Atomic Structure Most of the mass of an atom is concentrated in an extremely small, dense, and positively charges core called the nucleus The nucleus contains protons (positive charge) and neutrons (neutral) Electrons (negative charge) orbit the nucleus in shells

Subatomic Particles Proton Neutron Electron Charge +1 -1 Symbol p+ no -1 Symbol p+ no e- Location nucleus orbits around the nucleus Mass 1 0.0005 (negligible)

Atomic Information All atoms are composed of the same basic particles, yet not all atoms are exactly the same. Atoms of different elements have different numbers of protons. The atomic number of an element is the number of protons in the nucleus of that element. Atoms of the same element have the same number of protons.

(Number of Protons)

Elements in the periodic table are arranged by increasing atomic number

Protons have a positive charge. Electrons have a negative charge. Atoms are electrically neutral in nature, which means that the number of protons equals the number of electrons.

If electrons are added or taken away, the ratio between protons and electrons becomes imbalanced and the atom will have a charge (an ion)

Mass Number The number of protons plus neutrons is the mass number

Isotopes Atoms can also gain or lose neutrons to become more or less stable Atoms of an element with different numbers of neutrons are called isotopes Note: Atoms cannot gain or lose protons