Atomic radius
The atomic radius is a measurement that indicates the size of an atom, defined as the distance from the nucleus to the outer boundary of its electron shell. Due to the unpredictable nature of electron positioning, scientists typically calculate atomic radius by measuring the distance between the nuclei of two bonded atoms and then dividing that distance by two. Different types of chemical bonds, such as covalent and ionic bonds, influence how atomic radii are determined, with covalent bonding allowing for simpler calculations compared to the complexities introduced by ionic bonding.
In the periodic table, atomic radii exhibit distinct trends: they generally decrease from left to right across a row, as increased nuclear charge pulls electrons closer to the nucleus, while they increase from top to bottom in a column, due to the addition of electron shells. For instance, hydrogen has an atomic radius of 53 picometers, while cesium has one of the largest at 298 picometers. Understanding atomic radius is essential in the study of chemical properties and interactions, as it impacts how atoms bond and behave in different chemical environments. This knowledge provides foundational insights into the structure and characteristics of matter.
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Atomic radius
An atomic radius is a measurement used to determine the size of an atom. Technically, it is meant to gauge the distance between the center of the atom’s nucleus and its outer shell of electrons. However, because of the variable properties of electrons, the outer boundary of the electron shell is difficult to determine. As a result, scientists calculate the radius of an atom by measuring the distance between the two nuclei in atoms that are bonded together. This measurement can be determined in a number of ways, depending on the type of atom and the type of chemical bond. In general, the relative size of a specific element’s atomic radius can be seen in the rows and columns of a periodic table. The atomic radius of chemical elements decreases from left to right across the rows of the table. The radius increases from top to bottom down the columns of the table.
Background
An atom is the smallest particle of matter that makes up chemical elements and still maintains the properties of that element. Atoms are the primary building blocks of all known matter in the universe. Atoms are themselves made up of three basic subatomic particles: protons, neutrons, and electrons. Protons are positively charged particles, electrons have a negative charge, and neutrons have no charge at all. The nucleus of an atom contains the vast majority of its mass and is made of protons and neutrons. The number of protons in an atom determines its chemical properties. For example, hydrogen has one proton, carbon has six, oxygen has eight, and so on.
Electrons are much smaller and less massive than protons and neutrons. They orbit the nucleus of an atom in an electron cloud, similar to how planets circle the sun. Electrons move around the nucleus so fast that their exact positions at a given time can only be estimated. They also orbit the nucleus in shells, with the number of electrons increasing as the number of shells increases. The shells are not set at fixed distances from the nucleus but can vary depending on their energy level. Electrons in higher-energy shells are farther from the nucleus, while electrons in lower-energy shells are closer. Each shell can hold only a certain number of electrons. For example, the first shell can hold up to two electrons, the second shell eight, and the third eighteen. If an atom has less than the maximum number of electrons in its outer shell, that atom is considered unstable and can bond with other atoms more easily. Stable atoms have full outer shells and are less likely to bond with other atoms.
Overview
In geometry, a radius is the measure of a circle or sphere from its center to its circumference, our outer perimeter. While, theoretically, an atomic radius would be the measure of an atom from its center to its outer boundary, the movement and properties of electrons do not allow such a simple calculation. The position of electrons in the outer shell of an atom is impossible to determine with certainty, so scientists use a formula that measures the distance between the nuclei of two atoms bonded together. The specific formula used and the result it provides depends on the type of bond between atoms.
A covalent bond occurs when two atoms share a pair of electrons in their outer shells. This sharing of electrons fills the outer shells, making the atoms stable. When two atoms of the same element are locked together in a covalent bond, the atomic radius can be determined by measuring the distance between the two nuclei and dividing that number in half. For example, the distance between two bonded hydrogen atoms is 74 picometers—a measure corresponding to one-trillionth of a meter. Using this formula, the covalent radius of a hydrogen atom would be 37 picometers.
An ionic bond occurs when atoms of different elements either share electrons or transfer electrons from one atom to the other. This method of determining atomic radius is more complicated because the atoms may be of different sizes. Atoms may also lose or completely share an outer shell in the process of bonding, or the remaining electrons may be pulled closer to the nucleus, reducing its radius. To determine an atomic radius from an ionic bond, the distance between the nuclei must first be measured and the respective sizes of the atoms taken into account when dividing the distance.
As a general reference, scientists often use a standard estimate of atomic radius based on the mean values derived from numerous calculations. The values are a simplified way of estimating the true radius of an atom in its free state and not bonded with another atom. Using these calculated values, hydrogen has an atomic radius of 53 picometers. Helium has the smallest atomic radius at 31 picometers, while cesium has the largest at 298 picometers.
The relative sizes of atomic radii can be observed by looking at a periodic table, a chart of the elements organized by the number of protons in the atoms, electrons in their shells, and other chemical properties. The horizontal rows of the table are known as periods. Elements in each period have the same number of shells circling their nuclei, but the number of protons and electrons in the atoms increases from left to right. Despite containing more subatomic particles, the atomic radii of the elements actually decreases from left to right across the rows. This occurs because the increased electrons are actually packed tighter towards the nucleus. For example, lithium—the first element in the second row—has an estimated atomic radius of 167 picometers; neon—the last element on the right of the row—has an estimated radius of 38 picometers.
The vertical columns on the table are known as groups. Atoms in each new row in a group have an extra outer shell, meaning the atomic radius of atoms increases going down the chart from top to bottom. This is why sodium, which is one step down in the same group as lithium, has an atomic radius of 190 picometers; the element below sodium, potassium, has a radius of 243 picometers.
Bibliography
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