1. 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.

2. (Number of Protons)

3. 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.

4. Ions If electrons are added to the atom, then there are more negative charges than positive charges. The atom becomes negatively charged and is called an anion. If electrons are removed from the atom, then there are less negative charges than positive charges. The atom becomes positively charged and is called a cation. The size of the charge is easily calculated. Net charge equals protons minus electrons.

5. When an atom has a charge, it is written on the upper right-hand corner of the element symbol. Calcium ion has 20 p + and 18 e - – Charge is 20 – 18 = +2 » Symbol is Ca 2+ Bromine ion has 35 p + and 36 e - – Charge is 35 – 36 = -1 » Symbol is Br 1-

6. The Neutron Early models of the atom only considered two particles of the atom, the proton and the electron. The neutron has a mass about equal to the proton, but has no charge. Having neutrons would not pose a problem if all atoms of a particular element had the same number of neutrons – but this is not the case! The number of neutrons can vary.

7. Isotopes Isotopes are atoms of the same element that have different masses, due to the different number of neutrons. The mass number is the number of protons plus the number of neutrons in the nucleus. The mass number is useful when designating isotopes.

9. To write an isotopic symbol, the mass number goes on the top left-hand corner of the element symbol. The number of protons (atomic number) goes on the bottom left-hand corner of the element symbol. Oxygen – 17 – Means that the mass number is 17. Oxygen has 8 protons, which means that there are 9 neutrons. 17 8 O Magnesium – 23 – Means that the mass number is 23. Magnesium has 12 protons, which means that there are 11 neutrons. 23 12 Mg

10. Atomic Notation Example: Number of Protons Number of Electrons Number of Neutrons 17 20 Cl 37 17 Neutrons = Mass Number – Protons _______ = 37 - 17 20