Atoms 1.1 Flashcards
Atomic Nuclei
Atomic nuclei are the number of protons in the nucleus that defines the type to atom, and therefore which element it belongs to. For example, all hydrogen (H) atoms have 1 proton in their nucleus, helium (He) atoms have 2 protons, lithium (Li) atoms have 3 protons and so on. Scientists refer to the number of protons in the nucleus as the atomic number. The total number of protons and neutrons in the nucleus is the atom’s mass number.
Isotopes
Atoms of the same element may have different numbers of neutrons. For example, most helium atoms have 2 protons and 2 neutrons. These atoms have a mass number of 4 and so are known as helium-4. However, helium-3 atoms also exist. Helium-3 atoms contain 2 protons but only 1 neutron, so their mass number is 3. Atoms that have the same number of protons but different numbers of neutrons are referred to as isotopes.
Almost every element has two isotopes and sometimes many more. Hydrogen has three isotopes: hydrogen-1, hydrogen-2 and hydrogen-3. The most common isotope is hydrogen-1, which has a single proton as its nucleus. It makes up 99.98% of hydrogen atoms on Earth and is sometimes referred to as protium. Hydrogen-2 is more commonly known as deuterium and has 1 proton and 1 neutron. Hydrogen-3 is known as tritium and has 1 proton and 2 neutrons. These isotopes of hydrogen are used in nuclear power plants to make the generation of power more efficient.
Electrons and the Nucleus
The number of electrons surrounding the nucleus of an atom is exactly equal to the number of protons in the nucleus. As a result, atoms are charge-neutral (have no charge) because the positive charge of the protons is exactly balanced by the negative charge of the electrons.
Although each electron is 1800 times smaller than a proton, together with the electrons form ‘clouds’ around the nucleus. The clouds can be 100 or even 1000 times wider than the nucleus. This means that if the nucleus was the size of a golf ball, the electrons would form clouds the size of a football stadium. It also means that most of an atom is empty space.
Electron Shells
The electrons in an atom are attracted to the nucleus by the positive charge of the protons. However, the electrons never fall into the nucleus. This is because the electrons are trapped inside the electron shells, which surround the nucleus like the layers of an onion.
Many of the electron shells in an atom are empty. The biggest known element at present has 118 electrons in 6 shells. The 1st electron shell is the innermost shell. It is the smallest electron shell and can only contain 2 electrons. Once the 1st electron shell is full, electrons start to fill the 2nd electron shell. which can hold up to 8 electrons. The 3rd electron shell holds up to 18 electrons. The 4th shell can hold 32 electrons.
The number of electrons in each shell of an atom is known as its electron configuration. For example, carbon has 6 protons and therefore 6 electrons. The first 2 electrons fill the 1st electron shell, and the remaining 4 electrons go into the 2nd shell. Therefore the electron configuration for carbon is 2,4.
The electron configuration for a sodium atom is 2,8,1. This is because sodium atoms contain 11 electrons. The first 2 electrons fill the 1st electron shell, the next 8 electrons fill the 2nd electron shell and the remaining electron goes into the 3rd electron shell.
Atoms in Elements and Compounds
When atoms stick together they can form either cluster of atoms known as molecules or large grid-like structures known as crystal lattices. For example, water (H2O) is made up of molecules. Eery water molecule is identical and contains two hydrogen atoms (H) and one oxygen atom (O). On the other hand, a grain of beach sand is a crystal lattice of silicon (Si) and oxygen (O) atoms. the number of atoms in the lattice depends on the size of the grain of sand.
