1. Location The protons and neutrons make up the dense nucleus of the atom. The electrons are found in the space around the nucleus called the electron cloud.
Electron cloud Nucleus
2. Mass The mass of sub-atomic particles is measured in a unit called an atomic mass unit (a.m.u.). Mass is normally measured in grams. However, atomic mass units are used because the particles are extremely tiny: 1 a.m.u. = 1.66 x 10-30
Fig. 3 The location of the nucleus and the electron cloud
g (or 0.00000000000000000000000000000166 g). As
you can see, it is much simpler just to call that number 1 atomic mass unit! Protons and neutrons have an equal mass of 1 a.m.u. Electrons have a lot less mass. Nearly
2,000 electrons have the same mass as one proton. This means that practically all the mass of the atom is concentrated in the nucleus where the protons and neutrons are.
3. Charge From the table you can see that both protons and electrons carry an electrical charge. Protons carry a positive charge (represented by +1) and electrons carry an equal but opposite charge (represented by –1). The difference in charge creates an attraction between the electron cloud and the nucleus. For an atom to be neutral (to have no overall charge) it must have equal numbers of protons and electrons. Neutrons do not affect the overall charge because they are neutral and do not have a charge.
Calculating the number of protons and electrons Every element listed on the Periodic Table has two numbers listed with it. These are called the atomic number and the mass number.
He 2
4
Atomic number = the number of protons (= the number of electrons in a neutral atom)
Atomic symbol
Mass number = the total mass of an atom of the element
The atomic number gives the number of protons in one atom of that element. There are two protons in an atom of helium. If the atom is neutral then there is the same number of electrons as protons in the atom. A neutral atom of helium contains two electrons.
The atomic
number of an atom is the
number of protons in its nucleus.
The mass
number of an atom is the combined total of protons and neutrons in its nucleus.
