An atom is the smallest building block of everything that exists. The smallest object you can see under a microscope has more than 10billion atoms .
There are over a hundred different types of atoms, that we call elements. They form the world we live in.
Well known elements are hydrogen, oxygen, iron or lead. When two elements get together they form a compound. For example,water has two atoms of hydrogen and one atom of oxygen.
Atoms all have about the same size, but they have different weights. Plutonium is one of the heaviest atoms , about 200 times heavier than hydrogen , which is the lightest element.
Parts of the Atom
An atom consists of three parts : protons, neutrons, electrons.
Protons and neutrons are in the centre of an atom, which is called the nucleus. It is very small if you compare it with the whole atom and it has almost all of an atom’s mass. If an atom had a diameter of about 6 km the nucleus would only be as big as a tennis ball. The rest of the atom outside the nucleus is mostly empty.
Electrons fly around in an atom very, very quickly. They have almost no mass and travel around the nucleus millions of times every second.
The parts of an atom have electrical charges. Each proton carries a positive electrical charge and each electron has a negative electrical charge. Neutrons have no charge. In most cases an atom has the same number of protons and electrons. It is electrically neutral.
The energy of the nucleus keeps the electron inside the atom—just like the Earth keeps the moon in its orbit. But electrons have energy themselves. They want to break away from the nucleus. If an electron has a lot of energy it moves around farther away from the nucleus.
Electrons move around the nucleus in up to seven round paths , called shells. The first shell is closest to the nucleus. It can hold two electrons. The second shell can hold 8, the third 18 and the fourth 32 electrons. In most atoms, the outer shells are never completely filled with electrons. The inner electrons travel fastest, the outer ones are the slowest.
