All matter is made up from tiny particles called atoms. An atom is the smallest part of an element and consists of protons, neutrons and electrons. Protons, which are positively charged and neutrons, which are neutral, join together at the centre of the atom to form the nucleus. Around the nucleus negatively charged particles called electrons orbit, a little like planets orbiting the Sun. These electrons are arranged in layers like the layers of an onion. These layers are called shells. More than 99.9% of an atom is empty space occupied by electrons.
In an atom of any particular element, there is always the same number of electrons and protons. It is the number of protons that gives the element its atomic number. So, for example, an atom of carbon has six protons in its nucleus, has six electrons surrounding it and has an atomic number of six. The element is also electrically neutral since the number of positively charged protons and negatively charged electrons are the same (the neutron is neutrally charged so does not affect the electrical charge of the atom). All the atoms of an element have the same number of protons. Some however have different numbers of neutrons. These are called isotopes. For example, carbon atoms usually have six protons and six neutrons in their nuclei with an atomic mass of 12. However, if the atom has eight neutrons, it would be an isotope. In this case, it would be called C-14, which is a radioactive isotope of carbon.
The atoms of different elements vary, with different masses and different colours. Atoms of one element can combine with atoms of another to form a new substance called a compound. Compounds are held together by ionic bonding, covalent bonding or metallic bonding. The larger particles in compounds are called molecules. For example, when two atoms of hydrogen (H) combine with one atom of oxygen (O) we get the compound water (H2O). Atoms can also be brought together in a mixture where the atoms are not combined chemically but physically.
