18. Explain how the atomic number of an element identifies the element.
19. How can atomic number and mass number be used to find the numbers of protons, electrons, and neutrons?
20. An atom is identified as platinum-195
18. The atomic number of an element is a unique identifier for that element. It represents the number of protons in the nucleus of an atom. Each element on the periodic table has a different atomic number, which corresponds to a specific number of protons. For example, hydrogen has an atomic number of 1, meaning it has one proton, while carbon has an atomic number of 6, indicating it has six protons. Therefore, by knowing the atomic number of an element, you can determine which element it is.
To find the atomic number of an element, you can refer to the periodic table. The atomic numbers are usually listed above or below each element's symbol.
19. Both atomic number and mass number can be used to determine the number of protons, electrons, and neutrons in an atom.
The atomic number (Z) of an element gives you the number of protons in the nucleus. Since atoms are electrically neutral, the number of protons is equal to the number of electrons orbiting the nucleus. So, if you know the atomic number of an element, you can directly determine the number of protons and electrons.
The mass number (A) of an atom represents the total number of protons and neutrons in the nucleus. To find the number of neutrons, subtract the atomic number (Z) from the mass number (A). Neutrons don't have a charge, so they don't affect the electrical neutrality of the atom.
For example, if you have an atom with an atomic number of 6 (Z = 6) and a mass number of 12 (A = 12), you know that it has 6 protons and 6 electrons. To find the number of neutrons, subtract the atomic number from the mass number: 12 - 6 = 6. Therefore, this atom has 6 neutrons.
20. An atom identified as platinum-195 means that its mass number is 195. The mass number represents the total number of protons and neutrons in the nucleus. In the case of platinum-195, it means there are 195 protons and neutrons combined.
To determine the number of protons, you can refer to the atomic number of platinum, which is 78. The atomic number gives the number of protons, so there are 78 protons in platinum-195.
To find the number of neutrons, subtract the atomic number from the mass number: 195 - 78 = 117. Therefore, in platinum-195, there are 78 protons and 117 neutrons.
Note: The number of electrons in an atom is the same as the number of protons in a neutral atom. So, for platinum-195, there would also be 78 electrons.