Electrons are by far the smallest of the three main, stable parts of every atom; the other two parts are protons and neutrons (see atoms). In a normal atom there are the same number of electrons as protons.
Electrons are 1836 times as small as protons and have a mass of just 9.109 x 10 -31 kg. 10-31 means there are 30 zeros after the decimal point. So they weigh almost nothing.
Electrons were discovered by English physicist Joseph John Thomson in 1897 as he studied the glow in a cathode-ray tube (see television). This was the first time anyone realized that the atom is not just one solid ball.
Each atom has a different number of electrons. Its chemical character depends on the number of electrons in its outer shell. Atoms with only one electron in their outer shell, such as lithium, sodium and potassium, have many properties in common.
Electrons are packets of energy. They can be thought of either as a tiny vibration or wave, or as a ball-like particle. They travel as waves and arrive as particles.
You can never be sure just where an electron is. It is better to think of an electron circling the nucleus not as a planet circling the Sun but as a cloud wrapped around it. Electron clouds near the nucleus are round, but those farther out are other shapes.
Electrons have a negative electrical charge. This means they are attracted to positive electrical charges and pushed away by negative charges. Electrons cling to the nucleus because protons have a positive charge equal to the electron’s negative charge. Electrons have so much energy that they whizz round too fast to fall into the nucleus. Instead they circle the nucleus in shells (layers) at different distances, or energy levels, depending on how much energy they have. The more energetic an electron, the farther from the nucleus it is. There is room for only one other electron at each energy level, and it must be spinning in the opposite way. This is called Pauli’s exclusion principle.
Electrons are stacked around the nucleus in shells. Each shell is labelled with a letter and can hold up to a particular number of electrons. Shell K can hold up to 2, L 8, M 18, N 32, 0 about 50, and P about 72.