Chapter 3: Elements and the Periodic Table Section 1: Introduction to Atoms

Intro Questions: A.) What does all matter consist of? B.) How many elements are there? C.)What are elements made of? D.)What is a molecule? E.)How many atoms are in a water molecule? F.)How many atoms would fit in a period at the end of a sentence? G.)Can we see an atom?

Structure of an Atom Nucleus: very small center core of an atom where protons and neutrons are located; contains most of the mass of the atom Proton: positively charged subatomic particle found in the nucleus Neutron: neutral (no charge) subatomic particle found in the nucleus Electron: negatively charged subatomic particle found moving rapidly outside the nucleus in electron cloud

Electron Cloud Sphere-shaped area outside the nucleus where electrons move Negatively charged Electrons may be anywhere within it Electrons with lower energy usually move in the space (orbitals) near the atom’s nucleus and ones with higher energy are further from the nucleus Accounts for most of the volume of the atom (if nucleus were the size of a pencil eraser in the middle of a baseball stadium, the electrons could be as far away as the top row of seats)

Checkpoint 1 What are the three subatomic particles?

Checkpoint 1 What are the three subatomic particles? Protons, neutrons, and electrons

Comparing Particle Masses Protons and neutrons have approximately equal masses Electrons have a mass 1/2000th that of a proton or neutron Almost all of the mass of an atom is in the nucleus even though it is so much smaller than the electron cloud (nucleus is very dense!) Atoms measured in atomic mass units (amu)

Checkpoint 2 Which is the smallest of the subatomic particles?

Checkpoint 2 Which is the smallest of the subatomic particles? electron

Development of Atomic Theory over Time 1808: John Dalton concluded that each element is made of atoms that are all alike. He thought that different elements have atoms of different mass and imagined atoms as tiny, solid balls.

Development of Atomic Theory over Time 1897: J.J. Thomson discovered the electron. Later proposed that an atom is a positively charged sphere with electrons embedded in it. Called the plum pudding model.

Development of Atomic Theory over Time 1904: Hantaro Nagaoka proposed a model of the atom that had a large sphere in the center with a positive charge. His model showed the electrons revolving around this sphere like the planets around the sun.

Nagaoka Model

Development of Atomic Theory over Time 1911: Ernest Rutherford concluded that the atom is mostly empty space. Electrons orbit randomly around a small, positively charged nucleus.

Development of Atomic Theory over Time 1913: Niels Bohr proposed a model that showed electrons moving in specific layers, or shells, rather than randomly. He said that atoms absorb or give off energy when the electrons move from one shell to another.

Development of Atomic Theory over Time 1932: James Chadwick discovered the neutron. The existence of neutrons explained why atoms were heavier than the total mass of their protons and electrons.

Development of Atomic Theory over Time Present model: Electrons form a negatively charged cloud around the nucleus. It is impossible to determine exactly where an electron is at a given time.

Checkpoint 3 Who discovered that electrons are found in shells?

Checkpoint 3 Who discovered that electrons are found in shells? Niels Bohr

Atoms and Elements Each element consists of atoms that differ from atoms of all other elements An element can be identified by the number of protons in the nucleus (all hydrogen atoms have 1 proton, all helium atoms have 2, etc) Atomic number: the number of protons in the nucleus (usually big number in box on periodic table)

Atomic Number

Isotopes All atoms of same element have same number of protons, but can have different number of neutrons Isotope: atoms with the same number of protons and different number of neutrons are called isotopes

Isotopes of Helium

Mass Number Mass number: sum of the protons and neutrons in the nucleus of an atom Used to identify isotopes of the same elements Ex. Carbon-14 (carbon with a mass number of 14) has 6 protons (as do all carbon atoms) and so, must have 8 neutrons.

Modeling Atoms Atoms hard to study because of size (a sheet of paper is 10,000 atoms thick) Scientists create models to describe them Model: a diagram, mental picture, mathematical statement, or object that helps explain ideas about the natural world

Checkpoint 4 Oxygen has an atomic number of 8. How many neutrons does the isotope oxygen-15 have?

Checkpoint 4 Oxygen has an atomic number of 8. How many neutrons does the isotope oxygen-15 have? Mass number – atomic number = neutrons 15 – 8 = 7 neutrons

Post-lesson Quiz 1. An element is identified by its atomic number. What is atomic number equal to? a. neutrons b. electrons c. protons d. quarks

Post-lesson Quiz 2. Which two subatomic particles are located in the nucleus? a. electrons and neutrons b. protons and neutrons c. electrons and protons d. protons and quarks

Post-lesson Quiz 3. Which number is equal to the number of protons and neutrons? a. atomic number b. mass number c. atomic mass d. weight number

Post-lesson Quiz 4. What are atoms with the same number of protons and different number of neutrons called? a. atoms b. ions c. molecules d. isotopes

Post-lesson Quiz 5. What do scientists call a diagram, mental picture, mathematical statement, or object that helps explain ideas about the natural world? a. a model b. a graph c. a formula d. an equation