Category: My Classroom Material

What is an Atomic Orbital?

Orbitals and Orbits 

When the a planet moves around the sun, you can plot a definite path for it which is called an orbit. A simple view of the atom looks similar and you may have pictured the electrons as orbiting around the nucleus. The truth is different, and electrons in fact inhabit regions of space known as orbitals. If an electron is in a particular orbital it will have a particular definable energy.

Each orbital has a name.

The orbital occupied by the hydrogen electron is called a 1s orbital. The “1” represents the fact that the orbital is in the energy level closest to the nucleus. The “s” tells you about the shape of the orbital. s orbitals are spherically symmetric around the nucleus – in each case, like a hollow ball made of rather chunky material with the nucleus at its centre.

Hydrogen’s electron – the 1s orbital   

The orbital on the left is a 2s orbital. This is similar to a 1s orbital except that the region where there is the greatest chance of finding the electron is further from the nucleus – this is an orbital at the second energy level.

2s (and 3s, 4s, etc) electrons spend some of their time closer to the nucleus than you might expect. The effect of this is to slightly reduce the energy of electrons in s orbitals. The nearer the nucleus the electrons get, the lower their energy.

3s, 4s (etc) orbitals get progressively further from the nucleus.

p orbitals

Not all electrons inhabit s orbitals (in fact, very few electrons live in s orbitals). At the first energy level, the only orbital available to electrons is the 1s orbital, but at the second level, as well as a 2s orbital, there are also orbitals called 2p orbitals.

A p orbital is rather like 2 identical balloons tied together at the nucleus. The diagram on the right is a cross-section through that 3-dimensional region of space. Once again, the orbital shows where there is a 95% chance of finding a particular electron.

Unlike an s orbital, a p orbital points in a particular direction – the one drawn points up and down the page. At any one energy level it is possible to have three absolutely equivalent p orbitals pointing mutually at right angles to each other. These are arbitrarily given the symbols px, py and pz. The p orbitals at the second energy level are called 2px, 2py and 2pz. All levels except for the first level have p orbitals.

d and f orbitals

In addition to s and p orbitals, there are two other sets of orbitals which become available for electrons to inhabit at higher energy levels. At the third level, there is a set of five d orbitals (with complicated shapes and names) as well as the 3s and 3p orbitals (3px, 3py, 3pz). At the third level there are a total of nine orbitals altogether.

Fitting electrons into orbitals

You can think of an atom as a very bizarre house (like an inverted pyramid!) – with the nucleus living on the ground floor, and then various rooms (orbitals) on the higher floors occupied by the electrons. On the first floor there is only 1 room (the 1s orbital); on the second floor there are 4 rooms (the 2s, 2px, 2py and 2pz orbitals); on the third floor there are 9 rooms (one 3s orbital, three 3p orbitals and five 3d orbitals); and so on. But the rooms aren’t very big . . . Each orbital can only hold 2 electrons.

A convenient way of showing the orbitals that the electrons live in is to draw “electrons-in-boxes”.

“Electrons-in-boxes”

Orbitals can be represented as boxes with the electrons in them shown as arrows. Often an up-arrow and a down-arrow are used to show that the electrons are in some way different. The diagram (not to scale) summarizes the energies of the orbitals up to the 4p level. Notice that the s orbital always has a slightly lower energy than the p orbitals at the same energy level, so the s orbital always fills with electrons before the corresponding p orbitals.

The order of filling orbitals

Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible.

Osmosis Through the Cell Membrane of an Egg
by Brett Helms

Introduction:
An egg can be used to show the process of osmosis through the egg’s cell membrane. Osmosis is the process in which water diffuses across a cell membrane from an area of high concentration to an area of low concentration. Osmosis occurs when the cell needs to release water to the exterior of the cell membrane or it can occur when water needs to diffuse into the cell. Diffusion is the movement of molecules from an area of high concentration to an area of lower concentration. Diffusion and osmosis are both types of passive transport. A passive transport is the movement of molecules across a cell membrane without the cell having to exert any energy. Water will move into the cell when the solution is hypotonic to the cytosol. A hypotonic solution is defined as a solution whose solute concentration is lower than that inside a cell. The cell membrane is the thin outer layer of a cell. It allows certain substances to diffuse across, into and out of the cell. When a solution is hypertonic the solute concentration is higher than that inside a cell. This would result in molecule movement into a cell. When a solution is isotonic, the solute concentration equals that inside a cell. This would result into no movement of molecules across the cell membrane. Molecules that are very small but not soluble in lipids may diffuse across the membrane by moving through the pores of the cell membrane.

Hypothesis:
The water and the vinegar will diffuse through the cell membrane more easily then the syrup.

Materials:
The materials used included paper, pencil, 200ml beakers, vinegar, distilled water, syrup, 2 eggs, tray, plastic wrap, colored marker, tape, and a digital balance.

Methods:
The two eggs were weighed on a digital balance and their initial masses were recorded. The eggs were then placed in a beaker of vinegar to dissolve the calcium shell and leave the cell membrane exposed. The eggs were weighed after they were removed from the vinegar and their masses were recorded. The two eggs were then placed in a beaker of syrup.  The eggs were once again weighed on the digital balance and their masses recorded. The eggs were then placed a beaker of distilled water. After the eggs were removed and their weights and appearance recorded.

Results:

Results of Osmosis
Table for Egg 1

Results of Osmosis
Table for Egg 2

1. When the egg was placed in the water, in which direction did the water molecules move? The water molecules moved into the egg.

2. On what evidence do you base this? The egg mass increased.

3. How do you explain the volume of liquid remaining when the egg was removed from the syrup? It was less because some had moved into the egg.

4. When the egg was placed in the water after being removed from the syrup, in which direction did the water move? The water moved into the egg.

Error Analysis:
This experiment is based upon changes in egg mass in various solutions. The balance used was not checked for accuracy with two standard masses. The changes in masses may or may not be correct.

Discussion and conclusion:
There were significant increases in egg mass when the eggs were submerged in vinegar and water. However there was a significant decreased in egg mass when the eggs were submerged in syrup causing the egg to appear shriveled. The reason for the increasing and decreasing is called diffusion, which is the movement of molecules from an area of high concentration to an area of lower concentration. That is why when the egg was submerged in syrup much of the water in the egg diffused out because of the higher amount of water in the egg compared to the smaller amount in the syrup. That is also why when the egg was placed in vinegar and in distilled water its mass increased meaning that more water molecules moved into the eggs because the solution outside the egg was lower in water than that inside the egg. These results support the hypothesis that water diffuses from an area of high to an area of low concentration across a cell membrane. This experiment demonstrated the process of osmosis through the egg cell membrane.