Section 1: Introduction to Atoms

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?

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

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)

What are the three subatomic particles?

What are the three subatomic particles?

Protons, neutrons, and electrons

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)

Which is the smallest of the subatomic particles?

Which is the smallest of the subatomic particles?

electron

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.

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.

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.

1911: Ernest Rutherford concluded that the atom is mostly empty space. Electrons orbit randomly around a small, positively charged nucleus.

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.

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

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.

Who discovered that electrons are found in shells?

Who discovered that electrons are found in shells?

Niels Bohr

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)

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

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.

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

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

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

1. An element is identified by its atomic number. What is atomic number equal to?

a. neutronsb. electronsc. protonsd. quarks

2. Which two subatomic particles are located in the nucleus?

a. electrons and neutronsb. protons and neutronsc. electrons and protonsd. protons and quarks

3. Which number is equal to the number of protons and neutrons?

a. atomic numberb. mass numberc. atomic massd. weight number

4. What are atoms with the same number of protons and different number of neutrons called?

a. atomsb. ionsc. moleculesd. isotopes

5. What do scientists call a diagram, mental picture, mathematical statement, or object that helps explain ideas about the natural world?

a. a modelb. a graphc. a formulad. an equation