We observe that some objects or particle are easier to move than others. The resistance to changing the state of motion (either stopping or accelerating) is called inertia.
We can quantify the inertia of various objects with a parameter called, "the inertial mass" or just "mass". An object with more mass has more inertia and is thus harder to stop if it is already moving or harder to accelerate if it is not moving.
The unit for mass is the kilogram (denoted) kg. This is the SI unit. SI stands for Systeme International. This is the standard unit used in science and what we will often use in this class.
To give you a sense of scale, a typical human as a mass of 75 kg. Since the atoms are really small, they have a correspondingly very small mass. What is the mass of an atom? First lets look at an atom's parts.
The simplest of all atom is hydrogen. It is also the most common atom in the Universe (~ 70% of all atoms in the Universe are hydrogen). Ironically, you cannot find pure hydrogen on Earth (there is a -physics- reason for that -- see your book!).
There are two basic components of an atom. The nucleus is at the center and it is made of protons and neutrons (except for hydrogen where the nucleus is just a proton). The nucleus is heavy. Almost all the mass of an atom is in the nucleus.
Around the nucleus, electrons are moving around. In what is called the "Bohr model" of the atom, the electrons are going around in circles (orbits) around the nucleus. This is the picture of the atom that most people are used to. It is not quite right and we will see a better picture later on but the important thing to remember is that atoms are mostly made of nothing or void . Atoms are almost all empty space. The nucleus is very very small and the image above of Hydrogen is deceiving. If it was to scale, you would not be able to see the nucleus. A clever website illustrating the void between the proton and electron in Hydrogen is KeithCom's Hydrogen atom .
The reason why electrons orbit the nucleus is because of their electric charge. They have a negative electric charge while the nucleus contain protons which have positive electric charge. Opposite attracts and so the electron wants to stick around the proton. It is always moving though so the electron does not actually collide with the nucleus (which is very very small) and instead it just hovers and orbits around it.
In addition to protons (with positive electric charge) and electrons (with negative electric charge), the nucleus contains neutrons who are neutral. The picture above shows a nucleus with neutrons and protons together.
Isotopes: Most elements have an equal numbers of neutron and protons, although this vary (e.g. hydrogen has one proton but no neutron). The number of protons in atom is what decides its name and a lot of his properties. Any atom with 6 protons in the nucleus is called carbon . An isotope is an element with the same number of protons but different number of neutrons. Deuterium, often called heavy hydrogen, is a common isotope of hydrogen. The nucleus has 1 proton and 1 neutron and this is a perfectly stable element. Because the nucleus is twice as heavy as normal hydrogen, deuterium is often called heavy hydrogen. When deuterium is used to make water we call the water, heavy water. This type of water is used to cool nuclear power plant for example. Tritium is another isotope of Hydrogen. It has 1 proton and 2 neutrons. It is unstable and radioactive and thus much rarer on our planet.
Ions: Finally, an ion is an atom which has more or fewer electrons than the number of protons. If the atom has one more electron than protons, it will have a net negative charge If one electron is taken away, it will have a net positive charge.