Atoms and Periodic Trends: Your MCAT Guide

Atoms: The Building Blocks of Matter

Atoms are the basic building blocks of matter, and they are made up of three types of particles:

1. Protons - Positively Charged
2. Electrons - Negatively Charged
3. Neutrons – Neutral/No Charge

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Protons and neutrons are found in the Nucleus of the atom – which is the center of the Atom, while electrons orbit the nucleus in a specific path, known as shell or orbit.

The remaining two particles of the Nucleus i.e. – protons and neutrons – are located inside the Nucleus, and are tightly packed together. The number of protons in the Nucleus (atomic number of an element) determines the type of element, and as such, each element has its own unique properties. Hence, the number of protons in the Nucleus became the basis of arrangement of elements in the periodic table, as the atomic number of elements reflects the periodic nature of their electron configurations, determining their chemical and physical properties.

As we move across the rows and columns of the table, we can observe distinct patterns known as periodic trends. These trends help us predict the properties of new elements as they are discovered and provide insight into the relationships between elements. But before we proceed further, let’s understand what the periodic table is, and how elements are arranged on it.

The Periodic Table: Understanding the Organization of the Elements

The periodic table is a chart that shows all the known elements arranged in order of their atomic number (the number of protons in the nucleus of an atom). It consists of a grid of rows (called periods) and columns (called groups) that group elements with similar properties together. During your test, you'll have access to the MCAT periodic table.

The arrangement of elements in the periodic table exhibits particular trends of their properties, which are known as periodic trends. Let's learn more about these trends.

Periodic Trends: Properties of the Elements

Periodic trends are the regular patterns in the properties of elements in the periodic table. Some of the most significant periodic trends include atomic size, ionization energy, electron affinity, and electronegativity. Understanding these trends is crucial for predicting and explaining the chemical behavior of elements.

One example of periodic trends can be seen when comparing the elements Lithium (Li), Sodium (Na), and Potassium (K), which are all in the first column (group 1) of the periodic table. As we move down the column, the atomic size of these elements increases due to the addition of electron shells. This results in a decrease in the ionization energy, meaning it takes less energy to remove an electron from an atom.

Periodic trends refer to the regular patterns of chemical and physical properties of elements that occur across the periodic table. These trends result from the arrangement of electrons in an atom’s outermost energy level, or valence shell. By examining these patterns, we can gain a better understanding of the properties and behavior of different elements and predict the properties of undiscovered elements.

Let’s take a look at some of the major periodic trends and how they relate to the properties of atoms and elements:

1) Atomic Radius:

The atomic radius refers to the size of an atom. Imagine that each atom is a ball, and the atomic radius is the size of the ball; just as a ball can vary in size, so can atoms. For example, a basketball might represent an atom with a large atomic radius, a tennis ball an atom with a medium-sized atomic radius, and a ping pong ball an atom with a small atomic radius.

When they know the size of atoms, scientists can predict how they will interact with other atoms in chemical reactions. For example, an atom with a small radius might be more likely to form a chemical bond with an atom with a large radius because the small atom can fit into the spaces between the larger atoms. Conversely, atoms with similar radii might be less likely to form chemical bonds with each other because they would have a harder time sharing electrons.

Atomic Radius Periodic Trend from Left to Right:  The atomic radius tends to decrease when going from left to right on the periodic table.

Reason: The number of protons in the nucleus increases, creating a stronger attraction to the electrons in the outermost shell, which results in a smaller atomic radius.

Atomic Radius Periodic Trend from Top to Bottom: The atomic radius tends to increase when going from top to bottom on the periodic table.

Reason: The number of energy levels increases down the group in the periodic table, which means that the outermost electrons are farther from the nucleus, resulting in a larger atomic radius.

Elements that Are Exceptions to the Atomic Radius Periodic Trend: There are some exceptions to the periodic trend of atomic radius, such as the noble gases in Group 18, which have a smaller atomic radius than expected due to their full valence electron shells.

Additionally, elements in the same period (row) of the periodic table may not follow the trend exactly due to differences in electron configurations.

Element With the Smallest Atomic Radius: The element with the smallest atomic radius is helium (He), which is the smallest element on the periodic table.

Element With the Largest Atomic Radius: The element with the largest atomic radius is francium (Fr), which is the largest element on the periodic table. This is due to its large number of electrons and energy levels, as well as the shielding effect of its inner electrons.

2) Ionic radius:

Ionic radius is similar to atomic radius, but it applies specifically to ions, which are atoms that have gained or lost electrons. Two kinds of ions exist in nature: cations and anions.

Cations are positively charged ions that form when atoms lose electrons, while anions are negatively charged ions that form when atoms gain electrons.

Ionic Radius Periodic Trend from Left to Right: The ionic radius of cations tends to decrease when going from left to right on the periodic table.

Reason: This is because the number of electrons decreases, resulting in a stronger attraction between the remaining electrons and the nucleus. As a result, the remaining electrons are held more tightly, causing a smaller ionic radius.

Ionic Radius Periodic Trend from Top to Bottom: The ionic radius of cations and anions tends to increase when going from top to bottom on the periodic table.

Reason: This is because the number of energy levels increases down the group, which means that the outermost electrons are farther from the nucleus, resulting in a larger ionic radius.

Elements that Are Exceptions to the Ionic Radius Periodic Trend: The trend in ionic radius can be affected by various factors, such as the charge of the ion, the presence of d-orbitals, and the coordination number of the ion in a crystal lattice. Anions from the noble gases in Group 18, cations from transition metals, anions from some nonmetals, and cations from elements in Group 13 don’t follow the ionic radius periodic trend in the periodic table.

Element With the Smallest Ionic Radius: The element with the smallest cationic radius is helium (He), which does not form cations. The element with the smallest anionic radius is fluorine (F), which has a small atomic radius and a strong attraction for electrons, resulting in a small anionic radius.

Element With the Largest Ionic Radius: The element with the largest cationic radius is francium (Fr), which has a large atomic radius. The element with the largest anionic radius is cesium (Cs), which has a large atomic radius.

3) Electronegativity:

Electronegativity refers to the ability of an atom to attract electrons when it forms a chemical bond with another atom.

Electronegativity is like the sharing of candy between friends. Imagine you and your friend are sharing a bag of candy. If you both like the same types of candy equally, you will probably share the candy equally. But if one of you likes a certain type of candy more than the other, they might take more of that candy and leave less for the other person.

In the same way, atoms in a molecule share electrons like candy. Some atoms, like oxygen, really like to have more electrons and are considered “greedy” or “electronegative.” So, they will pull the shared electrons closer to them, leaving less for the other atoms in the molecule.

Electronegativity Periodic Trend from Left to Right: The electronegativity of atoms tends to increase when going from left to right on the periodic table.

Reason: An increase in the number of protons in the nucleus creates a stronger attraction to shared electrons.

 Electronegativity Periodic Trend from Top to Bottom: The electronegativity of atoms tends to decrease, when going from top to bottom on the periodic table.

Reason: The outermost electrons are farther from the nucleus and experience a weaker attraction from the parent nucleus.

 Elements Exceptional to Electronegativity Periodic Trend: Noble gases like helium (He), neon (N), and argon (Ar) have very low electronegativities, despite being on the right side of the periodic table. Moreover, oxygen (O) has a higher than expected electronegativity, mainly because in addition to the nuclear attraction, it has two unpaired electrons in its valence shell, which attract electrons from other atoms.

Element With the Highest Electronegativity: The element with the highest electronegativity is fluorine (F).

Element With the Lowest Electronegativity: The element with the lowest electronegativity is francium (Fr).

4) Ionization Energy:

Ionization energy is the energy required to remove an electron from an atom.

Ionization energy is like trying to separate a piece of metal stuck to a magnet. Just as the magnet requires a certain amount of energy to release the metal, an atom requires a certain amount of energy to release an electron. The stronger the magnet, the more energy it takes to remove the metal, and the higher the ionization energy of an atom, the more energy it takes to remove an electron.

Ionization Energy Periodic Trend from left to right: The ionization energy of atoms tends to increase when going from left to right on the periodic table.

Reason: An increasing number of protons in the nucleus creates a stronger attraction to the outermost electrons.

Ionization Energy Periodic Trend from top to bottom: The ionization energy of atoms tends to decrease when going from top to bottom on the periodic table.

Reason: As the outermost electrons are farther from the nucleus, the shielding effect of inner electrons reduces the effective nuclear charge felt by the outermost electrons.

Elements Exceptional to Ionization Energy Periodic Trend: Certain elements may have lower ionization energies than expected due to the presence of d-orbitals or other factors, and the trend is a generalization, meaning individual elements may not fit the trend perfectly.

Element With the Highest Ionization Energy: The element with the highest ionization energy is helium, which has a very high ionization energy due to its small size and the fact that its electrons are in the 1s subshell. The noble gases in Group 18 also have high ionization energies due to their complete valence shells.

Element With the Lowest Ionization Energy: The element with the lowest ionization energy is francium, which has very low ionization energy due to its large size and the fact that its valence electrons are far from the nucleus. Additionally, the alkali metals in Group 1 also have low ionization energies due to their large size and relatively weak attraction to the valence electrons.

5) Electron Affinity:

Electron affinity is the energy released when an electron is added to an atom. Think of a magnet trying to pull a piece of metal toward it. Some magnets are stronger than others and can pull the metal closer with more force. Similarly, some elements have a stronger attraction to electrons than others and can pull them in more easily.

Electron affinity is another important factor in determining the reactivity of elements. Elements with high electron affinities tend to be more reactive because they are more likely to gain an electron and form a negative ion. Elements with low electron affinities tend to be less reactive because they are less likely to gain an electron.

Electron Affinity Periodic Trend from Left to Right: The electron affinity of atoms tends to become more negative when going from left to right on the periodic table.

Reason: This is because the atomic radius tends to decrease, which means that the outermost electrons are held more tightly by the nucleus.

Electron Affinity Periodic Trend from Top to Bottom: The electron affinity of atoms tends to become less negative or more positive going down a group on the periodic table.

Reason: This is because the number of energy levels increases down the group, which means that the outermost electrons are farther from the nucleus.

Exceptions to the Electron Affinity Periodic Trend: Sulfur (S) and selenium (Se) have lower electron affinities than would be expected based on their position in the periodic table. In comparison, fluorine (F) and chlorine (Cl) have higher electron affinities than expected.

Element With the Highest Electron Affinity: The element with the highest electron affinity is chlorine (Cl).

Element With the Lowest Electron Affinity: The element with the lowest electron affinity is helium (He).

6) Metallic Character:

Metallic Character refers to the properties of metals. Metallic character is a chemical property of elements that describes how easily an element can lose its valence electrons to form positively charged ions.

Metals are good conductors of heat and electricity, have high melting and boiling points, and are malleable and ductile.

Metallic Character Periodic Trend from Left to Right: The metallic character of elements tends to decrease when going from left to right on the periodic table.

Reason: The effective nuclear charge felt by the outermost electrons increases, which makes it more difficult for the outermost electrons to move freely and participate in metallic bonding.

Metallic Character Periodic Trend from Top to Bottom: The metallic character of elements tends to increase when going from top to bottom on the periodic table.

Reason: As we move down a group, the number of energy levels increases, which means more electrons are available to participate in metallic bonding.

Elements Exceptional to Metallic Character Periodic Trend: Certain transition metals, such as copper (Cu), silver (Ag), and gold (Au), have exceptionally high metallic character. Some metalloids, such as boron (B) and silicon (Si), have lower metallic character than would be expected based on their position on the periodic table.

Element With the Most Metallic Character: Francium (Fr) is the most reactive metal and has a high metallic character.

Element With the Least Metallic Character: The element with the least metallic character is helium (He), which has no metallic character at all.

High-Yield MCAT Terminologies:

Here are some high-yield terms to ensure that you are aware of the basic terms of atoms and periodic trends or to refresh your memory. You’ll come across these terms multiple times during your MCAT study process, so make sure that you know each and every one.

Atoms:

• Proton: a positively charged particle found in the nucleus of an atom
• Neutron: an uncharged (electrically neutral) particle found in the nucleus of an atom
• Electron: a negatively charged particle found in the electron cloud surrounding the nucleus of an atom
• Atomic number: the number of protons in the nucleus of an atom, which determines the identity of the element
• Mass number: the total number of protons and neutrons in the nucleus of an atom
• Isotope: atoms of the same element that have different numbers of neutrons in the nucleus
• Valence electron: an electron in the outermost energy level of an atom that is involved in chemical bonding
• Electron configuration: the arrangement of electrons in the energy levels of an atom

Periodic trends:

Periodic table: an arrangement of elements in order of increasing atomic number and grouped according to their chemical and physical properties

Period: a horizontal row of elements in the periodic table

Group: a vertical column of elements in the periodic table

Effective nuclear charge: the net positive charge experienced by an electron in an atom or ion, which takes into account the attraction between the positively charged nucleus and the negatively charged electrons and the repulsion between the electrons themselves

Shielding effect: the reduction in effective nuclear charge experienced by an electron in an atom due to the presence of other electrons in the inner energy levels

Coulomb’s law: an equation that describes the electrostatic attraction or repulsion between two charged particles, such as an electron and a nucleus

Aufbau’s principle: the principle that electrons fill the lowest available energy levels before occupying higher levels

Multiple Choice Questions With Explanations:

To excel on the MCAT, you can test yourself with practice questions.

Based on what you have learned in this article, try to answer the 10 questions below. The answer and explanation are given below each question, but remember, no peeking before attempting the question yourself!

Conclusion

From the sizzling reactivity of alkali metals to the sparkly luster of noble gases, there’s no shortage of surprises in the atomic world. Knowing more about the tiny building blocks that make up everything around us and how they come together to form the amazing variety of elements on the periodic table is fundamental to comprehending the fascinating realm of chemistry better. By breaking down some of these concepts and identifying groups and trends, you will start to see patterns. Instead of simply memorizing the elements and their placement on the periodic table, you can use these patterns as mnemonic devices to prompt you and guide your answers to chemistry questions on the MCAT.

To your success,

Your friends at BeMo

BeMo Academic Consulting

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