Trillium International School. Chemistry for Grade 9. Unit 1. Objectives. After completing this unit, you will be able to. Copperhen Dalton's Atomic Theory and Modern Atomic Theory. Understand the discovery of the electron and the nucleus. Know the terms like the atomic number, mass number, atomic mass, isotope, and electron configuration. Develop skills in colon determining the number of protons, electrons, and neutrons of atoms from atomic numbers and mass numbers. The Greek philosopher Democritus proposed what we consider as the first atomic theory. He believed that all matter consists of very small, indivisible particles. He named the particles atoms, meaning uncutable or indivisible. John Dalton's work in atomic theory marked the start of modern chemistry. His concept of an atom was far more detailed and specific than Democritus. In our current view of atomic theory, atoms consist of smaller particles, electrons, protons, and neutrons. The structure of an atom is basically a cloud of electrons surrounding a small, dense region, called the nucleus. An electron is a negatively charged particle with negligible mass. The nucleus was discovered by Ernest Rutherford. Rutherford suggested that the charge in mass of an atom are concentrated in its nucleus and that the nucleus has a positive charge. In current views, the nucleus of an atom contains protons and neutrons. Protons have a positive charge, and neutrons have no charge. Each element has a unique number of protons in its atoms. This number is the atomic number of the element and is designated by z. The total number of protons and neutrons in the nucleus of an atom is the mass number of the element. The mass number is designated by a. Atomic number and mass number for an atom of an element, x, is designated by. A neutral atom is an atom of any element that has no electric charge over it. This implies that it has the same number of protons and electrons in it. For a neutral atom, the number of protons equals the number of electrons equals atomic number. The number of protons plus the number of neutrons equals mass number. In this video, we're going to have a quick look at atoms. In an atom, the number of electrons is equal to the number of protons, and this means that an atom has no overall electric charge. All atoms have a particular element in the same number of protons and you can see this on the here-look table. This is lithium. All lithium atoms have three protons, and we call the number of protons the atomic number. When you look at the periodic table, you will see another number above the element number. This is the mass number, and it's the total number of neutrons and protons combined. As well as using a diagonal of an atom to determine the number of protons and neutrons, you can also use the periodic table to do a structural arrangement. Sodium has an atomic number of 11, meaning it has 11 protons. The mass number of sodium is 23, so therefore to find out the number of neutrons, you simply need to take away the atomic number from the mass number, even 12, the number of neutrons. Remember our first word, for the atom, the number of electrons is equal to the number of protons, so therefore we know that sodium has 11 electrons. Though the number of protons in a particular element cannot change, you can change the number of neutrons in electrons. Changing the number of neutrons makes an isotope. Next take carbon here is an example. Normally carbon has a mass number of 12 consisting of 6 protons and 6 neutrons. By adding an extra neutron, we can make an isotope called carbon 13. Another common isotope of carbon is carbon 14, which has 2 additional neutrons. As well as changing the number of neutrons, you can also change the number of electrons. As electrons have a negative charge, losing one gives the atom an overall positive charge. In this example sodium lost one electron and thus gained an overall charge with plus 1. Here however, barilium has lost 2 electrons and therefore gained an overall charge with plus 2.