GED · Science · 6 min read
Matter, Atoms, and Chemical Reactions
- Identify the parts of an atom, including subatomic particles, atomic numbers, mass numbers, and valence electrons.
- Distinguish between the three primary states of matter and explain phase changes in terms of thermal energy and particle motion.
- Differentiate physical changes from chemical reactions and apply the law of conservation of mass to balanced equations.
Atomic Structure and Elements
All matter around us is made of basic building blocks known as atoms. An atom is the smallest unit of an element that retains the chemical properties of that element. Atoms contain protons and electrons, and most also contain neutrons; the most common isotope of hydrogen has no neutron. Protons carry a positive electrical charge, electrons carry a negative electrical charge, and neutrons carry no electrical charge and are neutral.
These subatomic particles are arranged in specific regions within the atom. An atom's protons and any neutrons are grouped together in its dense, central nucleus. Protons and neutrons have approximately equal masses and account for nearly all of an atom's mass. In contrast, electrons move within the electron cloud surrounding the atomic nucleus and have negligible mass compared to protons and neutrons.
The number of protons in an atom's nucleus is called the atomic number of an element. This atomic number uniquely determines the identity of an element. In an electrically neutral atom, the number of negatively charged electrons equals the number of positively charged protons. If you add together the total count of protons and neutrons in an atom, you get the mass number. Atoms of the same element that share the same number of protons but contain different numbers of neutrons are known as isotopes.
An atom's electrons are arranged at various distances from the nucleus, and the outermost energy level contains valence electrons. These valence electrons largely determine how an atom bonds and reacts with other atoms. Because chemical reactions involve interacting outer boundaries of atoms, valence electrons govern the chemical behavior of elements.
Common mistakes
- Changing chemical subscripts instead of coefficients when balancing equations, which incorrectly alters the identity of the substances involved.
- Assuming that dissolving salt in water or changing a substance’s state creates a new chemical substance, rather than recognizing these examples as physical changes.
- Believing that mass was destroyed during an open-container reaction when the apparent loss was simply due to gaseous products escaping into the air.
- Treating the appearance of a new odor as absolute proof of a chemical change, when odor alone does not prove that a chemical reaction occurred.
Key points
- Atoms have a central nucleus containing protons and, in most cases, neutrons, surrounded by an electron cloud of negatively charged electrons whose mass is very small compared with that of protons and neutrons.
- The atomic number equals the number of protons and uniquely identifies an element, while valence electrons in the outermost level govern how atoms bond.
- Matter exists primarily as solids, liquids, and gases, with particles vibrating in fixed positions, sliding past one another, or moving rapidly across large distances.
- Temperature measures the average kinetic energy of particles, and adding or removing thermal energy drives physical phase changes such as melting, freezing, vaporization, condensation, and sublimation.
- Physical changes alter appearance or state without modifying chemical identity, whereas chemical changes form new substances and rearrange chemical bonds.
- The law of conservation of mass dictates that matter is neither created nor destroyed, meaning the total mass of reactants equals the total mass of products in a closed system.
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