- plot the independent variable on the horizontal axis and the dependent variable on the vertical axis
- label the axes, ensuring that the labels include units
- choose an appropriate scale for each axis
- give the graph an appropriate title
- draw a line of best fit
### Titles
The title of a graph should clearly indicate what the graph represents under what conditions in **title case**, so that any onlooker should be able to identify the experiment. It should not include "vs.". Any legends present should be located under the graph.
??? example
"Effect of Cat Deaths on Suicides in New Zealand"
### Error bars
Please see [SL Physics#Error bars](/sph3u7/#error-bars) for more information.
### Line of best fit
Please see [SL Physics#Uncertainty of gradient and intercepts](/sph3u7/#uncertainty-of-gradient-and-intercepts) for more information.
- The **effective nuclear charge** ($Z_\text{eff}$) is the net positive charge (attraction to the nucleus) experienced by an electron in an atom.
- **Electron shielding** describes the decrease in the effective nuclear charge of an electron because of the repulsion of other electrons in lower-energy shells.
**Atomic notation** is used to represent individual atoms or ions. It is written in the form $^M_Z \text{Symbol}^\text{Charge}$, where $M$ is the mass number of the particle and $Z$ is the atomic number of the particle.
!!! example
- $^1_1 \text{H}^{+}$ is the atomic notation for the most common hydrogen ion.
- $^{16}_8 \text{O}^{2-}$ is the atomic notation for the most common oxygen ion.
- $^{20}_{10} \text{Ne}$ is the atomic notation for a neon atom.
### Isotopes
Isotopes are atoms of the same element but with different masses, or alternatively, atoms with the same number of protons but with different numbers of neutrons.
**Radioisotopes** are isotopes that are unstable (will spontaneously decay, are radioactive). Unstable atoms **decay** (break down) into one or more different isotopes of a different element. The **half-life** of a radioisotope is the time it takes for 50% of a sample's atoms to decay.
!!! warning
Radioisotopes are dangerous! They emit radiation, which is not at all good for human health in the vast majority of cases. However, there are also useful applications for radioisotopes today. For example, Cobalt-60 is used in radiation therapy to kill cancer tumours by damaging their DNA.
### Atomic mass
The mass of every atom is represented relative to 1/12th of a carbon-12 atom. This mass is either unitless or expressed in terms of **atomic mass units (amu or u)**. On the periodic table, the **relative atomic mass** ($A_r$) is shown, which is a weighted average of each the mass of each natural isotope combined with their natural abundance (%occurence).
$$A_r = \text{%occurrence}×\text{mass number of isotope}$$
### Atomic radius
The atomic radius of an atom is the distance from the centre of the nucleus to approximately the outer boundary of the electron shell. This cannot be directly measured by scientists.
### Models
Please see [SL Physics#Models](/sph3u7/#models) for more information.
### Periodic trends
Some trends in the periodic table include:
- atomic radius decreases when going across a period
- atomic radius increases when going down a group
- ionic radius decreases when going across a period for groups 1–13, then sharply increases and then increases for groups 15–17
- ionic radius increases when going down a group
- electron affinity increases when going across a period
- electron affinity decreases when going down a group
- ionisation energy increases when going across a period
- ionisation energy decreases when going down a group
When explaining these trends in the periodic table, it is best to use basic concepts to build on to larger points.
!!! example
To explain why there is a trend of decreasing atomic radius across a period:
- As the number of protons and electrons increase together, but the number of electron shells does not change, the effective nuclear charge of each electron increases, while the effect of shielding remains unchanged.
- This increased effective nuclear charge reduces the atomic radius compared to other atoms before it.
