The answer is D. element
It's not a mixture, just a pure element. Copper (Cu) is found on the Periodic Table of the Elements, where only ELEMENTS are organized.
The difference in elements??
The positive effect of using chemistry is the generation of energy without fossil fuels.
Chemistry is the branch of science that deals with the study of the nature, properties and composition of matter as well as the changes that matter undergoes. Chemistry has really contributed positively to the advancement of the society.
Lately, there has been a lot of emphasis on alternative sources of energy apart from fossil fuels. One such options is the use of hydrogen fuels. Chemistry has been at the forefront of this research.
Hence, one positive effect of using chemistry is that energy can be generated without burning fossil fuels.
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The electron configuration that represent an excited state for an atom of calcium is 2, 8, 7, 3.
Calcium atom has an atomic number of 20 and its electronic configuration is 2, 8, 8, 2. An atom is said to be in an excited state if it gains energy and move to an higher energy level. For the calcium atom given above, there are 20 electrons which are distributed into four shells. But in the excited state [option 3], one of the 8 electrons in the third shell gains energy and move to the fourth shell. Thus, the number of electrons in the third shell reduced by 1, while the number of electrons in the fourth shell increase by 1.