What is an atom?
An atom is the smallest unit of ordinary matter that keeps the properties of an element. Every atom has a dense central nucleus made of positively charged protons and neutral neutrons. Negatively charged electrons occupy regions of space around the nucleus called orbitals.
Atomic number & mass number
The atomic number (Z) equals the number of protons and defines the element. The mass number (A) is protons + neutrons. Neutral atoms have equal numbers of protons and electrons.
Electron shells
Electrons live in energy levels (shells). The first shell holds up to 2 electrons, the second and third up to 8. Chemical behavior is mostly decided by the outermost shell — the valence electrons.
Isotopes
Atoms of the same element with different neutron counts are isotopes. Carbon-12 and Carbon-14 are both carbon (6 protons) but with 6 and 8 neutrons respectively. Carbon-14 is radioactive and is used in dating fossils.