The chemical properties of atoms are largely determined by the number of protons in the atom’s nucleus, which determines the atom’s element. Other properties, such as color, depend on the electron configuration and orbital structure.
Atoms can be either diatomic (containing two atoms) or monatomic (containing only one atom). When a diatomic gas atom interacts with another Diatomic gas atom, they can form a compound. When a monatomic gas atom interacts with another monatomic gas atom, they form a vapor.
The periodic table is organized by atomic number — the number of protons in an atom’s nucleus. This determines the element and its properties. The rows below each other are called periods, and each period increases by one proton in the nucleus. The columns to each side are called groups, and each group increases by one inner shell electron count.
This article will discuss how atomic radius changes from top to bottom in a group in the periodic table.
Radius varies throughout each period
As mentioned earlier, atomic radius varies depending on the element. All elements have a different size atoms, which affects how many atoms can fit in a certain diameter.
Radius also varies between periods in the periodic table. For example, let’s look at the difference in radii between transition metals and inner transition metals.
Transition metals have a larger radius than inner transition metals due to their electronic configuration. Electrons are located closer to the atom’s core due to the higher number of electrons in the atom.
This causes the outer electrons to feel a stronger force of attraction from the atomic core, pulling them closer. This does not change the number of electrons in the atom, but distributes them differently.
Radius also varies between elements in a period
The radius of an atom is the distance from the center of the atom to its edge. How big an atom is affects its chemical properties.
Atoms with smaller radii are more reactive because it is easier for other atoms or molecules to get inside it and chemically interact with it.
Atoms with larger radii are less reactive because it takes more effort for other atoms or molecules to get inside it. This can be helpful in processes where you want to keep the inner components separate, like in solid materials.
Radius can change either by changing the number of protons or neutrons an atom has, or by changing the shape of the atom. Changing these values will change the overall radius of an atom.
Group 1 elements have a small radius due to their single electron. Adding more electrons increases the radius due to there being more shielding against electric force fields.
The radius increases as you move down a group
As mentioned earlier, the atomic radius determines the size of an atom. How does the radius of an atom change as you move from one element to the next?
Atomic radii vary depending on the element and its place in the periodic table. For example, carbon has a larger atomic radius than lithium due to its higher electron shell.
The electron shells are arranged in orders: innermost shell, outermost shell, and sometimes a few shells in between. These determine the size of the atom; how wide its borders are.
To simplify this concept, think of an orange. An orange is round and has defined edges like an atom.
The radius increases as you move to the right in a period
As mentioned before, atomic radius is the distance between the nucleus and the outermost electron. This can be thought of as the distance between the atom’s edge and its electron ring.
Atomic radius typically increases as you move to the right in a period. This is because heavier elements have larger nuclear masses, so it is harder to pull an electron away from them.
Radius decreases as you move down a group. This is because atoms in lower groups have fewer electrons, and it is easier to pull one away. A few examples will be discussed later in this article.
Ionic radius refers to the average distance between atoms or ions. Ionic radius can change more dramatically than atomic radius within a period or group. This is because it depends on how many electrons are present, and how easily they are pulled away.
Helium has the smallest radius and gold has the largest radius
Because of the number of electrons in a neutral atom, the size of an atom is determined by its radius. The greater the number of electrons, the smaller the radius will be due to electron repulsion.
Atomic radii typically decrease when moving left to right across a period and increase when moving down a group. This is because electron shells are arranged with increasing distance between them, forcing atoms to have smaller radii.
Radius decreases by 2–3 atomic diameters moving down a group and 1–2 atomic diameters left to right across a period. This is because the number of electrons in an atom increases by one when moving down a group and left to right across a period. These new electrons increase the electron repulsion, making the atoms less dense and thus decreasing the radius.
Atomic radii can only vary so much due to quantum effects.
The presence of additional electrons affects the atomic radius
As we saw earlier, atoms with a larger number of protons in their nucleus have a larger atomic radius because they are heavier. Atoms with a greater number of electrons in their outermost orbit also have a larger atomic radius.
Atoms with more electrons have a greater electron density, which means that the atoms would feel more solid when touched. This is because there are more electrons moving around within it, pushing against other things.
Atoms with fewer electrons have a smaller atomic radius and feel softer when touched because there are fewer moving particles inside it. This is why some elements are classified as noble gases; they feel very soft when touched.
The difference in electron density and number between atoms in a given element can affect their atomic radii.
Ionization energy affects atomic radius
So how does atom size change as we move down a column or group in the periodic table? Ionization energy, or the energy it takes to remove an electron from an atom, affects atomic radius.
Ionization energy increases with larger atoms. This is because it takes more energy to pull an electron away from a heavier atom. Since electrons surround the nucleus in shells, this increase in ionization energy also increases the shell size.
A larger shell requires more space for all of the electrons it holds. Since atoms tend to share electrons evenly, when one atom gains an electron, its neighbor must lose one to maintain electrical neutrality. This makes it harder to pull an electron away from the larger atoms in general.
In general, moving down a column in the periodic table decreases atomic radius because atoms get bigger. Moving from left to right across a period increases atomic radius because atoms get smaller.
Electron cloud size affects atomic radii
As mentioned earlier, the first two periods of the periodic table—groups 1 and 2—are classified as those with the smallest atoms. Atoms in group 1 are lithium, sodium, and potassium, while group 2 contains calcium, barium, and magnesium.
Because group 1 atoms have one inner electron shell and one outer electron shell, the size of these shells affects the atomic radius. The inner shell is smaller due to its single electron, making the atom slightly smaller.
Group 2 atoms have an outer electron shell that is slightly larger than the inner shell, making the atom slightly larger. This is why calcium is slightly larger than lithium.
Moving down to group 3, beryllium has an inner electron shell that is larger than its outer one. Because of this, it has a smaller atomic radius than lithium does.
