Survey of the Natural Sciences Study Guide for the DAT

General Chemistry (continued)

Periodic Properties

The periodic table is arranged in such a way that the rows (periods) and columns (groups) have distinct and representative characteristics. This allows scientists to predict element behavior and bonding patterns. Groups represent elements of the same valence electron structure.

Representative Elements

Representative elements include group IA through VIIA elements with \(\text{s}\) or \(\text{p}\) orbitals as their outermost shell. The transition elements, lanthanide, and actinides are non-representative elements. Hydrogen behaves in a manner that is all its own.

The group IA elements are known as alkali metals, and group IIA are the alkaline earth metals. Two other notable groups include the halogens in group VIIA and the noble gases in group VIIIA. Halogens are very reactive nonmetals because they need only one additional electron to complete their outer shell, whereas noble gases are inert because their outer shell is full.

Transition Elements

Groups IB to VIIIB are the transition elements. They are unique because their oxidation state can vary and they can form complex ions. They have characteristically high melting points and boiling points and are therefore used as catalysts in numerous chemical reactions.

Periodic Trends

From left to right across a period, the elements have:

• increasing effective nuclear charge
• decreasing atomic radius
• increasing ionization energy
• increasing electronegativity

From top to bottom down a group, the elements have:

• decreasing effective nuclear charge
• increasing atomic radius
• decreasing ionization energy
• decreasing electronegativity

effective nuclear charge \(Z_{eff}\)	net positive charge experienced by an electron
atomic radius	half the distance between two atoms in a bond
ionization energy	energy needed to remove an electron from an atom
electronegativity	tendency to attract an electron
electron affinity	energy released when a neutral molecule accepts another electron, increases with \(Z_{eff}\)

Descriptive Chemistry

In descriptive chemistry, groups and periods are used to describe the nature of the containing elements in reactions as well as their electron structures. They also tend to describe the chemical reactions they will undergo with other chemicals or elements. By understanding electron configuration, oxidation states, and periodic trends, a majority of chemistry and chemical reactivities can be predicted.

Nuclear Reactions

Nuclear chemistry is based on radioactivity, which is when a nucleus is unstable, meaning it may spontaneously emit particles. Through various techniques of bombarding the nucleus with subatomic particles (e.g., electrons, protons, and neutrons), nuclear reactions release or absorb large amounts of energy, causing the elements to change from one to another.

Balancing Equations

Nuclear reactions also follow conservation laws. They are balanced in two ways:

1. The sum of the mass numbers of the reactants equals the sum of the mass numbers of the products.

2. The sum of the charges of the reactants equals the sum of the charges of the products.

If the atomic number and the mass number of all but one of the particles in a nuclear reaction are known, we can identify the particle by balancing the reaction.

Balancing Equations for Nuclear Reactions

The reaction of an \(\alpha\) particle with magnesium-25 (\(\phantom{}^{25}_{12}\text{Mg}\)) produces a proton and a nuclide of another element. Identify the new nuclide produced.

Solution

The nuclear reaction can be written as:

\[\phantom{}^{25}_{12}\text{Mg} + \phantom{}^4_2 \text{He} \rightarrow \phantom{}^1_1 \text{H} + \phantom{}^A_Z \text{X}\]

where \(A\) is the mass number and \(Z\) is the atomic number of the new nuclide, \(X\). Because the sum of the mass numbers of the reactants must equal the sum of the mass numbers of the products, we have:

\[25 + 4 = A + 1\text{ or }A = 28\]

Similarly, the charges must balance, so:

\[12 + 2 = Z + 1\text{ and }Z = 13\]

Check the periodic table: The element with nuclear charge \(Z = +13\) is aluminum. Thus, the product is \(\phantom{}^{28}_{13}\text{Al}\).

Adapted from: https://openstax.org/books/chemistry-atoms-first-2e/pages/20-2-nuclear-equations

Binding Energy

Binding energy is the energy required to break a nucleus down into the composing neutrons and protons. Binding energy is calculated using:

\[E = mc^2\]

where \(m\) is the mass defect (in \(\text{g/mol}\)) and \(c\) is the speed of light.

Decay Processes

Radioactive decay happens spontaneously in nature when certain nuclei emit particles without being stimulated to do so. The various types of decay are described in the table below:

Retrieved from: https://openstax.org/books/chemistry-atoms-first-2e/pages/20-3-radioactive-decay

Particles

There are various particles that are relevant in nuclear chemistry, but the main ones are summarized in the table below:

Name	Description
Alpha particle	(High-energy) helium nuclei consisting of two protons and two neutrons
Beta particle	(High-energy) electrons
Positron	Particles with the same maas as an electron but with 1 unit of positive charge
Proton	Nuclei of hydrogen atoms
Neutrons	Particles with a mass approximately equal to that of a proton but with no charge
Gamma ray	Very high-energy electromagnetic radiation

Adapted from: https://openstax.org/books/chemistry-atoms-first-2e/pages/20-2-nuclear-equations

Terminology

The difference in the actual mass of the particles assembled in a nucleus and the mass of the individual parts added separately is called the mass defect. Fusion is when small nuclei are combined to form a larger nucleus. This process, known as a thermonuclear reaction, occurs at extremely high temperatures . Fission is when large atoms are split into multiple, smaller and more stable nuclei. Sometimes chain reactions are induced during fission as neutrons are emitted, causing other molecules to continue undergoing fission.

Laboratory Practices

A large component of chemistry education includes time spent in the laboratory observing chemical principles in action. There are many basic rules of thumb when working in a laboratory to keep everything in harmony. These rules help keep all nearby scientists safe and ensure the accuracy and precision of laboratory experiments.

Basic Techniques

Calorimetry is a technique used to measure the energy of a system, either measured at constant pressure or constant volume. Titrations are used to slowly add liquids dropwise and are often used to record titration curves from which pH information can be obtained. Distillation and filtration are basic separation techniques used to purify solutions and remove unwanted compounds.

Equipment

Glassware is the preferred type of container due to glass’s strength, transparency, and ability to tolerate a range of temperatures without interacting with the chemicals inside. Sometimes, the glassware is calibrated to be used in measurements, like a graduated cylinder, and sometimes it is non-calibrated for storage like Erlenmeyer flasks and test tubes.

Balances are used to measure weight. They need to be tared to the weight of the weighing container prior to measuring the amount of chemicals to ensure accuracy. Determining pH is performed with a litany of equipment, including pH meters, burets, and volumetric flasks.

Error Analysis

Accuracy is how close to the correct value measurements are achieved. Precision is how repeatable the measurements are. Both of these values are important when analyzing the success of an experiment. Scientists must ensure that their instruments are well calibrated to improve both of these values.

Safety

To practice safety in a laboratory setting, it is important to wear safety goggles and close-toed shoes. In addition to this, appropriate gloves should be worn with special attention being paid to the materials being handled, as this will determine the proper glove type. Sometimes. chemicals expel harmful fumes, and for these, you must handle them in a properly ventilated fume hood. Depending on the chemical and reaction being studied, face masking may also be necessary.

For emergencies, all laboratories should be equipped with an eyewash station and shower station as well as a fire extinguisher. Additionally, all labs must have multiple exits. It is also important to have proper equipment for the disposal of chemicals.

Data Analysis

To properly record data, careful consideration must be given to make sure that all instrumentation is calibrated to reduce error. Data analysis can be performed via computer monitoring via sensors (e.g., pH sensors). Additionally spreadsheet programs like Excel can aid in making graphics of information. During data analysis, only a certain amount of significant figures needs to be preserved. The results of calculations only need to contain the same amount of significant figures as the measurement with the least number of significant figures.