Elements, Atoms, and Ions

Elements

• 118 known: 88 found in nature, rest in lab
• You need to know the names and symbols before you can do chemistry
• Could mean a single atom of an element (Ar or H)
• Could mean many molecules of the same element (H2)
• Or atoms of an element in some form (sodium in the human body)

Symbols for Elements

• Each has 1 or 2 letters
• 1st letter is capitalized, 2nd is not
• Usually consists of 1 or 2 letters from names
    * Oxygen = O
• Sometimes taken from Greek or Latin name
    * Gold = Au (aurum)

Dalton’s Atomic Theory

1. Most natural materials are mixtures of pure substances
2. Pure substances are either elements or combinations of elements called compounds
3. A given compound always contains the same mass of the elements

Law of Constant Composition

A given compound always has the same composition, regardless where it came from.

Dalton’s Atomic Theory (1808)

• Elements are made up of tiny things called atoms
• All atoms of the same element are identical (proven to be false)
• Atoms are unique from other atoms of a different element
• Atoms of one element can combine with atoms from another to create a compound
• Atoms are indivisible in chemical processes. They are not created or destroyed in a chemical reaction, it simply changes the way they are grouped (proven to be wrong)

Formula of Compounds

• Compound - distinct substance of 2 or more elements, has the same masses of those elements
• Chemical Formulas - expresses the types of atoms and the number of each

Rules for Formulas

• Each atom is represented by its element symbol
• The number of each is indicated by a subscript written to the right of the symbol
• No subscript is written if there is just one atom

J.J. Thomson (1898-1903)

Postulated the existence of electrons using cathode-ray tubes

The atoms must also contain positive particles that balance exactly the negative change carried by particles we now call electrons

William Thomson (Plum Pudding Model)

Reasoned that the atom might be thought of as a uniform pudding of positive charge with enough negative electrons scattered within to counterbalance the positive charge

Ernest Rutherford (1911)

Explained the nuclear atom

Atom has a dense center of positive charge called the nucleus

Electrons travel around the nucleus at a relatively large distance

A proton has the same magnitude of charge as the electron, but its charge is positive

Rutherford and Chadwick (1932)

Mostly nuclei; contains neutron

A neutron is slightly more massive than a proton, but has no charge

Modern Atom Structure

Electrons: tiny, negatively charged equal to protons, outside nucleus

Protons: positively charged, same size as neutron, inside nucleus

Neutron: no charge, found in nucleus

Different Chemical Properties in Atoms

Caused by electrons

Electrons intermingle when atoms combine into molecules

Number of electrons determines chemical behavior

Isotopes

Atoms with same number of protons, ==different number of neutrons==

Shows almost identical chemical properties

Most elements contain isotopes in nature

X = symbol of element

A = mass number (number of protons and neutrons)

Z = atomic number (number of protons)

Periodic Table

In order of increasing atomic number

Elements with similar properties are in same column, called groups or families

Periods - horizontal rows of elements

Physical Properties of Metal

Conductivity of heat and electricity

Malleability

Ductility

Lustrous

Physical Properties of Nonmetals

Lack of metal properties

Variations in properties

Can be solid, gas, or liquid

Physical Properties of Metalloids

Exhibits properties of metals and nonmetals, i.e. silicon

Natural States of the Elements

Most are very reactive

Usually not found in unconfined form

Exceptions are: noble metals - gold, platinum, silver, and noble gases - Group 8

Allotropes

Different forms of the same element

Ex: Solid carbon occurs in 3 forms - diamond, graphite, and buckminsterfullerene

Ions

Atoms can form ions by gaining or losing electrons

Metals tend to lose one or more electrons to form positive ions called cations

Cations are generally named by using the parent name followed by ion

Nonmetals want to receive electrons to form negative ions called anions

Anions are named using the root of the atom name followed by the suffix -ide, i.e. Oxygen → Oxide ion

Periodic Table Locations

The ion that a particular atom will form can be predicted from the periodic table

Compounds that Contain Ions

Ions combine to form ionic compounds

Properties: high melting points, conduct electricity (if melted or dissolved in water)

Electrically neutral

The charges the anion and cation in the compound must equal to zero

Formula for Compounds

Write the cation element symbol followed by the anion

The number of cations and anions must be correct for their charges to equal to zero

Mg(2+) + Cl(1-) + Cl(1-) = 0 charge

Cation Anions