Explain why Fluorine is more reactive than Iodine.
Fluorine is higher up in the halogens than Iodine is. The higher up the group, the fewer electron shells the element has. (We also know that Fluorine has fewer electrons than iodine, and hence fewer electron shells, because of the period it is in and the atomic number.) Because Fluorine has fewer electron shells than Iodine their is less shielding, ie. the attractive force between the nucleus' positive charge and the valence electrons is greater. Because the Halogens are group 7 we know they need 1 electron to complete their outer shell of electrons. Therefore, having the ability to strongly attract an electron makes an element more reactive. Iodine is less reactive because there are more shells of electrons shielding the attractive forces between the nucleus and the valence electrons. Therefore, Iodine has less of an ability to attract a new electron and is thus less reactive than Fluorine.
How are cacti adapted to their environment?
Cacti have spiky needle-like leaves which reduce the surface area in contact with the air and in turn reduces the effect of water loss by transpiration. They also have a thick waxy cuticle which also reduces the rate of water loss. Cacti have long shallow roots which means they can gather water from a large area as quick as they can. Cacti have pleats which allow for the stem to enlarge when water is taken up.
What are the units of frequency? How does the frequency of light relate to energy?
Hz, E=hf ( where E is the energy of the photon, h is Planck's constant and f is the frequency)