Matter and the Periodic Table of Elements Study Guide (page 2)
Science attempts to understand and explain our observations in nature and the universe. Chemistry, a branch of science, studies the chemical and physical properties of matter. Chemistry plays a fundamental role in understanding particular concepts of biology, physics, astronomy, geology, and other disciplines of science.Arguably, chemistry is the central science.
Matter is anything that occupies space and has mass. In other words, matter is everything in the universe. A concept map of matter is shown in Figure 1.1.
Heterogeneous mixture: A system of two or more substances (elements or compounds) that have distinct chemical and physical properties. Examples include mixtures of salt and sand, oil and water, crackerjacks, and dirt.
Homogeneous mixture (or solution): A system of two or more substances (elements or compounds) that are interspersed, such as the gases making up the air or salt dissolved in water. The individual substances have distinct chemical properties and can be separated by physical means.
Element: A substance that contains one type of atom and cannot be broken down by simple means.
Compound: A combination of two or more atoms of different elements in a precise proportion by mass. In a compound, the atoms are held together by attractive forces called chemical bonds and cannot be separated by physical means.
Molecule: A combination of two or more atoms.Molecules cannot be separated by physical means.
Atom: The basic unit of an element that retains all the element's chemical properties.An atom is composed of a nucleus (which contains one or more protons and neutrons) and one or more electrons in motion around it. Atoms are electrically neutral because they are made of an equal number of protons and electrons.
Proton: A particle that has a mass of 1 atomic mass unit (amu; 1 amu = 1.66 * 10–27 kg) and an effective positive charge of +1.
Neutron: A particle that has a mass of 1 amu with no charge.
Electron: A particle that is of negligible mass (0.000549 amu) compared to the mass of the nucleus and that has an effective negative charge of –1.
Matter can also be classified as one of four states: solid, liquid, gas, or plasma. To simplify, the discussion will be limited to solids, liquids, and gases (see Table 1.1). A solid is rigid and has a fixed volume.A liquid has a fixed volume but assumes the shape of its container. A gas has no definite shape or volume and can be compressed.
Table 1.1 States of Matter
Physical Versus Chemical Change
A physical change of a substance does not change its chemical composition. The boiling or freezing of water is an example of a physical change. It does not matter if H2O is a solid (ice), liquid, or gas (steam); it is still water. Popcorn popping is another example of a physical change. The oil heats the water in the popcorn kernel and converts the water into steam. The liquid water changing to steam causes the "explosion" and opening of the kernel, but no chemical change has occurred.
A chemical change or reaction is a process where one or more substances are converted into one or more new substances. The burning logs in a campfire are a chemical change that converts the wood (carbohydrates, etc.) into carbon dioxide, water, and other substances. The roasting of marshmallows over the campfire quickly becomes a chemical change if the marshmallows catch fire and the campfire converts its carbohydrates into carbon.
The Periodic Table of Elements
Approximately, 115 elements have been discovered to date. These elements are organized in a periodic table of elements. Two seventeenth-century chemists, Dmitri Mendeleev and Julius Meyer, independently organized the early periodic table that evolved into the modern periodic table of elements (see Figure 1.2).
The periodic table of elements is structured according to the properties of the elements.Mendeleev's early experiments classified the elements according to their properties and reactivity with oxygen and grouped the elements in octaves (eight). Elements have chemical symbols that are used for their representation in the periodic table (O is oxygen and Zn is zinc). Some of the symbols are based on their original names, usually their Latin origin (Fe is iron [ferrum] and Na is sodium [stannum]).
The elements are organized by periods (horizontal rows) and groups (vertical columns). Elements in vertical columns are groups (or families), they usually have similar chemical properties, and they are identified by group numbers 1 through 18 (newer labels). The groups labeled 1A through 8A (older labeling) are often called main group elements.Although the 1 through 18 labels have been recommended by the International Union of Pure and Applied Chemistry (IUPAC) and adopted by the American Chemical Society (ACS), this book will refer to the older,more commonly used 1A (or 1) through 8A (or 8) group designations. Elements in the same group also have (in common) the same number of electron(s) in their outermost shell (i.e., group 6 elements have six electrons in their outer shell). Groups 1A, 2A, 7A, and 8A have specific names based on their properties:
- Group 1A: Alkali metals (Li, Na, K, Rb, Cs, Fr)
- Group 2A: Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra)
- Group 7A: Halogens (F, Cl, Br, I, At)
- Group 8A: Noble gases (He, Ne, Ar, Kr, Xe, Rn)
Elements in the same period have the same number of electron shells (or levels). Seven periods can be found in the modern periodic table.
Elements in the middle and left side of the table are classified as metals (Na, Fe, Hg). A metal is an element that is shiny, conducts electricity and heat, is malleable (easily shaped), and is ductile (pulled into wires).Metals are electropositive, having a greater tendency to lose their valence electrons.
Elements in the upper-right corner of the table are classified as nonmetals (C, F, S). A nonmetal is an element with poor conducting properties. Nonmetals are electronegative, having a greater tendency to gain valence electrons.
A metalloid or semimetal is an element with properties that are intermediate between those of metals and nonmetals, such as semiconductivity. They are found between metals and nonmetals in the periodic table (see shaded elements in Figure 1.2).
The Scientific Method
In the everyday world, people want their lives to be unambiguous with clear-cut answers. Science does not always follow the "truth" that most citizens set out to find. Scientists use the scientific method as a procedure for developing the knowledge to classify concepts as a law, theory, principle, or model. The scientific method is a framework for the stepwise process to experimentation. The steps are as follows:
- Identify a problem.
- Research the problem.
- Form a hypothesis.
- Plan an experiment.
- Collect and analyze the data.
- Form a conclusion.
- Report the experiment.
In other words, you make an observation, develop a hypothesis, and perform experiments. The observations and results from the experimental procedure allow the scientist to review the hypothesis and plan new experiments. The interpretations of the results from many experiments can classify the observation as a theory or law. A law explains what happens. For example, Charles's Law explains that temperature and volume, at constant pressure, are proportional. If the Kelvin temperature of a gas doubles, then its volume doubles correspondingly.A law is a statement based on many experiments that produced the same result and conclusion. A principle would explain a more specific set of relationships of a law.
However, if a hypothesis or set of hypotheses, based on an observation of a natural phenomenon, cannot be tested or experiments fail to show a direct conclusion, it is identified as a theory. A theory explains why something happens. For example, the atomic theory of small electron particles orbiting around a dense nucleus containing protons and neutrons is based on indirect observations of the atom. This is only a possible explanation for the structure of an atom.A model is the description of the theory, such as the structure of an atom.
Practice Problems for these concepts can be found at - Matter and the Periodic Table of Elements Practice Questions