Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School


The atom explained in two minutes


  • An atom is the smallest particle of matter. We cannot break it down in simpler substances by chemical means (we can break it down in nuclear reactions)
  • Atoms are composed by SUB-ATOMIC PARTICLES.
  • An atom contains protons and neutrons in the nucleus (red in the picture) ,  surrounded by an electron cloud (yellow dots).
  • On the left, another representation of the atom, with protons and neutrons in the nucleus  ,  surrounded by an electrons.


  • Elements are substances which contain only one kind of atoms
  • There are 92 natural elements. Scientists were “producing” new elements in labs since they discovered how to, in 1936. The first man made
    element was technetium.
  • Elements are represented by symbols (First letter always capital)
  • Elements were named after people, places or Latin names
    • K = Kalium, latin name for Potassium
    • Po= Polonium, discovered by Marie Curie and named after


  • Elements are arranged in the Periodic Table. Elements are ordered in increasing number of protons in the nucleus.
  • Each column is called a Group or Family because they contain elements with similar chemical and physical properties. Each row is called a period
    • Most of the elements are metals.
    • Non metals are placed on the right of the periodic table above the  Stairs.
    • The elements just on the stairs are called metalloids. They act as metals or non metals depending of the reaction.

When representing an element, we use its symbol. Some elements exist only in groups of atoms called molecules. Some of them are diatomic molecules (2 atoms together).

How do we know the elements that form diatomic molecules

  • When representing an element, we use its symbol. Some elements exist only in groups of atoms called molecules. Some of them are diatomic molecules (2 atoms together).

Elements marked with a circle form diatomic molecules. Remember:

    • 7 elements
    • forming a number 7,
    • beginning in the element #7 (Nitrogen).
    • The seventh element is Hydrogen.

There only two more elements forming molecules, Phosphorus (with 4 atoms) and Sulphur (with 8 atoms)

The rest are represented as monoatomic.(1 atom)

Periodic Table Group Names

  1. Alkali metals
  2. Earth alkaline metals
  3. Group of boron.
  4. Group of Carbon
  5. Group of Nitrogen
  6. Chalcogens
  7. Halogens
  8. Noble gases (Also called “group 0”) 
  9. Transition metals
  10. Lanthanides (after Lanthanum)
  11. Actinides (after Actinium)


  • All elements in a group will have similar chemical properties because they have the same amount of electrons in the last level of energy

Inside the atom

  • Atoms contain a nucleus (center) with protons and neutrons. They are packed together thanks to the strong nuclear force.
  • Around the nucleus there is an electron cloud containing the same number of electrons than protons in the nucleus. That’s why the atom does not have any charge. The cloud has different shapes depending on the amount of electrons around the nucleus

Properties of Subatomic Particles

There are many subatomic particles that we know so far, but we will concentrate in the ones that are important for chemistry. 

Electrons are SO light that we do not count them when calculating the mass of the atom.

Atomic Number and Mass Number

Atomic number (Proton number)

    • Is the number of protons in the nucleus
    • The atomic number gives the identity to the atoms.
    • Atoms of different elements have different number of protons

Mass number (Nucleon Number)

    • The sum of protons and neutrons
    • The number of neutrons do not affect the atom’s identity

Note: protons and neutrons are the only HEAVY particles in the atom, that’s why we call the total number of protons and neutrons the MASS NUMBER

Representation of Atoms

  • X=Element Symbol (atom)

First letter in upper case, second (if any) in lower case

  • Z=Atomic Number (proton number) = Protons in the nucleus –

Always the smaller number

  • A= Mass number (nucleon number) = Protons + neutrons)

always the bigger number

Some elements lose electrons

Some elements gain electrons

How many protons neutrons and electrons?

How many p, n, e, in atoms and ions?

Metals and non-metals

  • Most of the elements in the periodic table are metals. (the ones in red)
  • The rest are non-metals or metalloids
  • The difference is in the properties of each.

General Properties of metals and non-metals.