## Atomic Size

Atomic radii generally decrease along each period (row) of the periodic table and increase down each group (column).

### Learning Objectives

Discuss the trend of atomic size in the periodic table.

### Key Takeaways

#### Key Points

• Atomic radii vary predictably across the periodic table. For instance, the radii generally decrease along each period (row) of the table from left to right and increase down each group (column).
• As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost shell, causing the atomic radius to decrease due to the increasing nuclear charge.
• When moving down a group of the periodic table, the atomic radius increases because of the presence of additional principal energy levels, which are further away from the nucleus.

#### Key Terms

• noble gas: Any of the elements of group 18 of the periodic table, being monoatomic and (with very limited exceptions) inert.
• period: A horizontal row in the periodic table, which signifies the total number of electron shells in an element’s atom.
• atomic radius: A measure of the size of an atom. Assuming atoms have a spherical shape, the radius of the sphere describes the size of the atom.

The atomic radius of a chemical element is a measure of the size of its atoms. It represents the mean distance from the nucleus to the boundary of the surrounding cloud of electrons.

Atomic radii vary in a predictable manner across the periodic table. Radii generally decrease along each period (row) of the table from left to right and increase down each group (column). These trends in atomic radii (as well as trends in various other chemical and physical properties of the elements) can be explained by considering the structure of the atom.

Atomic Sizes (Radii): The atomic size trends across a period and down a group (‘family’ in this figure) of the periodic table are shown in this figure.

### Moving Across the Periodic Table

As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost principal energy level (also known as valence level). This can be predicted to lead to

1. an increase in atomic size because of additional repulsions between electrons,
2. a decrease in size because of the additional protons in the nucleus,
3. no effect at all as the two opposing tendencies of electron repulsion and nuclear attraction balance each other out.

Experiments have shown that the first case is what happens: the increase in nuclear charge overcomes the repulsion between the additional electrons in the valence level. Therefore, the size of atoms decreases as one moves across a period from left to right in the periodic table.

### Moving Down the Periodic Table

The principal energy levels hold electrons at increasing radii from the nucleus. In a noble gas, the outermost level is completely filled; therefore, the additional electron that the following alkali metal (Group I) possesses will go into the next principal energy level, accounting for the increase in the atomic radius. Therefore, atomic size, or radius, increases as one moves down a group in the periodic table.

## Types of Bonds

Ionic bonds can form between nonmetals and metals, while covalent bonds form when electrons are shared between two nonmetals.

### Learning Objectives

Describe the types of bonds formed between atoms.

### Key Takeaways

#### Key Points

• Nonmetals can form different types of bonds depending on their partner atoms. Ionic bonds form when a nonmetal and a metal exchange electrons, while covalent bonds form when electrons are shared between two nonmetals.
• An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal.
• A covalent bond involves a pair of electrons being shared between atoms.
• Atoms form covalent bonds in order to reach a more stable state.
• A given nonmetal atom can form a single, double, or triple bond with another nonmetal. Which type of bond is formed between the atoms depends on their numbers of valence electrons.

#### Key Terms

• covalent bond: A type of chemical bond where two atoms are connected to each other by the sharing of two or more electrons.
• ionic bond: A type of chemical bond where two atoms or molecules are connected to each other by electrostatic attraction.

Nonmetals can form different types of bonds depending on their partner atoms. Ionic bonds form when a nonmetal and a metal exchange electrons, while covalent bonds form when electrons are shared between two nonmetals.

### Ionic Bonds

An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some degree of covalent bonding. Thus, an ionic bond is considered a bond where the ionic character is greater than the covalent character. The larger the difference in electronegativity between the two atoms involved in the bond, the more ionic ( polar ) the bond is. Bonds with partially ionic and partially covalent character are called polar covalent bonds.

Formation of Sodium Flouride: The attraction of oppositely charged atoms and the transfer of electrons leads to the formation of an ionic compound. In this case, NaF.

### Covalent Bonds

A covalent bond involves electrons being shared between atoms. The most stable state for an atom occurs when its valence electron shell is full, so atoms form covalent bonds, sharing their valence electrons, so that they achieve a more stable state by filling their valence electron shell.

### Polar covalent bonds

Some covalently bounded compounds have a small difference in charge along one direction of the molecule. This difference in charge is called a dipole, and when the covalent bond results in this difference in charge, the bond is called a polar covalent bond.

These kinds of bonds occur when the shared electrons are not shared equally between atoms. If one atom has a higher electronegativity, the electrons will be drawn closer to the nucleus of that atom, resulting in a small net charge around each nucleus of the atoms in the molecule.

If the atoms in the molecule have the same electronegativity (for example, if the atoms are the same, as in N2), then the shared electrons will not be drawn towards one nucleus more than another, and the bond will be nonpolar. Similarly, the higher the difference in electronegativity, the more unequal the sharing of electrons is between the nuclei, and the higher the polarity of the bond.

### Number of Bonds Between Covalently Bonded Atoms

A given nonmetal atom can form a single, double, or triple bond with another nonmetal. Which type of bond is formed between the atoms depends on their numbers of valence electrons.

### Comparison of Covalent and Ionic Compounds

Compounds that are built from covalent bonds have, in general, some differences in physical properties (ex.  solubility in water, conductivity, boiling point, and melting point) when compared to ionic compounds.

The boiling and melting point of covalent compounds is, in general, higher than for ionic compounds. They are also less soluble and conductive. A rule of thumb is that covalent compounds are more difficult to change than ionic compounds.