Atoms, Ions and Molecules 2.1

1. Atoms, Ions and Molecules 2.1 Democritus asked, about 2500 years ago, “suppose you continuously break something into two. Would you eventually reach a point at which the item can no longer be divided?” He believed that there was a limit. He called this smallest fragment atomos, meaning “unable to be cut.”

2. Today an atom is considered the smallest part of an element that retains all of the properties of the element. Atoms are incredibly small. If you placed 100,000,000 atoms side by side, they would make a row about 1 cm long.

3. As small as an atom is, it is made out of even smaller sub-atomic parts. In the center of an atom, occupying a space equivalent to that of a marble in a football field, is a nucleus, which contains two kinds of particles, protons and neutrons. Protons and neutrons have the same mass*.

4. Each proton, has one positive electric charge, whereas a neutron has no charge. A third type of atomic particle is the electron. Electrons are so light that their mass is usually considered to be zero. An electron is 1/1840 the mass of a proton. Each electron has one negative electric charge.

5. Electrons move so rapidly through the area around the nucleus that they occupy most of the space in an atom; this space is referred to as the atom's electron shells. The number of protons equals the number of electrons in an atom, so that an atom has a net electric charge of zero.

7. All the atoms of a particular element have the same number of protons in their nuclei; this number is the atomic number of the element. carbon sodium

8. carbon sodium Protons - 6 Protons -11 Electrons - 6 Electrons - 11 Neutrons - 6 Neutrons - 11

9. Because each element has a different number of protons, it also has a different atomic number from all other elements. For example, the atomic number of carbon is six, because it has six protons.

10. What is the atomic number of helium?

11. Helium has two protons. Its atomic number is 2.

12. The mass of an atom, the atomic mass, is determined by adding the number of its protons and the number of its neutrons, since protons and neutrons have the same mass. What is helium’s mass?

13. There are two protons, and two neutrons. Helium has an atomic mass of 4.

14. Some elements occur in two or more different forms, or isotopes. Isotopes of the same element have different numbers of neutrons, but the same number of protons. They have the same atomic number (same element), but a different atomic mass.