Physical Science: GED Test Prep
Physical science includes the disciplines of chemistry (the study of matter) and physics (the study of energy and how energy affects matter). The questions on the physical science section of the GED will cover topics taught in high school chemistry and physics courses. This article reviews the basic concepts of physical science—the structure of atoms, the structure and properties of matter, chemical reactions, motions and forces, conservation of energy, increase in disorder, and interactions of energy and matter.
The Structure of Atoms
You and everything around you are composed of tiny particles called atoms. The book you are reading from, the neurons in your brain, and the air that you are breathing can all be described as a collection of various atoms.
History of the Atom
The term atom, which means indivisible, was coined by Greek philosopher Democritus (460–370 B.C.). He disagreed with Plato and Aristotle—who believed that matter could infinitely be divided into smaller and smaller pieces—and postulated that matter is composed of tiny indivisible particles. In spite of Democritus, the belief that matter could be infinitely divided lingered until the early 1800s, when John Dalton formulated a meaningful atomic theory. It stated:
- Matter is composed of atoms.
- All atoms of a given element are identical.
- Atoms of different elements are different and have different properties.
- Atoms are neither created nor destroyed in a chemical reaction.
- Compounds are formed when atoms of more than one element combine.
- A given compound always has the same relative number and kind of atoms.
These postulates remain at the core of physical science today, and we will explore them in more detail in the following sections.
Protons, Neutrons, and Electrons
An atom is the smallest unit of matter that has a property of a chemical element. It consists of a nucleus surrounded by electrons. The nucleus contains positively charged particles called protons, and uncharged neutrons. Each neutron and each proton have a mass of about 1 atomic mass unit, abbreviated amu. An amu is equivalent to about 1.66 × 10–24 g. The number of protons in an element is called the atomic number. Electrons are negatively charged and orbit the nucleus in what are called electron shells.
Electrons in the outermost shell are called valence electrons. Valence electrons are most responsible for the properties and reaction patterns of an element. The mass of an electron is more than 1,800 times smaller than the mass of a proton or a neutron. When calculating atomic mass, the mass of electrons can safely be neglected. In a neutral atom, the number of protons and electrons is equal. The negatively charged electrons are attracted to the positively charged nucleus. This attractive force holds an atom together. The nucleus is held together by strong nuclear forces.
The number of protons in an element is always the same. In fact, the number of protons is what defines an element. However, the number of neutrons in the atomic nucleus, and thus the atomic weight, can vary. Atoms that contain the same number of protons and electrons, but a different number of neutrons are called isotopes. The atomic masses of elements in the periodic table are weighted averages for different isotopes. This explains why the atomic mass (the number of protons plus the number of neutrons) is not a whole number. For example, most carbon atoms have 6 protons and 6 neutrons, giving it a mass of 12 amu. This isotope of carbon is called "carbon twelve" (carbon-12).But the atomic mass of carbon in the periodic table is listed as 12.011. The mass is not simply 12, because other isotopes of carbon have 5, 7, or 8 neutrons, and all of the isotopes and their abundance are considered when the average atomic mass is reported.