Atoms, Molecules and Ions Chapter 2

Properties of the Electron, Proton & Neutron amu = atomic mass units 1 amu = 1.66x g Mass of carbon-12 = 12 amu The Modern View of Atomic Structure

The nucleus of an atom has a structure –It is composed of protons and neutrons The proton  a nuclear particle  positive charge equal to that of the electron  mass more that 1800 times that of the electron. The neutron  a nuclear particle  having a mass almost identical to that of the proton  but no electric charge.

Atomic number (Z) = number of protons in the nucleus of an atom. Isotopes, Atomic Numbers and Mass Numbers Mass number (A) = the total number of protons and neutrons in a nucleus. An element is a substance whose atoms all have the same atomic number.

Isotopes have the same atomic number (p + ), but different mass numbers (p + +n o )

Nuclide symbols You write the nuclide symbol for the naturally occurring sodium nuclide as follows: Na Mass number  Atomic number  A nuclide is an atom characterised by a definite atomic and mass number.

Alternate Example Write the nuclide symbol for the nucleus that has 19 protons and 20 neutrons.

Periodic table of the elements A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements. Mendelev

Molecules and Molecular Compounds A molecule is a definite group of atoms that are chemically bonded together - that is tightly connected by attractive forces. Molecular compounds  compounds that consist of molecules

The chemical formula of a substance is a notation that uses atomic symbols with numeric subscripts to convey the relative proportions of atoms of the different elements in the substance. eg. Alumina = Al 2 O 3 Salt = NaCl Ethanol = C 2 H 6 O A molecular formula gives the exact number of different atoms of an element in a molecule eg. Ethanol = C 2 H 6 O

Some molecules

Elements that can exist as molecules H 2, O 2, N 2, F 2, Cl 2, Br 2, I 2,

Polymers are very large molecules that are made up of a number of smaller molecules repeatedly linked together. Monomers are the small molecules that are linked together to form the polymer e.g. ethylene to polyethylene

Ions and Ionic Compounds Not all substances are molecular, some are made up of ions. An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons. Anion - a negatively charged ion Cation - a positively charged ion An ionic compound is a compound composed of cations and anions, eg NaCl

NaCl - Lattice

The empirical formula of a substance is the relative number of atoms or ions symbolized in the formula.  Use the smallest set of integers to express the correct ratios of atoms present. Eg. Consider hydrogen peroxide: Molecular formula = H 2 O 2 Empirical formula =

Writing an ionic formula What is the formula of magnesium nitride?