Kaplan MCAT General Chemistry Review

Although you may have encountered in your university-level chemistry classes such subatomic particles as quarks, leptons, gluons, and other particles whose names sound as if they were picked up from a Star Trek episode, the MCAT’s approach to atomic structure is much simpler. There are three subatomic particles that you must understand.

Figure 1.1

PROTONS

Protons are found, along with neutrons, in the nucleus of an atom. Each proton has an amount of charge equal to the fundamental unit of charge (1.6 × 10^-19 C), and we denote this fundamental unit of charge as “+1” for the proton. Protons have a mass of approximately one atomic mass unit, or amu. The atomic number (Z) of an element is equal to the number of protons found in an atom of that element. The atomic number is like your Social Security number; it acts as a unique identifier for each element because no two elements have the same one. All atoms of a given element have the same atomic number, although, as we will see, they do not necessarily have the same atomic mass.

NEUTRONS

Neutrons are the Switzerland of an atom; they are neutral, which means that they have no charge. A neutron’s mass is only slightly larger than that of the proton, and together, the protons and the neutrons of the nucleus make up almost the total mass of an atom. Every atom has a characteristic mass number, which is the sum of the protons and neutrons in the atom’s nucleus. The number of neutrons in the nuclei of atoms of a given element may vary; thus, atoms of the same element will always have the same atomic number but will not necessarily have the same mass number. Atoms that share an atomic number but have different mass numbers are known as isotopes of the element. The convention is used to show both the atomic number (Z) and the mass number (A) of atom X.

ELECTRONS

If you think of the nucleus as a game of checkers, the electrons would be children who express varying degrees of interest in playing or watching the game. Electrons move around in pathways in the space surrounding the nucleus and are associated with varying levels of energy. Each electron has a charge equal to that of a proton but with the opposite (negative) charge, denoted by “-1.” The mass of an electron is approximately that of a proton. Because subatomic particle masses are so small, the electrostatic force of attraction between the unlike charges of the proton and electron is far greater than the gravitational force of attraction based on their respective masses.

Bridge

The valence, or outer, electrons will be very important to us in both General and Organic Chemistry. Knowing how tightly held those electrons are will allow us to understand many of an atom’s properties and how it interacts with other atoms.

Going back to our checkers analogy, consider how children form rough circles surrounding a game of checkers; the children sitting closer to the game are more interested in it than the children who are sitting on the periphery. Similarly, electrons are placed in pathways of movement that are progressively farther and farther from the nucleus. The electrons closer to the nucleus are at lower (electric potential) energy levels, while those that are in the outer regions (or shells) have higher energy. Furthermore, if you’ve ever seen children sitting around a game, you know that the “troublemakers” are more likely to sit on the periphery, which allows them to take advantage of an opportunity for mischief when it arises—and so it is also for electrons. Those in the outermost energy level, or shell, called the valence electrons, experience the least electrostatic draw to their nucleus and so are much more likely to become involved in bonds with other atoms (filling empty spaces in other atoms’ valence shells). Generally speaking, the valence electrons determine the reactivity of an atom. In the neutral state, there are an equal number of protons and electrons; a gain of electron(s) results in the atom gaining a negative charge, while a loss of electron(s) results in the atom gaining a positive charge. A positively charged atom is a cation, and a negatively charged atom is an anion.

Some basic features of the three subparticles are shown in Table 1.1.

Table 1.1

Example: Determine the number of protons, neutrons, and electrons in a nickel-58 atom and in a nickel-60 2+ cation.

Solution:⁵⁸Ni has an atomic number of 28 and a mass number of 58. Therefore, ⁵⁸Ni will have 28 protons, 28 electrons, and 58 - 28, or 30, neutrons.

In the ⁶⁰Ni²⁺ species, the number of protons is thet same as in the neutral ⁵⁸Ni atom. However, ⁶⁰Ni²⁺ has a positive charge because it has lost two electrons; thus, Ni²⁺ will have 26 electrons. Also the mass number is two units higher than for the ⁵⁸Ni atom, and this difference in mass must be due to two extra neutrons; thus, it has a total of 32 neutrons.