The Atom and The Periodic Table of Elements

naturally occurring / made by scientists We can classify (arrange) elements in different ways: naturally occurring / made by scientists solid/liquid/gas metal/non-metal

Elements are pure substances that cannot be broken down into any simpler substance. Elements are the basic building blocks of all matter. Compounds are any substances that are formed by the chemical combination of two or more elements and acts like a single substance

The Periodic Table is a system of classifying all of the known elements and was first developed by Dmitri Mendeleev. Metals are found to the left of the zigzag line of the periodic table. Nonmetals are found to the right of the zigzag line of the periodic table.

Metalloids are metal or metal-like elements found on both sides of the zigzag line. Metalloids have properties of both metals and non metals. EXAMPLES: Boron, Silicon, Arsenic.

A vertical column of elements on the periodic table is called a group. The elements in the same group of the periodic table have similar chemical properties.

Periods are the horizontal rows of elements on the periodic table. They do not have similar properties.

Every element is made up of very small particles called atoms. Atoms of different elements have a different number called the atomic number. Atoms have a very small, positively charged nucleus, with negatively charged electrons outside the nucleus in energy levels.

Protons (positively charged) The nucleus of every atom (except hydrogen) contains two particles: Protons (positively charged) Neutrons (no charge) In energy levels outside the nucleus we find: Electrons (negatively charged)

Atoms are neutral because the positive charge of the nucleus is equal to all of the negative charges of the electrons added together. 11 electrons 11 negative charges 11 protons 11 positive charges Atoms are neutral because the numbers of protons and electrons are equal - the opposite charges cancel.

Nuclide notation – how many protons, neutrons, and electrons are in atoms. Mass number (protons + neutrons) Cl 35 Atomic number (number of protons) 17 As atoms have no charge, the number of electrons is the same as the number of protons. This atom has 17 electrons. (Electrons = Protons)

Scientists from our past. Dmitri Mendeleev – developed the first periodic table in 1869. In 430 BC Greek Philosopher Democritus gave the name to the smallest piece of matter the atom.

Joseph John “JJ” Thomson discovered the electron in 1897 Joseph John “JJ” Thomson discovered the electron in 1897. His model is sometimes called the “plum pudding” model. According to his model, the atom was made of a pudding-like positively charged material throughout which negatively charged electrons were scattered.

In 1911 English scientist Ernest Rutherford disproved Thomson’s model In 1911 English scientist Ernest Rutherford disproved Thomson’s model. He reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged electrons were scattered outside the nucleus around the atom’s edge. (credited for splitting the first atom) Nucleus – the tiny, extremely dense, positively charged region in the center of an atom.

In 1913 Danish Scientist, Niels Bohr, discovered that electrons move or orbit around the nucleus. The current atomic theory states that electron clouds are where electrons are most likely to be in the space around the nucleus.

Subatomic Particles are particles that are smaller than an atom Subatomic Particles are particles that are smaller than an atom. The three main subatomic particles are protons, neutrons, and electrons. Electron Cloud is the space in which electrons are likely to be found (think of a beehive).

The four forces that account for the behavior of subatomic particles are: Electromagnetic Force – a force that can attract or repel the particles in an atom. This force holds the electrons around the nucleus. Example: If there were 2 protons (same charge) the E.F. is repulsion. If there were 1 electron and 1 proton (opposite charge) the E.F. is attraction.

Strong Force – opposes the electromagnetic force of repulsion between protons. It binds protons together to form the nucleus. Weak Force – responsible for a process known as radioactive decay. A neutron in the nucleus changes into a proton and an electron.

4. Gravity – the force of attraction between all objects in the universe. Of the four forces, it is the weakest force. Why? The masses of the particles in atoms are so small, the force of gravity within atoms is very small.