neutrons, while ions are formed when atoms gain electrons (anions) or lose electrons (cations).
- The periodic table organizes elements by increasing atomic number into periods (rows) and groups (columns); elements in a group often have similar chemical properties, and main-group elements generally have the same number of valence electrons, except helium.
- Elements are pure substances of one atom type, compounds consist of elements chemically combined in fixed ratios, and mixtures are physically combined substances that retain their identities.
- An ionic bond is the attraction between oppositely charged ions, often formed after electrons transfer from a metal to a nonmetal; a covalent bond forms when atoms share electrons and can produce molecules or larger networks.
- Matter exists as solids, liquids, gases, and plasma; phase changes such as melting, freezing, vaporization, condensation, sublimation, and deposition occur as heat is absorbed or released.
- Temperature measures the average kinetic energy of particles, and density is a physical property calculated by dividing mass by volume.
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