Question

List the number of elements in periods 3 and 4 of the periodic table. Why does each period have a different number of elements?

Short answer

Period 3 has 8 elements, and Period 4 has 18 elements. Each period has a different number of elements due to the increasing number of atomic orbitals that are filled as you move down the periodic table.

Step-by-step solution

Counting Elements in Period 3

To determine the number of elements in Period 3, count all the elements starting with sodium (Na) and ending with argon (Ar) in the periodic table. This represents the third row in the periodic table.

Counting Elements in Period 4

For the number of elements in Period 4, start counting from potassium (K) and end with krypton (Kr). This comprises the fourth row of the periodic table.

Explaining the Difference

The difference in the number of elements between periods occurs due to the filling of electron orbitals. Period 3 fills the 3s and 3p orbitals while Period 4 starts filling the 4s, then the 3d, and finally the 4p orbitals. Electrons fill according to the Aufbau principle, which accounts for the additional elements in Period 4.

Key concepts

Electron Orbitals

Electron orbitals are like the 'parking spaces' for the electrons in an atom. Imagine a large parking lot with multiple levels; these levels are like the energy levels in an atom. Each parking space within those levels is like an orbital where an electron can 'park' itself. Electrons reside in these orbitals and depending on their energy, they choose different levels or 'shells'.

There are various types of orbitals - s, p, d, and f - with different shapes. For example, the 's' orbital looks like a sphere, while the 'p' orbitals resemble dumbbells. In a period, such as period 3, traditionally the 3s and 3p orbitals get filled. Interestingly, these orbitals don't fill up haphazardly. Instead, there's a specific order to it, which brings us to the Aufbau principle.

Aufbau Principle

The Aufbau principle is like a rule book that electrons follow when deciding where to 'sit' in an atom. Aufbau is German for 'building up' and the principle guides the way electrons fill up orbitals, from the lower energy levels to the higher ones. It dictates that we start at the ground floor (the nucleus) and build our way up.

The order is electron orbitals fill up from lower to higher is 1s, 2s, 2p, 3s, and so on. However, sometimes the electrons do something a little unexpected. For instance, the 4s orbital gets filled up before the 3d orbitals, even though '4' is larger than '3'. This is because the 4s orbital has a slightly lower energy than the 3d. So the electrons, always looking to be in the lowest energy state possible, will fill the 4s before the 3d. This quirk in electron filling is what leads to the difference in the number of elements in periods 3 and 4, which our next section will explore further.

Chemical Elements

Chemical elements are the building blocks of everything around us. Each element is distinguished by the number of protons in its nucleus, known as the atomic number. In the periodic table, these elements are laid out in rows called periods and columns called groups. The periods tell us how many energy levels an atom has. So back to our exercise, period 3 elements all have their highest energy electrons in the third shell, while period 4 elements start to fill the fourth shell.

Yet, as we move from period 3 to 4, more orbitals become available, like the d subshell that period 3 doesn't have, resulting in period 4 having more elements. Essentially, the periodic table not only orders elements by atomic numbers but also reflects their electron configurations, a map of where the electrons are located around the nucleus. Understanding this structure gives insight into an element's chemical behavior - why some are reactive, some are inert, and why they form the types of bonds they do.