Atomic Radius

A neutral atom must have the same number of _______ and _______.

• protons

• neutrons

• electrons

Answer explanation

The net charge of an atom is # protons - # electrons. When protons = electrons, the net charge is zero.

What is the charge of an atom's nucleus?

• positive

• negative

• neutral

• none of these

Answer explanation

The nucleus has protons (+) and neutrons (0), so they add to be positive!

What law describes the force relationship between protons in the nucleus and the electrons orbiting around the nucleus?

• Ideal Gas Law

• Combined Gas Law

• Coulomb's Law

• Biot-Savart Law

Answer explanation

Coulomb's Law states that the force between two charged particles is proportional to both of their charges, and inversely proportional to the distance between them.

Coulombic Attraction is the ​ (a)   between two ​ (b)   charges, often used to describe the interaction between ​ (c)   in an atom.

• force
• opposite
• protons and electrons
• energy
• like
• protons and neutrons

Answer explanation

Like charges repel, and opposite charges attract! Coulombic attraction refers to the latter.

Adding a proton to an atom ​ (a)   coulombic attraction between protons and electrons, which causes the electrons to be ​ (b)  

• increases
• pulled closer toward the nucleus
• decreases
• pushed further away from the nucleus

Increasing the number of protons in an atom within a row ​ (a)   the atomic number​ and (b)   the radius of the atom.

• increases
• decreases
• does not affect

Answer explanation

Adding more protons causes coulombic attraction to increase and pulls the electrons closer, decreasing the radius of the atom.

Why does an atom's radius increase when entering a new period (row)?

Examples: He -> Li, Ne -> Na, Ar -> K

• A new electron shell is formed

• More protons are added to the atom

• More neutrons are added to the atom

• The increased number of electrons increases repulsion forces

Answer explanation

Electrons are stored in shells or energy levels! Entering a new period requires a new shell to be added, drastically increasing the atomic radius.

Select two!

The trend for atomic radius is that it:

• Increases across a period (row)

• Decreases across a period (row)

• Increases down a group (column)

• Decreases down a group (column)

Answer explanation

Write this one down! The atomic radius tends to decrease across a period, and increase down a single group. This is explained with coulombic attraction and electron shells.

Get your periodic table out!

Which element's atoms are LARGER?

• Silicon

• Sulfur

Which element's atoms are SMALLER?

• Potassium

• Calcium