CHAPTER 18 Properties of Matter

18-1 Atomic Structure Inside the Atom All atoms contain three smaller particles called Subatomic Particles: Protons = Positive Charge (+) Neutrons = Neutral (0) Electrons = Negative Change (-)

Atomic Structure cont.. Protons and neutrons cluster together in the atom’s center, called the Nucleus.

Electrons move in space around nucleus

The Atomic Theory In 1908, John Dalton published the Atomic Theory. It states that all matter is composed of tiny particles called atoms. John Dalton was one of the first scientists to gather evidence for the idea.

The Atomic Theory cont… This theory contains the following important points: 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical. 3. Atoms of different elements have different properties, including mass & chemical reactivity.

The Atomic Theory cont… 4. Atoms are not changed by chemical reactions, but merely rearranged into different compounds. 5. Compounds are formed when atoms of more than one element combine. 6. A compound is defined by the number, type (element) and proportion of the constituent atoms.

Historical Events Ernest Rutherford (1871-1937) identified that the center of the atom is made up of mostly empty space and located a tiny core called the nucleus. James Chadwick (1891-1974) identified the subatomic particle, the neutron. Niels Bohr (1885-1962) described how electrons move around the nucleus in fixed orbits, which became known as “Bohr’s Model.”

18-2 Comparing Atoms Atomic Number The number of protons distinguishes an atom of one element from the atom of another element. If atoms bond or break apart during chemical reactions, the number of protons in each atom always remain the same!!

Atoms Atomic Number is the number of protons in an atom. Mass (weight) Number is the total number of protons and neutrons in the nucleus of the atom.

Atoms cont… Neutrons add mass to the atom, but do not change its atomic number (or charge). Chemists arrange the elements in a table called the Periodic Table of Elements.

Periodic Table of Elements

Isotopes Many elements have atoms with different numbers of neutrons. Isotopes are atoms of the same element that have different number of neutrons.

Isotopes cont… Hydrogen1 1 proton, 0 neutron (mass #= 0) Hydrogen3 1 proton, 2 neutrons (mass #= 3)

What holds the nucleus of an atom together? The nucleus stays together because there is another force acting that is stronger than the repulsion of the protons for each other. This is called Strong Nuclear Force.

How are electrons arranged in atoms? Electrons are never all in one place at the same time! They “buzz” around the nucleus at a very fast rate. Scientists refer to the entire space that electrons occupy as the Electron Cloud.

The Electron Cloud & Particles

Electron Cloud cont… The electron cloud is divided into energy levels. The first energy level is closest to the nucleus and has the lowest energy. As energy levels extend farther from the nucleus, they get larger in diameter and can hold more electrons.

Electron Cloud cont… Each energy level can hold up to a certain number of electrons! 1st energy level holds 2 electrons. 2nd energy level holds 8 electrons. 3rd energy level holds 8 electrons. 4th energy level holds 18 electrons. The outermost region of the electron cloud contains the Valence Electrons & is called “Valence Shell.”

Electron Cloud cont… orbital and the electrons in it are associated with a specific amount of energy, and the farther an electron is from the nucleus the greater its energy. Each element on the periodic table has specific numbers in their energy levels.

Electron Shells for a Sodium Atom (atomic # is 11)

18-3 Periodic Table of Elements Groups of Elements Elements that are part of the same group act alike. Chemists called these similar elements a Group of Elements.

Reading the Periodic Table The Chemical Symbol is an abbreviation of the element’s name. For example: carbon- C, Neon- N, Sulfur- S, etc… The first letter is upper case and the second (if there is one) is lower case. For example: Helium- He, Lead- Pb, Sodium- Na, etc…

Fill in what’s missing!

The End